NBSIR 74-581

Development of a Solid Sorption

Tube for Sampling Hydrogen Fluoride

in the Work Atmosphere

James Wing and John K. Taylor

National Bureau of Standards

Department of Commerce

Wastiington, D. C. 20234

October 1974

Final Report

i

Prepared for National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati Ohio 45202

NBSIR 74-581

DEVELOPMENT OF A SOLID SORPTION

TUBE FOR SAMPLING HYDROGEN FLUORIDE

IN THE WORK ATMOSPHERE

James Wing and John K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

October 1974

Final Report

Prepared for National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

U. S. DEPARTMENT OF COMMERCE. Frederick B. Oent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

CONTENTS

Page

1. Introduction 1

1.1 Scope of the Work 1 1.2 Method of Approach 4

1.3 Work Coverage 4

2. Development of Methodology 4

2.1 General Consideration 4

2.2 Gas Dilution System 6

2.3 Gas Sampling System 6

2.4 Selection of Solid Sorbents for Hydrogen Fluoride 8

2.4.1 Literature Survey on Solid Sorbents. ... 8

2.4.2 General Considerations 9

2.4.3 Collection Efficiencies 10

2.5 Construction of Sorption Tubes for Hydrogen. . . 12

Fluoride

2.5.1 Specifications of Sorption Tubes 13

2.5.2 Impurities in Sodium Acetate 16

2.6 Analytical Methods for Hydrogen Fluoride .... 16

2.6.1 Brief Review of Analytical Methods .... 16

2.6.2 General Considerations 17

2.6.3 Operation of Fluoride Selective Electrode. 18

Instrument

2.6.4 Procedure for Measurement of Amount of . . 20

Fluoride Collected in Sorption Tubes

2.6.5 Calibration of Fluoride Selective 20

Electrode

3. Experimental Results * .... 21

3.1 Collection Efficiencies of Sodium Acetate. ... 21

3.2 Breakthrough and Capacity of the Sorption Tube . 23

3.3 Effects of Elevated Temperatures 25

3.4 Effects of Humidity 25

3.5 Precision, Bleed-out, Storage, and Shipment. . . 28

Studies

3.6 Glass versus Polyethylene Tubes 30

4. Summary and Conclusions. 30

5. Bibliography 32

APPENDIX: ANALYTICAL PROCEDURE: HYDROGEN FLUORIDE IN THE

ATMOSPHERE

i
TABLES

Page

1. Collection Efficiencies of Various Sorbents for . . H

3 ppm Hydrogen Fluoride Gas

2. Collection Efficiencies of Sodium Acetate for ... 22

Hydrogen Fluoride Gas

3. Capacity of Sorption Tubes of Different Sizes ... 24

4. Effects of Temperature on Collection of Hydrogen. . 26

Fluoride

5. Effects of Hymidity on Collection of Hydrogen ... 27

Fluoride

6. Precision, Bleed-out, Storage, and Shipment Studies 29

7. Glass versus Polyethylene Tubes 31

FIGURES

1. Gas Generating and Sampling Systems for HF 7

2. Sorption Tube for HF , 14

3. Pressure Drop Measurement Apparatus 15

4. Calibration Curve of Fluoride Standard Solutions. . 19

11
DEVELOPMENT OF A SOLID SORPTION TUBE FOR SAMPLING

HYDROGEN FLUORIDE IN THE WORK ATMOSPHERE

ABSTRACT

The efficiencies of several solid substances in

a sorption tube for collecting trace quantities of

hydrogen fluoride gas in work atmosphere have been

evaluated. Reagent-grade sodium acetate crystals proved

to be a very efficient sorbent for hydrogen fluoride.

Its solubility in water and pFI buffer properties are

highly advantageous for subsequent fluoride determination by the ion selective electrode. Experimental

results are presented for collection efficiencies of

hydrogen fluoride gas at several concentration levels,

flow rates, and collection times; the effects of elevated

temperatures and humidity; and also breakthrough, bleedout, storage and shipment studies. Specifications for

construction of the sorption tubes are also given.

1. Introduction

This report summarizes work on the development of a sampling procedure and a device for collecting hydrogen fluoride gas in the work atmosphere, and subsequent quantitative

determination of the collected gas in the laboratory. The

work is part of the program under a Memorandum of Agreement

between the National Bureau of Standards and National

Institute for Occupational Safety and Health. The program

calls for a development of procedures and devices for personal sampling applications, and analytical methods suitable

for routine laboratory use. The gas sampling medium should

consist of only solid substances.

1.1 Scope of the Work

As stated in the Memorandum of Agreement, the scope of

the work includes:

A. Development of sampling procedures and devices for the

collection of hydrogen fluoride gas in the industrial atmosphere, including:

1. Evaluation of various solid sorbent materials.

2. Selection of an appropriate sorbent based on the

following criteria. A suitable sorbent shall:

a. Be stable upon exposure to air flows, water vapor,

temperature extremes, shipping, and storage.

1
b. Retain the contaminant quantitatively during

sampling, shipment, and storage up until the

time of laboratory analysis.

c. Not react or degrade the contaminant in such a way as to prevent identification or change the

concentration.

d. Be inexpensive to fabricate and purchase.

e. Be capable of manufacture in a manner so that

reproducible collection and desorption properties can be obtained from batch to batch.

f. Be suitable for use in conjunction with personal

samplers

.

g. Be suitable in all respects for quantitative

analysis of the collected materials.

h. Retain the substance of interest in the presence of other substances commonly present in

industrial atmospheres.

i. Provide for specificity of measurement of the

substance of interest, either by selective

collection or by compatibility with selective

analytical procedures.

3. Design of a suitable collection device containing

the solid sorbent. The collection device should

meet the following criteria:

a. The collection device should be as small in

size and as light in weight as possible so

that it can be attached to a worker's collar or

lapel to collect a breathing zone sample.

b. The resistance of the collection device to air

flow should not exceed 2.54 cm of mercury pressure drop at a flow rate of one liter per minute

through the device, or 6.35 cm of water pressure drop at 200 cm^ per minute.

c. There should be a method to verify that no breakthrough has occurred during sampling.

d. The collection device should have a sufficient

capacity for collection of 4- to 8-hour samples

at a flow rate between 50 and 200 cm^ per minute,

and for 15-minute samples at a flow rate of 1

liter per minute, for concentration ranges of

2
1/5 to 5 times the threshold limit value (TLV)

concentrations of hydrogen fluoride.

B. Provision of suitable methods for retrieving the contaminant from the collection device and for analyzing the level

of contaminant which was collected. The present state-ofthe-art of analytical methods for hydrogen fluoride shall

be investigated. Candidate methods will be experimentally

evaluated for such factors as accuracy, reproducibility,

and criticality of steps. A procedure will be written for

the recommended method suitable for routine laboratory use.

C. Thorough evaluation of the recommended sampling and

analytical methods. The scope of the evaluation shall

include:

1. Assessment of the collection efficiency of the

collection device.

2. Assessment of the effects of temperature and humidity on the breakthrough characteristics of the

collection device.

3. Measurement of the effects of shipping and storage

of samples in adverse conditions of temperature and

pressure on percent recovery and analytical accuracy and precision.

4. Measurement and quantitation of any bleed-out

effects during extended sampling periods.

5. Determination of the overall accuracy and precision

of the recommended sampling and analytical methods,

compared with known air concentrations.

6. Assessment of the ruggedness of the recommended

analytical methods according to the method of

W. F. Youden given in the Association of Official

Analytical Chemists paper, "Statistical Procedures

for Collaborative Tests", where applicable.

7. Determination of the accuracy and precision of the

recommended analytical method according to

American Society for Testing and Materials

recommended practices.

8. Determination of the useful range of the recommended

sampling and analytical methods.

3
1.2 Method of Approach

For use as a personal sampler, the collection device

containing solid sorbent materials should function with

high efficiency and possess a large capacity for the contaminant hydrogen fluoride. The contaminant so collected

should be completely retrievable for subsequent analysis.

These three processes, sorption, retrieval, and analysis,

as outlined on p 5 could be either specific or non-specific

for hydrogen fluoride. For example, capture of volatile

contaminants can be achieved by the mechanisms of physical

adsorption/absorption or chemical reaction, each of which

may or may not be specific for hydrogen fluoride. The

recovery of the contaminant or its reaction product may

involve specific or non-specific chemical or physical processes. Finally, analytical methods are available for

specific and non-specific determination of hydrogen fluoride.

The only necessary condition here is that one of the processes should be specific for hydrogen fluoride.

1.3 Work Coverage

The work in this report involves construction of apparatus for continuous generation of hydrogen fluoride gas at

trace levels; construction of apparatus for sampling hydrogen fluoride gas at calibrated flow rates; evaluation and

selection of solid sorbents for efficient collection of

hydrogen fluoride; development of a sorption tube for personal sampling applications; development of a procedure for

collection and subsequent quantitative analysis of hydrogen

fluoride; determination of collection efficiencies of the

sorption tube at several concentration levels of hydrogen

fluoride, flow rates,* and collection times; and study of

the effects of temperature, humidity, breakthrough, bleedout, storage, and shipment.

This report also provides specifications for construction of the sorption tube and a recommended procedure for

collection and determination of hydrogen fluoride gas.

2. Development of Methodology

This section describes the development of methods for

continuous generation and sampling of hydrogen fluoride

gas, and of procedures for collection and analysis of the

gas. A brief review of the literature on solid sorbents

and analytical methods for hydrogen fluoride is also given.

2,1 General Consideration

To approximate the industrial work atmosphere, a relatively concentrated mixture of hydrogen fluoride gas was

4
Sorption

Chemical Reaction Physical Adsorption/Absorption

1 specific non-specific specific non-specific

Retrieval

Chemical Reaction Physical Methods

specific non-specific specific non-specific

Anally SIS

specific non-specific

5
diluted with air rather than pure nitrogen. The hydrogen

fluoride gas samples were drawn through sorption tubes by

vacuum to simulate sampling by a personal sampler. Consideration was made in the selection of sorbent materials and

specification of the sorption tube to conform to the work

scope as outlined in Section 1.1, The recommended analytical

method was one that required minimal training for efficient

performance.

2.2 Gas Dilution System

To carry out many tests required for the selection of

the solid sorbents, a reliable source of contaminant is

needed. The concentration level of hydrogen fluoride required in this work ranges from 0.6 to 15 ppm, i.e., 1/5 to

5 times TLV (1). Although hydrogen fluoride gas mixtures

at this concentration range are available commercially, past

experience indicated instability of such gas mixtures for

extended periods. A cylinder of hydrogen fluoride gas mixture in nitrogen was used as a bulk source of the contaminant

The concentration of the gas at the time of use was found to

61 ppm. This gas mixture was then dynamically diluted with

in-house compressed air to the desired concentrations in a gas dilution system. Several systems of this type have been

developed and used successfully for gas dilution (2). This

dilution system can also be conveniently used to study the

effects of humidity on the collection of the contaminant gas.

Figure 1 is a schematic drawing of the gas dilution

system. It consisted essentially of a bubbler containing

distilled water to saturate the in-house air, two differential flow controllers (one had a stainless steel diaphram

for hydrogen fluoride) , two rotameters for measurement of

flow rates of air and hydrogen fluoride, and stainless steel

mixing and sampling manifolds. Either thick-wall Teflon or

stainless steel hardware was used wherever liydrogen fluoride

gas passed. Prior to the water saturator, the in-house air

was filtered through glass wool. Proper adjustment of the

two rotameters R-1 and R-2 by valves V-2 and V-5, respectively, produced hydrogen fluoride gas mixtures of the

desired concentrations. The use of valves V-3 and V-4 provided a choice of oil-free dry air and water - saturated air

for mixing with hydrogen fluoride. The rotameters were

calibrated with a wet test meter,

2.3 Gas Sampling System

The sampling rates required in this work range from

50 cm^ per minute to 1 liter per minute. These flow rates

were obtained with the gas sampling system shown schematically in figure 1. The system consisted essentially of a three-way ball valve, two sorption tubes joined in series

6
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by Cajon connectors, a brass in-line filter, a differential

flow controller, a rotameter, and a vacuum pump.

Gas samples were withdrawn under vacuum from the

sampling manifold. When adjusting valve V-7 for the proper

sampling rates, the three-way valve V-6 was opened to the

atmosphere. The flow rates of the hydrogen fluoride-air

mixture in the gas mixing manifold were always maintained

at three or more times the gas sampling rates. The rotameter was calibrated with a wet test meter.

2.4 Selection of Solid Sorbents for Hydrogen Fluoride

2.4.1 Literature Survey on Solid Sorbents

Several solid substances have been reported in the literature for sorption of hydrogen fluoride in air. These

include sodium bicarbonate (3), sodium carbonate (4-7),

limestone (8), alumina (9), sodium formate (10,11), sodium

hydroxide- carbonate (12, 13), potassium-sodium carbonates

(14,15), potassium hydroxide (16), silica gel (17), and

calcium oxide (18). Various collection devices have been

developed, such as impregnated filter papers (3, 15, 18-20),

fiber filters (7, 14, 21), impregnated polystyrene filters

(11), coated tubes or beads (5,6,18,22), and packed towers

(8). The Drager detector tube (23) for hydrogen fluoride

uses zirconium alizarin lake on silica gel as the sorbent

and color indicator.

The Intersociety Committee (18) described the use of

filter papers impregnated with calcium-based fixative agents

such as calcium oxide or other soluble alkalies, and cautioned that gaseous fluorides collected on glass fiber

filters cannot be quantitatively removed by leaching with

water. Thompson et al. at the Environmental Protection

Agency (21) employed fiber filters to collect gaseous and

particulate water-soluble fluorides from ambient air and

recovered the fluorides by reflux with water.

Bourbon and Alary (3) found that filter papers impregnated with 1 percent sodium bicarbonate gave better recovery

of fluoride than lime papers. Okita et al. (5) found that

the collection efficiency for gaseous fluorides was best in

a glass spiral coated with dried sodium carbonate. Hermann

(6) used silver balls coated with a thin layer of sodium

carbonate. A two-stage filter consisting of a membrane

filter and a second membrane filter impregnated with

1 percent sodium carbonate was developed in Japan (7).

Naoi (13) removed acidic gases by filters impregnated with

a mixture of sodium hydroxide and carbonate. Okita (14)

obtained 100 percent collection efficiency for particulate

fl uoride compounds by membrane filters impregnated with
sodium and potassium carbonates. Okita et al. (15) collected

gaseous hydrogen fluoride on filter papers impregnated with

1 percent sodium and potassium carbonates. At 0.05 to

1.0 ppm level, the collection efficiency was 94 percent at

40 percent relative humidity.

Gilbert et al. (8) investigated the effects of different types of limestone, gas velocity and temperature, size

of packing, and hydrogen fluoride concentrations on the

absorption coefficient. Lobos et al. (9) used a heated

alumina desiccator to absorb hydrogen fluoride, and found

that the gamma-type alumina absorbed over 2 percent fluoride

in a fraction of a second. Jahr (10) employed two successive filters, the first untreated and the second impregnated

with sodium formate. Particulate fluorides were collected

quantitatively on the first filter whereas hydrogen fluoride was found on both. However, the hydrogen fluoride on

the first filter could be completely transferred to the

second by heating.

Biheller and Resch (11) were able to trap more than

99 percent of hydrogen fluoride with polystyrene filters

which were impregnated with 10 percent sodium formate in

50 percent alcohol. Huygen (16) compared the efficiencies

of filter papers impregnated with 20 percent potassium

hydroxide-10 percent glycerol, filter papers impregnated

with 20 percent potassium hydroxide-10 percent triethanolamine, and bubblers containing 0.1 N sodium hydroxide solution. The freshly prepared filter papers had 100 percent

collection efficiency at humidities greater than 25 percent,

and 87 percent at zero humidity. Guilbault (20) described

paper tapes impregnated with Mg-8-hydroxylquinoline to give

ppb detection capability by quenching of fluorescence.

2.4.2 General Considerations

As hydrogen fluoride is chemically a very reactive

species and also a very strong acid when ionized, sorption

by chemical reaction received more consideration than physical adsorption/absorption (see Section 1.2). Many

inorganic fluorides are soluble in water. If the reaction

product in sorption is a water-soluble fluoride, retrieval

of hydrogen fluoride as a fluoride can be complete and

quite convenient. Furthermore, if the final chemical

analysis should be specific for fluoride, then the sorption

reaction need not be specific.

For personal sampler applications, the collection

device must be compact in size with high efficiency and

large capacity. It follows that packed columns or tubes

are more desirable than impregnated papers, tapes, membrane

filters and coated tubes.

9
2.4.3 Collection Efficiencies

In a preliminary screening of solid sorbents, the

collection efficiencies of sodium bicarbonate powders,

activated charcoal (roasted coconut shells) , activated charcoal soaked with saturated sodium bicarbonate solution, firebrick soaked with saturated sodium bicarbonate solution,

sodium citrate crystals, and sodium acetate crystals were

evaluated. These substances in granular form were packed in

glass tubings of 6 mm outside diameter so that the pressure

drop did not exceed 2.54 cm of mercury at the air flow rate

of 1 liter/minute.

Two tubes, connected in series, were used to evaluate

collection efficiency. After sampling a given hydrogen

fluoride gas mixture at a flow rate for a period, the amounts

of fluoride collected in the first and second sorption tubes

were determined by the method to be described in Section 2.6.

The collection efficiency, E, defined as

(Ai - Bi)

^ ^ CAi - BO - (Aa - B,) ^

where Ai = amount of fluoride collected in the first sorption tube,

A2 = amount of fluoride collected in the second sorption tube,

Bi = blank for the first sorption tube,

B2 = blank for the second sorption tube,

was calculated. Blanks were obtained under identical conditions but using the ambient air of the laboratory.

Table 1 is a summary of the collection efficiencies of

the solid sorbent systems studied. Although sodium bicarbonate showed a relatively high collection efficiency, it had

the disadvantage that it was available only in the powder

form, and the pressure drop of sorption tubes generally

exceeded 2.5 cm mercury. Also, its efficiency of collection

at the flow rate of 50 cmVmin was rather low.

It was difficult to leach sodium fluoride, the reaction product of sodium salts with hydrogen fluoride, from

activated charcoal and firebrick substrates. The fine

particulates of the substrates in the resultant solutions

had to be filtered off before analysis. No attempt was made

to determine the recovery of fluoride in the leaching process.

10
Table 1. Collection Efficiencies of Various Sorbents for

3 ppm Hydrogen Fluoride Gas

Collection Flow Rate Col lection

oO^ V* r~\ o n 'i~ I U C 11 L Time m ^ / m 1 K^lli / 111 1 il r:f f iciency

NaHCOg 15 min 1050 98.7- 99.8^

/I un 211 n A 0 y 0 . 0

6 li 211 95.8

8 h /L 1 X 99.8

QO n 50 7 7 7. / J . J

15 min J. U J u 93.1 , 97.4

u 211 Q n 7 4 h 41 82.8

8 1-1 50 52.5

Firebricks soaked with 15 min 1050 70.6

sat 'd NaHCOa sol 'n

Charcoal soaked with J. o III J. 1

1

1050 o o . u sat'd NaHCOa sol'n

Charcoal soaked with 15 min 1050 89.0, 90.3

sat'd NaHCOa sol'n +

5 percent glycerine

Na citrate crystals 15 min 1050 12.9

4 h 211 11.8

Na citrate in urethane 15 min 1050 31 . 5 + 5 percent glycerine

solution

Na acetate crystals 15 min 1050 93.9- 97.5^

total of 9 runs.

A total of 6 runs.

11
There was evidence that water was desirable for the

reaction of sodium in the solid sorbents with hydrogen fluoride, as shown by the higher collection efficiency (31.5

percent) of sodium citrate plus 5 percent glycerine in

water over the dry salt. Activated charcoal soaked with

saturated sodium bicarbonate solution also gave higher

collection efficiencies when 5 percent glycerine was present.

Sodium acetate was selected as the sorbent for hydrogen

fluoride on the following basis:

1. high collection efficiency,

2. acetate acts as a pH buffer in the fluoride selective electrode method of analysis,

3. readily available as reagent grade in the 20-40

mesh size,

4. relatively inexpensive,

5. chemical stability of both sodium acetate and the

reaction product sodium fluoride under normal

conditions ,

6. presence of three molecules of water of hydration

per formula weight, highly desirable for the reaction with hydrogen fluoride in air,

7. non-toxic, non-irritating, non-corrosive, nonobnoxious ,

8. high solubilities of both sodium acetate and

sodium fluoride in water,

9. high melting points of sodium acetate and sodium

fluoride, 58°C and 992°C, respectively.

2.5 Construction of Sorption Tubes for Hydrogen Fluoride

The sorption tubes were constructed according to the

procedure given below. The reagent-grade sodium acetate

used was pooled from bottles from three different suppliers.

Crystals of 20 to 40 mesh were separated from the bulk in

the reagent bottles by sieving. The only apparent difference

among these three sources of sodium acetate was in the distribution of particle size. An average of 10 percent of the

sodium acetate in the bulk was of 20 to 40 mesh. No attempt

was made to determine the distribution of particle size in

the 20 to 40 mesh crystals.

12
Figure 2 is a drawing of the sorption tube. Although

the specifications call for Pyrex glass tubing, polyethylene

tubing should be preferable to avoid reaction of hydrogen

fluoride with the glass surfaces. Subsequent studies showed

a slight loss of hydrogen fluoride when Pyrex tubes were used.

For use as a personal sampler, two sorption tubes may

be joined in series with a piece of polyethylene tubing so

that the second sorption tube acts as a breakthrough indicator. However, the high capacity of the collection tube

makes this virtually unnecessary.

2.5.1 Specifications of Sorption Tubes

1. Join two Pyrex glass tubings of 6 mm outside

diameter and 3 cm long to a Pyrex glass tube of 13 mm outside

diameter and 4 cm long. Make very slight constrictions at

the joints.

2. Introduce a wad of loose surgical cotton, about

5 mg, inside the tube.

3. Blow dry air inside the tube to arrest the cotton

at the "groin" of the tube.

4. With the aid of a small funnel and a piece of Tygon

tubing, fill the tube with 20 to 40 mesh reagent-grade

sodium acetate crystals, NaCaHaOa • 3H2O, (about 3 g) up to

the "neck". Gently tap the tube to help the crystals settle.

Do not use crystals that are lumped together.

5. Insert a wad of loose surgical cotton, about 4 mg

,

on top of the sodium acetate crystals.

6. Mark an arrow on the outside wall of the tube to

indicate the air flow direction, i.e., the same direction

as the air that was used in blowing the cotton.

7. Connect the sorption tube to an orifice which has

an opening to pass air at 1 1/min. Connect the orifice to

a manometer and vacuum with a tee. Figure 3 is a drawing

of the pressure drop measurement apparatus.

8. Turn the vacuum on to check the pressure drop.

Reject sorption tubes with a pressure drop over 2.54 cm Hg

.

9. When not in use, tightly seal both ends of the

sorption tube with plastic end caps. Do not store tubes at

temperatures above 40°C.

13
TOP
I

SORPTION

TUBE

CRITICAL FLOW

ORIFICE

J [

VACUUM

MANOMETER

Ficure 3. Pressure Drop Measurement Apparcfu:

15

1
2.5.2 Impurities in Sodium Acetate

The sodium acetate used in the experiments contained

the following impurities, as listed on the labels of the

reagent bottles. None of these impurities is expected to

interfere with the fluoride selective electrode analysis.

Impurities (in percent) Suppl ier A Supplier B Supplier C

Insoluble matter 0. 003 0. 005 0. 001

Chloride 0. 0004 0. 001 0. 0003

Phosphate 0. 0003 0. 0005 0. 0002

Sulfate 0. 001 0. 002 0. 0004

Ca, Mg, and R2O

Heavy metals as

3 precipitates 0. 005 0. 010 0. 003

Pb 0. 0002 0. 0005 0. 0002

Iron 0. 0002 0. 0005 0. 0001

Potassium 0. 001 0. 005 0. 005

Ammonium 0. 001

2.6 Analytical Methods for Hydrogen Fluoride

2.6.1 Brief Review of Analytical Methods

Many analytical methods for hydrogen fluoride are now

available. As the number of references on fluoride determination is quite large, only a few representatives are

cited here. In general, these methods may be classified as

ion selective electrode (21,24), colorimetry (18,23,24),

titrimetry (18), spectrophotometry (18,25), photometry

(6,22,24), fluorimetry (20,25,26), gas ionization (27),

bioassay (28), spot test (29,30), and nuclear activation

analysis (31,32). The first four methods are more applicable to the present work than the others. Below is a brief

description of their basic principles, interferences, and

range or limits of detection.

The fluoride selective electrode gives Nerstian

response to fluoride ion concentrations from above 1 M to

10 M, and only hydroxide ion seems to interfere with this

response. With the pH of the sample solution maintained

between 5 to 7 by a pH buffer and the presence of a polyvalent anion to break up any complexing agent, selectivity

is excellent. The limit of detection is 100 iJg/1.

The colorimetric method measures the change of color

intensity of a dye lake by the reaction of fluoride ions,

using either a color comparator or a photometer. In the

SPADNS method, sodium 2 - (parasulfophenylazo ) - 1

, 8 -dihydroxy- 3

,

6 -naphthalene disulfonate, also called 4 , 5-dihydroxy- 3-

(parasulfophenylazo) - 2

, 7 -naphthalene disulfonic acid trisodium salt, combine with zirconyl chloride to form the dye

lake. Sodium alizarinsulfonate , also known as alizarin
red S, may also be used. The range of applicability is 0.05-

1.4 mg/1. Interferences include alkalinity, aluminum, chloride, iron, hexametaphosphate , phosphate, and sulfate.

In titrimetry, the sample solution containing fluoride

and sodium alizarinsulfonate is titrated against thorium

nitrate. Interferences are aluminum, barium, calcium, iron,

thorium, titanic, vanadyl, zirconium, phosphate and sulfate

ions. The range and sensitivity are 5-50 pg in the total

sample for titration.

In spectrophotometry, the change in absorbance of the

solution containing a metal dye complex, such as zirconiumeriochrome cyanine R or z irconium-SPADNS , by fluoride is

measured. The range of applicability is 0.02-1.4 mg/1.

Lanthanum alizarin complexone, commonly called alizarin

fluorine blue, may also be used, and the range is lowered:

0.015-0.5 mg/1. Interferences include aluminum, iron,

phosphate, and sulfate.

2.6.2 General Considerations

The ion selective electrode method for fluoride determination was chosen on the basis of its excellent selectivity for fluoride and simplicity in operation. The

necessary equipment in this method includes a fluoride

selective electrode, a calomel reference electrode, a pH

meter with millivolt mode, and a magnetic stirrer. The

concentration of fluoride in a sample solution is obtained

from the millivolt reading and a calibration curve. The

latter is established from a plot of millivolt response

versus fluoride concentrations of sodium fluoride standard

solutions. The standard solutions are prepared from

known weights of dried sodium fluoride dissolved in 0.88 M

sodium acetate solution containing 0.3 g sodium citrate per

liter. The procedure for operation of the fluoride selective electrode instrument is given in the next section.

At the conclusion of gas sampling, the amount of

fluoride collected in the sorption tube may be determined

according to the procedure given in Section 2.6.4. In the

preliminary screening of solid sorbents, the sorbents

sodium bicarbonate and citrate were dissolved in sodium

nitrate solution to give the same ionic strength as the

standard solutions. For charcoal and firebricks, the

sodium fluoride was leached out from the substrates with

25 ml water ^ and the proper amounts of sodium acetate and

citrate were added to the solutions.

The amount of fluoride collected, in moles of fluorine,

was calculated from the following equation:

17
^ 1000

where A = amount of fluoride collected in the sorption

tube, and

C = molar concentration of fluoride.

The concentration of hydrogen fluoride, in parts per million

(ppm) , in the gas samples was calculated from the following

equation

:

Ax 24.5 X 10^

ppm HF = YIT^

where T = total collection time in minutes, and

R = flow rate of gas sample in liters per minute at

standard temperature-pressure condition.

To study the effects of sodium acetate concentration on

millivolt response, a series of sodium fluoride standards

in 0.55 M sodium acetate solutions was prepared. Figure 4

shows the calibration curves of the standard solutions in

both 0.88 M and 0.55 M sodium acetate solutions. The

effect of sodium acetate concentration was negligible.

2.6.3 Operation of Fluoride Selective Electrode Instrument

The following directions apply to the use of the ion

selective electrode:

1. Connect the fluoride and the reference electrodes

to the pH meter. Set the meter to "standby" position.

2. Transfer the sample solution to a 50-ml polyethylene

beaker. Add 2 drops of 0.5N HNO3 to adjust the

pH of the solution to 6.8,

3. Place the electrodes in the sample solution.

4. Place a Teflon coated magnetic stirrer in the

beaker and turn on the variable transformer to

provide gentle stirring of the solution.

5. At the end of 15 minutes, set the meter to millivolt

mode and record the millivolt reading.

6. Set the meter back to "standby" position.

18
19
7. Empty the contents o£ the beaker. Rinse the

electrodes thoroughly with distilled water. Wipe

the electrodes dry with soft tissue. The electrodes

are now ready for measurement of another sample.

When not in use, the electrodes should be immersed

in distilled water.

2.6.4 Procedure for Measurement of Amount of Fluoride

Collected in Sorption Tubes

Prepare a stock solution containing 0.3 g sodium

citrate per liter in distilled water, and use it for dissolution of the sodium acetate.

1. Disconnect the sorption tube from the sampling

apparatus

.

2. Use a stiff wire or a pair of straight corrugated

forceps to remove the top wad of cotton in the

sorption tube and place the cotton in a polyethylene

beaker

.

3. Insert the top end of the sorption tube into the

neck of a 25-ml volumetric flask, and empty the

sodium acetate crystals into the flask. Gently tap

the tube against the flask to get as many as possible of the crystals into the flask.

4. With a stiff wire, break loose the bottom wad of

cotton. Keep the cotton inside the sorption tube.

5. Rinse the inside walls of the sorption tube and the

cotton with 15-18 ml stock citrate solution. Make

sure all the rinse goes into the flask.

6. Remove the sorption tube from the neck of the

volumetric flask.

7. Rinse the cotton in the polyethylene beaker with

5 ml stock citrate solution. Add the rinse to the

25-ml volumetric flask.

8. Dissolve the sodium salts completely.

9. Add enough stock citrate solution to make 25 ml.

Mix thoroughly. The solution is now ready for fluoride concentration determination.

2.6.5 Calibration of Fluoride Selective Electrode

Prepare a series of sodium fluoride standard solutions

as follows:
1. Prepare a stock solution containing 120 g sodium

acetate and 0.3 g sodium citrate per liter in distilled water. Use this stock solution for dissolution and dilution of the sodium fluoride below.

2. Dissolve 420 mg, weighted to the accuracy of ±0.1

mg , of dried sodium fluoride and dilute it to 100.0

ml in a volumetric flask. This makes the lO"-^ M

solution.

3. Dissolve 420 mg, weighted to the accuracy of ±0.1

mg, of dried sodium fluoride, and dilute it to 1.000

liter in a volumetric flask. This makes the 10"^ M

solution.

_ 1 4. Pipette 10.0 ml of the 10 M solution and dilute

it to 1.000 liter in a volumetric flask. This

makes the lO"^ M solution.

5. Pipette 10.0 ml of the 10 ^ M solution and dilute

it to 1.000 liter in a volumetric flask. This

makes the lO"** M solution.

6. Pipette 1.00 ml of the lO"^ M solution and dilute

it to 1.000 liter in a volumetric flask. This

makes the 10 ^ M solution.

7. Pipette 1.00 ml of the 10"^ M solution and dilute

it to 1.000 liter in a volumetric flask. This makes

the 10"^ M solution.

Store all the sodium fluoride standard solutions in

polyethylene bottles. Use 25 ml of each of these solutions

for calibration periodically. Follow the procedure given

in Section 2.6.3. Plot the millivolt response against the

calculated concentrations of sodium fluoride.

3. Experimental Results

- 3.1 Collection Efficiencies of Sodium Acetate

Table 2 summarizes the collection efficiencies of

sodium acetate crystals for hydrogen fluoride at room

temperatures (23.9° to 25.5°C). These experimental runs

were made in random order and not in the sequence as shown

in table 2. It appears that 3 g of sodium acetate in 20 to

40 mesh was sufficient to remove completely hydrogen

fluoride from air having concentration of hydrogen fluoride

ranging from 0.5 to 16 ppm for the collection times and

flow rates specified in Section 1.1.

21
Table 2. Collection Efficiencies of Sodium Acetate for

Hydrogen Fluoride Gas

Collection

Time

15 min

15 min

15 min

15 min

15 min

15 min

15 min

15 min

15 min

15 min

15 min

15 min

4 h

4 h

4 h

4 h

4 h

Flow rate

cm ^ /min

916

916

916

916

916

1010

1010

1010

1010

1010

1010

1010

52

52

52

52

4.90

5.08

3.70

4.64

5.57

0.48

0.51

0.57

0.63

0.70

3.54

15.8

0.69

0.74

0.81

5.00

15.2

g. NaAc

1.87

1.87

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

3.0

Collection

Efficient

94 .

8

94.7

100

100

99.7

ICQ

100

100

100

100

100

100

100

100

100

100

100

h

h

h

h

h

188

188

198

198

198

4.93

5.11

0.57

3.09

15.4

87

0

3.0

3.0

3.0

99.3

100

100

100

100

h

h

h

h

h

h

h

h

52

52

52

52

18 8

198

198

198

0.74

2.21

4.64

16.4

5.43

0.57

4.37

15.8

3.0

3

3

3

3

3

3,

3,

100

100

100

100

100

100

100

100

22
3.2 Breakthrough and Capacity of the Sorption Tube

A gas mixture of 15 ppm hydrogen fluoride was passed

continuously through two sorption tubes at a flow rate of

1.01 l/min. At 15 minute intervals, the second tube was

replaced with another sorption tube and its fluoride content

analyzed. A total of 32 second sorption tubes was thus

obtained and analyzed. The first sorption tube had collected

5.89 mg hydrogen fluoride. No fluoride was found in any of

the 32 second sorption tubes in the 8 hours of collection.

In another similar experiment, the hydrogen fluoride

from the cylinder was fed without dilution through two sorption tubes in series, at the flow rate of 1.01 l/min. Every

4 hours, the second tube was replaced and analyzed. Again,

no breakthrough was detected in the second tubes after

collection of a total of 46 mg hydrogen fluoride in the

first sorption tube. However, the inside walls of the first

sorption tube above the sodium acetate crystals were etched

at the end of the 16 hour run, indicating reaction of

hydrogen fluoride with the glass surfaces.

The capacity of two smaller sorption tubes was also

studied. Hydrogen fluoride gas, without dilution, was

passed through two sorption tubes in series, each of which

was 4 cm long with 10 mm outside diameter and contained

1.4 g sodium acetate. The sample flow rate was 1.01 l/min.

At the end of 15 minutes, the fluoride contents of these two

tubes were determined. The experiment was repeated with

two sorption tubes of the same length but of 8 mm outside

diameter, each of which contained 0.8 g sodium acetate. The

sample flow rate was 198 cmViTiin and collection time was 5

hours. The results are tabulated in table 3.

These smaller sorption tubes also possessed 100 percent

collection efficiency, and no breakthrough was observed

after collecting more than 5 mg of hydrogen fluoride in the

10 mm tube and more than 17 mg in the 8 mm tube. However,

these tubes exhibited a pressure drop far higher than the

2.54 cm mercury specified in Section 1.1.

The amount of fluoride collected, column 6 of table 3,

was computed from the fluoride analysis results, and not

from the sample flow rate, collection time, and concentration

of the hydrogen fluoride gas. When the last two experiments

were performed, the pressure inside the hydrogen fluoride

cylinder was below 203 x 10^ pascal (15 pounds per square

inch above atmosphere) , and the concentration of hydrogen

fluoride was much higher than the original value of 61 ppm.

23
r—t

03

n3

O

4->

D o o S (/) 03

0) 5-1 0) T3 0) XI

•H +-> :3

t/l CD -M

03 <-H •H O

^3 3O

C >> o u

•H

C

M (D U -H o o o o

(L»U o o o o

I—I -H I—(

1—1 r-H .—1 O M-l U <D

0) ^ t/) 0) •H e N

•H o

CO

rH CD LO

<-H +-> oo o LO u • (D 1—1 0) LO

OS rH rH

d)

M I—1

<4-l

OO

M-l

H

U

•HQ

C

M-t o O •H LO W US/—.

oo o LO X3 I—1 +-> w r-H 1—I ou o

•H

+-> 00

I—1 1—1r-H Pi o +-> e o O O rH o E .—1 03 \ « . CO 03 <-H ^ I—(

I—t

I—1

I—1 O

CO ^ o

>s

4-> • H e o UO UO OO o

nJ (/)U I—I

Oi' CO ^ rg LO

cS CD i-H ^ Oh , O. O -M ^ 03 o o 00

rH o

00

24
3c3 Effects of Elevated Temperatures

A Pyrex glass tube of 13 mm outside diameter and 25 cm

long was placed in front of two sorption tubes, all in series

and all wrapped with a heating tape. Heat was generated at

a constant rate, and the temperature inside the 25 cm tube

was measured by a thermometer with air flowing at 52 crnVmin.

A hydrogen fluoride gas sample was withdrawn through this

assembly at a flow rate of 52 cm^/min. The experiment was

repeated with the heating tape removed and with two new

sorption tubes.

Table 4 is a summary of the experimental results for

two different collection times. Together with the experimental data presented in table 2, it is concluded that the

collection efficiency of sodium acetate for hydrogen fluoride was 100 percent at temperatures ranging from 23.9°C to

38.0°C. However, there was a loss of 9 percent of hydrogen

fluoride at the elevated temperatures, as shown in the last

column in table 4. Possibly, part of the hydrogen fluoride

reacted with the inside walls of the Pyrex tubes with a higher kinetic rate at the elevated temperatures than at the

room temperatures.

3.4 Effects of Humidity

With the water saturator turned on (see Figure 1), it

was possible to generate air-hydrogen fluoride mixtures x^^ith

various relative humidities by adjusting valves V-2 and V-5.

The samples were withdrawn through two sorption tubes in

series at a flow rate of 1.01 1/min for 15 minutes. The

experiment was repeated with the water saturator turned off

and with two new sorption tubes. The amount of fluoride in

the sorption tubes was determined after the collection.

The collection efficiencies in both the wet and dry

cases were 100 percent. However, the collection efficiency

as defined in Section 2.4.3 is a relative quantity. When

comparing the absolute amounts (Ai-Bi), of fluoride collected,

it was found that the fluoride collected from the wet air

was only 76.7 percent of that from the dry mixture.

In the next experiment, the stainless steel mixing and

sampling manifolds and tubings were replaced by polyethylene

manifolds and tubings. Again, the collection efficiencies

were 100 percent in all cases. But the amount of fluoride

collected from the wet air was now 96 percent of that from

the dry mixture. Finally, sorption tubes were constructed

from polyethylene tubing, and the experiments were repeated

at three relative humidities. The results are summarized in

table 5,

25
Table 4. Effects of Temperature on Collection

of Hydrogen Fluoride

Temperature

(°C)

Collection

t ime

(h)

Collection

efficiency

ppm

HF

Ratio of moles

Net moles F collected

F collected (elevated temp/

X 1 0

^ room temp)

36. 0

25.0

100

100

5.2

5.2

0.60

0. 66

0.91

38.0

25.4

3

.3

100 22.9 7.98

100 22.9 8.73

0.914

26
Table 5. Effects of Humidity on Collection of

Hydrogen Fluoride

Humidity Mixing and Ratio of

(Water Sampling Sorption Collection HF collected

saturation) Manifolds Tubes efficiency (wet/dry)

59.8 stainless steel Pyrex 100 0. 767

59.8 po lyethy lene Pyrex 100 0. 963

59.8 polyethylene polyethylene 100 1. 00

79.3 polyethylene polyethylene 100 1. 00

18.4 polyethylene polyethylene 100 1. 00

27
Apparently, part of hydrogen fluoride reacted with water

vapor, and the product(s) then reacted with the stainless

steel walls of the gas dilution and sampling systems. There

was indication that hydrogen fluoride also reacted with the • glass surfaces of the sorption tubes to the extent of 3,7

percent

.

3.5 Precision, Bleed-out, Storage, and Shipment Studies

A total of 20 collections of hydrogen fluoride gas was

made under identical experimental conditions, i.e., 15

minutes of collection time at a flow rate of 1.01 1/min and

hydrogen fluoride concentration of 3 ppm. The 20 sorption

tubes so obtained were distributed, in random order, for the

following studies.

Four were analyzed for the fluoride content immediately.

Four were stored for 35 days at room temperature and then

analyzed for fluoride content. Four were stored for 70

days at room temperature and then analyzed.

Four days after the collection, four sorption tubes were

sent to the National Institute for Occupational Safety and

Health. Two were returned immediately and the other two

were returned one week later.

Two sorption tubes were kept in a water- saturated

atmosphere at room temperature for 70 days, and then

analyzed.

One sorption tube was flushed with ambient air for 6

hours with a second tube in series. Another was flushed in

the same manner for 24 hours. The fluoride contents in the

first and the second tubes were analyzed.

The results are summarized in table 6, together with the

history of the sorption tubes. The samples were collected

in the alphabetical order of the sample labels, but they were

analyzed in random order. The solution resulting from each

sample was measured 7 times, and 'the average and estimated

standard deviation are listed in the second and third

columns respectively. For each group of samples receiving

the same or similar treatment, the average and standard

deviation are also computed and listed in table 6.

A statistical analysis of the data shows no significant

correlation among these 20 samples. A slight but inconclu^

sive trend is observed with storage. One sample (T) has a low value, but the overall average of 75,2 includes this

value.

28
Table 6. Precision, Bleed-out, Storage, and

Shipment Studies

Sample Average Standard

label mv reading deviation (niv) Hi story

A 7 c: A n A Analyzed immediately

c 75.3 0.4

H 74.8 0.3

T 73.2 0.6

74. 7 + 1. 0

u

7 A Q n x u , O oLoreu tor od uays at room

L 75.0 0.7 temperature

0 75.6 0.8

R 74. 7 0.7

75.6 + 1. 0

DD 7 A A U.J iJLUIcU. Q't'r^T^orl -LUl p/^Y* 7n/U /nQA7C Udyb dUdl" y*lUUlllm

E 75.9 0.7 temperature

J 76.4 0.7

M 75.3 0.7

7 A A / 0 . UnU . D

K 75.1 0.7 Sent to NIOSH, returned

P 74.3 1.0 immediately

G 75.5 1.0 Sent to NIOSH, returned one week

Q 74.4 0.7 later

74. 8

+ 0. 6

I 75.6 0.5 Flushed with air for 24 hours

N 74.2 0.9 Flushed with air for 6 hours

74. 9 + 1

.

0

F 74 . 5 0.6 Stored in water- saturated

S 74.8 0.9 atmosphere

74.6 + 1. 0

Average of 75.2 ± 0.87

2 0 s amp 1 e s

I

29
The overall standard deviation of ±0.87 mv corresponds

to a relative standard deviation of ±3,4 percent in fluoride

concentration. This is a measure of the sample variability.

No bleed-out was obtained in the sorption tubes that

were flushed with ambient air.

3.6 Glass versus Polyethylene Tubes

As mentioned in Sections 3.2, 3.3, and 3.4, there was

evidence that hydrogen fluoride reacted with the glass surfaces of the sorption tubes. To determine the extent of the

reaction more precisely, hydrogen fluoride gas, without dilution, was passed directly from the gas cylinder through

Teflon tubings into two sorption tubes in series. The flow

rate was 198 cm^/min and the collection time was 15 minutes.

The experiment was repeated alternatively using Pyrex glass

and polyethylene tubes. After collection, the fluoride content in the sorption tubes was determined. The results are

presented in table 7.

The difference of 0.7 mv (44.8 - 44,1), corresponds to

a loss of 3 percent of hydrogen fluoride in the glass tubes,

assuming no loss of hydrogen fluoride in the polyethylene

tubes. However, this may not be significant in view of the

relatively large standard deviations in the measurements,

4. Summary and Conclusions

A collection device consisting of 3 g sodium acetate

crystals of 20 to 40 mesh packed in a 13-mm OD tube has a very

high efficiency and capacity for removing gaseous hydrogen

fluoride from air. This device collects hydrogen fluoride

at the 1/5 to 5 times the TLV with 100 percent efficiency,

at air flow rates ranging from 50 cmVmin to 1 1/min. The

capacity of the device is at least 46 mg of hydrogen

fluoride. The recovery of the fluoride from the tube is

expected to be quantitative, although no determination was

made on the recovery.

This collection device is not specific for hydrogen

fluoride. It will probably trap particulate fluorides and

fluorine gas with high efficiency, and the recovery may

also be complete for water-soluble fluorides. The

analytical method, however, is specific for fluoride.

Therefore, any water-soluble compound that ionizes to fluoride and is trapped by this device will give a high analytical result for hydrogen fluoride, to the extent of the collection efficiency of such compound. On the other hand, no

interference is expected from any substance that will not

form ionizable fluoride.

30
Table 7. Glass versus Polyethylene Tubes

Tube mv reading

Glass 44.0

45.5

45.3

Polyethylene 43.6

44.3

44 . 3

Glass 45.0

44. 8

44.3

Average 44.8 ± 0.6 44.1 ± 0.4

31
Although no accuracy has been established for this

collection device, it is expected that the high collection

efficiency of sodium acetate for hydrogen fluoride, solubility

of sodium fluoride in water, and the specificity of the

fluoride electrode all contribute to a high degree of reliability. The sources of error include fluctuation in air

flow rates, inaccuracy in recording collection time, error

in volumetric dilution, instability of the instrument

response, and inaccuracy in weighing the sodium fluoride

standards.

At elevated temperatures such as 36° to 38°C, there is

a loss of 9 percent hydrogen fluoride, possibly resulting

from reaction of hydrogen fluoride with the glass surfaces

of the sorption tube. Even at room temperature, a loss of

hydrogen fluoride to the extent of 3.7 percent may be

realized at a 60 percent relative humidity. Such problems

are eliminated if the tubes were constructed of polyethylene.

The device is stable under normal conditions. No

significant deterioration was observed after storage for

70 days, shipment via air freight, and purging with air for

24 hours. The precision of the sampling and analysis is

±3.4 percent relative standard deviation at 3 ppm hydrogen

fluoride concentration.

The 3 grams of sodium acetate crystals must be packed

in a 13-mm outside diameter tube, so that the pressure drop

across the tube will be below 2.54 cm mercury at an air

flow rate of 1 1/min. A smaller diameter tube of the same

length (4 cm) exhibits a higher pressure drop for the same

flow rate. A shorter tube of the same 13 mm outside diam-.

eter containing less than 3 g sodium acetate has less than

100 percent collection efficiency.

5 . Bibl iography

1. American Conference of Government Industrial Hygienists,

"Threshold Limit Values for Chemical Substances and

Physical Agents in the Workroom Environment with Intended

Changes for 1972".

2. B. C. Cadoff, P. A. Pella, E. E. Hughes, E. P. Scheide,

A. A. Angotti, and J. K. Taylor, "Development of

Analytical Reference Materials and Contaminant Generation Systems", National Bureau of Standards Internal

Report, NBSIR 73-411 (1973).

3. P. Bourbon and J. Alary, Determination of Fluoride

Pollution by a Rapid Technique Utilizing Paper Impregnated with Soda, Bull. INSERM 24 (1), 23-30 (1969).

32
4. T. Okita, S. Yamashita, R. Sugai, H. Nakagawa, K. Kaneda,

and M. Mori, Calibration and Field Test of the Atmospheric Hydrogen Fluoride Recorder, J. Japan Soc. Air

Pollution 5 (1), 87 (1970).

5. T. Okita, R. Sugai, and K. Kaneda, Separation and

Subsequent Determination of Atmospheric Particulate and

Gaseous Fluorides, J. Japan Soc. Air Pollution _5 (1),

8'8 (1970).

6. P. Hermann, Measurement of Gaseous Fluoride Compounds

in the Outdoor Air and in Industrial Waste Gases, Tech

.

Ueberwach. (Duesseldorf

)

15 (2), 42-45 (1972).

7. Japan Environmental Agency, "Instructions for Air

Quality Measurements and Other Environmental Investigations", (1972).

8. N. Gilbert, I. A. Hobbs, and W. D. Sandberg, Absorption

of Hydrogen Fluoride in a Limestone-packed Tower, Chem

.

Eng. Progr. 49, 120-127 (1953).

9. J. S. Lobos, J. P. McGeer, and D. P. Sanderson,

Reactivity of Alumina towards Hydrogen Fluoride , American Institute of Mining, Metallurgical, and

Petroleum Engineering, New York, N. Y. (1971).

10. J, Jahr, A New Double Filter Method for the Separate

Determination of Hydrogen Fluoride and Dust-like Fluorides in the Air, Staub, Reinhaltung Luft 32 (6),

248-252 (1972).

11. J. Biheller and W. Resch, Tests with a Sampling and

Measurement Method for Routine Determination of Fluorine

in the Lower Atmosphere, Staub, Reinhaltung Luft _31^ (1),

9-11 (1971).

12. J. D, Navratil, "Disposal of Fluorine", Dow Chemical

Company, Golden, CO, AEC Contract AT ( 2 9- 1

) - 1 1 06 (1968).

13. Y. Naoi, Performance and Problems of Air Filters

Impregnated with Alkali, J. Japan Air Cleaning Assoc.

10_ (6) , 53-58 (1972) .

14. T. Okita, Measurement of Hydrogen Fluoride Using

Filter Method and Separate Determination of Gaseous and

Particulate Fluorides, Environmental Health Report,

No. 8, 19-22 (1972). ~

15. T. Okita, R. Sugai, and K. Kaneda, Collection of a Trace Amount of Hydrogen Fluoride using the Alkaline

Filter Method, Koash Eiseiin Kenkyu Hokoku 18 (2),

69 (1969).

33
16. C. Huygen, Sampling Hydrogen Fluoride in Air with

Impregnated Filter Paper, Anal. Chim. Acta 29 (5), 448

(1963) .

17. G. S. Salyamon, and M. V. Popelkovskaya , Methods for

the Determination of Fluoride Ions in Air and Water,

Gigiena i Sanit. No. 4, 65-67 (1973).

18. Intersociety Committee, Methods of Air Sampling and

Analysis , pp 246-281 , American Public Plealth Assoc. , Washington, DC (1972); Am. Ind. Hyg. Assoc. J . 33 (4),

269-278 (1972).

19. B. S. Marshall and R. Wood, The Determination of Total

Fluoride in Air by using a Microdif fus ion Technique,

Analyst 94 (1119), 493-499 (1969).

20. G, G, Guilbault, Practical Fluorescence , p 245,

Marcel Dekker, Inc., New York, NY (1973).

21. R. J. Thompson, T. B. McMullen, and G. B. Morgan,

Fluoride Concentrations in the Ambient Air, J . Air

Pollution Control Assoc . 2J_ (8), 484-487 (197Ty:

22. M. Buck and H. Stratmann, A Method to Determine Very

Small Concentrations of Fluorine Ions in the Atmosphere,

Brennstoff-Chem. (Essen) 46 (8), 231-235 (1965).

23. K. Leichnitz, "Drager Detector Tube Handbook".

24. American Public Health Association, American Water Works

Assoc., and Water Pollution Control Federation, Standard Methods for the Examination of Water and Wastewater , 13th ed. , pp 168-178, American Public Health

Assoc., Washington, DC (1971).

25. S. Ito, Methods and Tools for the Measurement of

Hydrogen Fluoride and Chlorine Gases, (Fukka suiso gasu,

enso gasu no sokuteiho to kiki) , PPM (Japan) _3 (11)

41-47 (1972).

26. J. 0. Ivie, D. Thomas, Moyer, and L. F. Zielenski,

Automatic Sensing and Recording of Air Pollutants on

Citrus Trees, Biomed. Sci. Instruments ]_, 45-52 (1964).

27. T. Hagiwara, Measuring Apparatus for Toxic Gases, Fuel

and Combustion 38 (12), 1217-1220 (1971).

28. A. van Raay, The Use of Indicator Plants to Estimate

Air Pollution by SU2 and HT^ Proc . First European

Congress Influence Air Pollution Plants Animals,

Wageningen, Netherlands (1968).

34
29. M. Kusumoto, and T. Toyama, The Micro Analysis of

Fluorine by the Linear Wash-out Type Oven and Its

Application, Japanese J. Ind. Health 1 3 (2), 120-127

(1971) .

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Company, New York (1972).

31. J. M. Bewers and F. C. Flack, Determination of Fluorine

by Prompt (Gamma) -Radiat ion from Proton Bombardment,

Analyst 94 (1114), 1-14 (1969).

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35
APPENDIX A

HYDROGEN FLUORIDE IN THE ATMOSPHERE

Tentative Method of the

Physical and Chemical Analysis Branch

and Engineering Branch

Analy te : Hydrogen Fluoride

Matrix : Air

Procedure : Ion selective

electrode

Date Issued:

Method No . P^CAM

Range: 0.03 to 5000 ppm

Precision ; Relative standard

. , deviation ±3.4 I Collaboratively Tested : No

Date Revised

1 . Principle of the Method

1.1 Hydrogen fluoride in air is collected by a sorption

tube containing sodium acetate crystals.

1.2 The fluoride, together with the sodium acetate, is

dissolved in water,

1.3 The fluoride content in the resultant solution is

measured by a fluoride ion selective electrode.

1.4 The amount of fluoride in the sample solution is

obtained from a calibration curve established with

sodium fluoride standards.

2 . Range and Sensitivity

2.1 At an air flow rate of 50cm^/min and a collection

time of 4 hours, the upper range of detection is

5000 ppm.

2.2 At an air flow rate of 200 cmVmin and a collection

time of 8 hours, the lower range of detection is

0.03 ppm.

2.3 The sensitivity is 2.5 yg of fluorine.

36
3. Interferences

3.1 In the presence of a pH buffer, such as sodium

acetate, and a polyvalent anion, such as citrate,

there is no interference in the analytical method.

3.2 Elemental fluorine is a positive interferrant , to

the extent of its collection efficiency on sodium

acetate and hydrolysis to form hydrogen fluoride.

4 . Precision and Accuracy

4.1 The relative standard deviation, computed from the

results of 20 samples, each with 7 determinations,

is ±3.4^ at 3 ppm hydrogen fluoride.

4.2 The accuracy has not been determined. No collaborative tests have been performed on this method.

However, the sodium acetate sorption tubes have

been shown to be 100% efficient in collecting

hydrogen fluoride, up to a total capacity of 46 mg.

The accuracy of the analytical method is dependent

upon the reliability of the sodium fluoride standards.

5 . Advantages and Disadvantages of the Method

5.1 Advantages: This method is highly selective for

fluoride ions and very simple in operation. It

involves few steps and requires a minimal amount

of training. The instrument is readily available

commercially and is relatively inexpensive. No

special chemical reagents are needed.

5.2 Disadvantages: This method requires a minimum of

20 minutes per run. This includes times for emptying the sorption tube (2 minutes), dissolution of

the sodium acetate (3 minutes), and stabilization

of the electrode (15 minutes).

6 . Apparatus

6.1 Fluoride ion selective electrode.

6.2 Calomel reference electrode.

6.3 pH meter with millivolt mode.

6.4 Magnetic stirrer with a variable transformer.

6.5 Analytical balance.

37
6.6 Polyethylene beakers, 50 ml.

6.7 Plastic coated forceps.

. 6.8 Straight corrugated forceps.

6.9 Triple beam balance.

6.10 1-liter polyethylene bottles, narrow mouth.

6.11 25-ml volumetric flasks.

6.12 Wash bottles . . .

•

6.13 Stiff wires.

6.14 Sampling apparatus.

6.14.1 Sorption tube for hydrogen fluoride.

6.14.2 Personal sampling pump.

7. Reagents '

7.1 Sodium acetate.

7.2 Stock citrate solution: 0.3 g sodium citrate

dissolved in one liter of distilled water.

7.3 Sodium fluoride, dried at 110°C for 24 hours and

kept in a desiccator.

7.4 0.5 N nitric acid.

8. Procedure

8.1 Collection of samples: hydrogen fluoride in air

is collected by the sorption tube with a personal

sampling pump. Plastic end caps are placed on the

sorption tube after sample is collected.

8.2 Analysis of samples.

8.2.1 The plastic end caps are removed from the

sorption tube.

8.2.2 Using a stiff wire, suitably bent, or a pair of straight corrugated forceps, the

, top wad of cotton in the sorption tube is

removed and placed in a polyethylene

beaker

.

38
8.2.3 The top end of the sorption tube is inserted

into the neck of a 25-ml volumetric flask,

and the sodium acetate crystals are emptied

into the flask. The sorption tube may be

gently tapped to get as many crystals as

possible into the flask.

8.2.4 Using a stiff wire, the bottom wad of cotton

is broken loose. The cotton should stay

inside the sorption tube.

8.2.5 The inside walls of the sorption tube and

the cotton are rinsed with 15-18 ml stock

citrate solution. Care must be taken that

all the rinse go into the volumetric flask.

8.2.6 The sorption tube is removed from the neck

of the volumetric flask.

8.2.7 The cotton in the polyethylene beaker is

rinsed with 5 ml stock citrate solution.

The rinse is added to the 25-ml volumetric

flask.

8.2.8 The solids in the volumetric flask are dissolved completely.

8.2.9 Stock citrate solution is added to the 25-ml

mark in the volumetric flask, and the solution is mixed thoroughly.

8.2.10 The sample solution is transferred to a 50-

ml polyethylene beaker, and 2 drops of 0.5

N nitric acid are added.

8.2.11 The fluoride selective electrode and the

calomel reference electrode are brought to

the sample solution.

8.2.12 The magnetic stirrer is placed in the beaker

containitig the sample solution, and the

variable transformer is turned on to provide

gentle stirring of the solution,

8.2.13 At the end of 15 minutes, the pH meter is

set to the millivolt mode and the reading

is recorded.

8.2.14 The pH meter is set to "standby" mode.

39
8.2.15 The sample solution is removed from the electrodes. The electrodes are rinsed with distilled water thoroughly and then wiped dry

with soft tissues. The electrodes are

immersed in distilled water when not in use.

Calibration and Standards

9. 1 Standards

9.1.1 A stock solution is prepared by dissolving

120 g sodium acetate and 0.3 g sodium citrate

in one liter of distilled water. This stock

solution is used for dissolution and dilution of the sodium fluoride standards.

9.1.2 Sodium fluoride, 420 mg weighed to the

accuracy of ±0,1 mg, is dissolved and diluted

to 100,0 ml in a volumetric flask. This

makes the 10 ^ M standard solution,

9.1.3 Sodium fluoride, 420 mg weighed to the

accuracy of ±0.1 mg , is dissolved and diluted

to 1.000 liter in a volumetric flask. This

makes the 10 ^ M standard solution.

9.1.4 10.0 ml of the 10'^ M standard solution is

transferred into a volumetric flask and

diluted to 1.000 liter. This makes the lO"^

standard solution.

9.1.5 10.0 ml of the 10"^ M standard solution is

transferred into a volumetric flask and

diluted to 1. 000 liter. This makes the lO"**

M standard solution.

9.1.6 1.00 ml of the 10~^ M standard solution is

transferred into a volumetric flask and

diluted to 1.000 liter. This makes the 10"^

M standard solution,

9.1.7 1,00 ml of the 10"^ M standard solution is

transferred into a volumetric flask and

diluted to 1,000 liter. This makes the 10'^

M standard solution.

9,2 Calibration Curve

9,2,1 A 25-ml sample of each of the standard solutions is used to obtain the millivolt reading for the fluoride concentration.

40
9.2.2 A plot is made o£ the millivolt reading

versus fluoride concentration. The latter

is plotted on logarithmic scale. The plot

should give a straight line down to 10 ^M.

1 0 . Calculations

10.1 The molar concentration of fluoride is read from

the calibration curve corresponding to the millivolt reading of the sample solution.

10.2 The amount of fluoride collected in the sorption

tube, in mg hydrogen fluoride, is calculated as

follows:

mg HF = 500 X molar concentration of fluoride.

11 . References

11.1 American Public Health Association, American Water

Works Association, and Water Pollution Control

Federation, "Standard Methods for the Examination

of Water and Wastewater", 13th ed., pp 168-178,

American Public Health Association, Washington,

DC (1971).

11.2 R. A. Durst, "Ion-Selective Electrodes", pp 389-

394, National Bureau of Standards Special Publication 314, National Bureau of Standards,

Washington, DC (1969) .

11.3 J. Wing and J. K. Taylor, "Development of a Solid

Sorption Tube for Sampling Hydrogen Fluoride in

the Work Atmosphere", National Bureau of Standards

Internal Report NBSIR 74-581, National Bureau of

Standards, Washington, DC (1974).

41
I NBS.114A (REV. 7-73)

U.S. DEr T. OF COMM. 1 . PU B L IC A TION OR R K PORT N O. 2. Gov't Accession

EI'^LIOGRAPHIC DATA No. SHEET NBSIR 74-581

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Development of a Solid Sorption Tube for Sampling

Hydrogen Fluoride in the Work Atmosphere

5. Publication Date October 1974

6. Performing Organization Code

7. AUTH0R(S1

James Wing and John K. Taylor

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE , , WASHINGTON, D.C. 20234

10. Hroiect/ 1 ask/ Work Unit No. 3104458

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Institute for Occupational Safety and Healt

Division of Laboratories and Criteria Development

v.) xnc xnna L X y unxo ^jz-kjc

13. Type of Report & Period

Covered

1 Final Report

14« Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most sigfiificant m formation. If document includes a significatst

bibliography or literature survey, mention it here.)

The efficiencies of several solid substances in a sorption tube for

collecting trace quantities of hydrogen fluoride gas in work atmosphere

have been evaluated. Reagent-grade sodium acetate crystals proved

to be a very efficient sorbent for hydrogen fluoride. Its solubility

in water and pH buffer properties are highly advantageous for subsequent

fluoride determination by the ion selective electrode. Experimental

results are presented for collection efficiencies of hydrogen fluoride

gas at several concentration levels, flow rates, and collection times,

the effects of elevated temperatures and humidity, and also breakthrough, bleed-out, storage and shipment studies. Specifications for

construction of the sorption tubes are also given.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

Air analysis; air sampling; gas analysis; hydrogen fluoride; industrial

hygiene; sodium acetate; sorber; solid; work atmosphere.

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