NBSIR 73-261

A Gas Dilution System for Hydrogen

Fluoride-Air Mixtures

Peter A. Pella, Ernest E. Hughes, and John K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

October 1973

Final Report

Prepared for National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

NBSIR 73-261

A GAS DILUTION SYSTEM FOR HYDROGEN

FLUORIDE-AIR MIXTURES

Peter A. Pella, Ernest E. Hughes, and John K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

October 1973

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. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

A Gas Dilution System for Hydrogen FluorideAir Mixtures

ABSTRACT

A gas-blending system was constructed for

producing hydrogen fluoride-air mixtures in the

concentration range from 1 to 20 ppm for the

purpose of calibrating analytical monitoring

devices. The system has been tested by measuring

these HF concentrations using a fluoride ionspecific electrode.

1 . INTRODUCTION

A gas dilution system was constructed which permits the

dilution of relatively high concentrations of hydrogen fluoride

in air to known low concentrations in the range from 1 to 20

ppm. The high concentration bulk mixture and diluent air are

mixed in a chamber at accurately controlled flow rates. The

diluted mixture then passes to a sampling manifold where the

known concentration of HF in air can be sampled or analyzed.

The sampling manifold can be directly attached to a collector

or other device for purposes of monitoring the HF concentrations produced.

2 . EXPERIMENTAL

2 . 1 Description of Dilution System

Figure 1 shows a schematic diagram of the dilution system.

The HF working standard is admitted into the system through

a stainless steel (s.s.) ON-OFF valve (V-1). The flow rate

is established with a s.s. differential flow rate controller

(DFC-1) and a s.s. flow control valve (FCV-1). The flow rate

is measured with a plastic bodied rotameter (constructed of

acrylic material). Similarly, the diluent air is admitted into

the system and mixed in the mixing chamber with the HF bulk

mixture. This system is designed to produce dilutions from

zero to a hundredfold of the HF bulk mixture. For determina-
tion of the HF concentration in the manifold, a sample is taken,

using a s.s. absorption tube and analyzed with a fluoride-ion

selective electrode analytical system. The metal components

for the diluent air are constructed of brass and 1/4" o.d. copper

tubing. The regulator used on the HF bulk mixture cylinder is

constructed of non-corrosive Monel metal. The mixing chamber,

sampling manifold, and all tubing connections on the HF side

of the dilution system are constructed of stainless steel.

Photographs of the dilution system are shown in Figures 2 to 4.

2 . 2 Description of Analytical System

The HF concentrations are determined by passing a knov/n

volume of the mixture through a known quantity of 0.1 M NaOH.

The HF is rapidly neutralized to form NaF. The fluoride-ion

concentration of the resulting solution is measured with a fluoride ion-selective electrode.

The apparatus consists of an ion-selective electrode, a calomel reference electrode, a glass electrode for measuring

the pH of the solution, and a pH meter. A schematic diagram

of the apparatus is shown in Figure 5.

The glass electrode is necessary for adjusting the pH of

the NaOIl absorbing solution. Below pH = 4 the NaF hydrolyzes

to form undissociated HF yielding low results. Above pH 8,

hydroxyl ion interferes and yields high results. The glass

electrode is disconnected after the pH adjustment and the

fluoride-ion concentration is measured with the same meter,

using the ion-selective electrode.

2 . 3 Calibration of Measurement System

The ion - se lective electrode was calibrated with standards

prepared as follows

A 0.0500 M stock solution of NaF was prepared by dissolving

2.100 grams of reagent-grade NaF, previously dried at 110 °C,

in distilled water in a 1 liter volumetric flask. Standard

- 2 - 5 solutions from 1.00 x 10 to 1.00 x 10 M were prepared by

accurate dilution of the stock solution with 0.1 M KNO^ in

2
100-ml volumetric flasks. The KNO^ is used to maintain the

same ionic strength as the 0.1 M NaOH absorbing solution. The

NaF solutions are stored in polyethylene containers. Figure 6

shows a typical calibration curve. These solutions were

measured periodically and the millivolt readings obtained are

presented in Table 1.

Table 1 Stability o£ Calibrating Solutions

Concentration Date, 1975 Millivolts

1.0xlO"^M 4/3 +127.4

. 4/7 +125.

8'

1.0x10 M 4/3 +67.6

4/7 +66.3

^ 4/30 +66.0

l.OxlO'-^M 4/3 +7.5

4/7 +7.0

4/30 +6.3

5/15 +7.5

These results indicate that a new calibration curve is not

required every time a series of fluoride ion measurements are

to be made. However, it is recommended that a calibration be

performed with at least two solutions prior to the start of

any fluoride ion measurements. This is particularly important

if the electrode has not been used for several weeks.

2 . 4 Preparation of HF-air Bulk Mixture

To evaluate the dilution system it was desirable to

prepare an HF bulk mixture in the range from 50 to 100 ppm.

This was done by adding 200-220 mm Hg pressure of hydrogen

fluoride vapor from a cylinder of liquid hydrogen fluoride to

a large evacuated cylinder. Air was then added to a pressure

of 1000 psi. Subsequent analysis by the method described in

Section 2.5 yielded an average concentration of 61 ppm. The

difference in concentration behavior predicted, 4000 ppm, and

the observed value is due to reactor of some HF with the

cylinder material or with foreign materials with the cylinder.
2 . 5 Procedure for Measuring HF-Air Mixtures

The hydrogen fluoride concentration of the bulk mixture

was determined as follows:

15.0 ml of 0.1 M NaOH was added to the collection tube

which was then connected to the dilution system. The exhaust

end of the collection tube was connected to a wet-test meter

and the diluent air control (V-2) turned to the "off" position.

An accurately measured volume of 10 to 20 liters was bubbled

through the 0.1 M NaOH solution at a rate from 0.5 to 1.0 1/min.

Valve V-1 was turned off, and the solution was transferred to

a 50-ml polyethylene beaker. The pH of this solution was

adjusted to values between 5.5 and 6.5 with three drops of

concentrated nitric acid followed by dropwise addition of 1:1

nitric acid. The pH meter was calibrated in this range using

a standard buffer. No volume dilution correction was made if

the added acid did not exceed four drops (i.e. about 0.2 ml).

The fluoride-ion concentration, Cp, of this solution was then

measured, allowing about 5 to 10 minutes for the electrode

reading to stabilize. The concentration of the solution was

determined by reference to the calibration curve in Figure 6.

The concentration of hydrogen fluoride, Cj_^p(ppm), in a gaseous mixture is calculated by the relationship

Cp V E

^ ^IIF^PP"^^ = -TOOOV ^ ^0

where Cp = fluoride ion concentration (from i on -se lective

electrode measurement) (mol/1)

.

V = volume of test solution (ml)

.

V = volume of gas sampled (1).

E = molar volume of gas (24.5 1 at 25 °C and 760 mm).

The concentration of HF in the diluted gas was determined

similarly, except the sampling is done from the manifold.

4
2 . 6 Stability Study of HF Bulk Mixtures

In order to use the dilution system described herein

effectively it is essential to have a bulk mixture where the

concentration remains unchanged over a reasonable period of

time. For this reason the HF bulk mixture was analyzed

periodically over a period of several weeks. Results of

measurements over a 6-week period are plotted in Figure 7,

together with the mean value of all determinations. No

significant decrease in the HF concentration level occurred

during this 6-week interval.

3. EVALUATION OF THE PERFORMANCE OF THE GAS DILUTION SYSTEM

Various dilute mixtures in the range of 1 to 20 ppm were

prepared by mixing the bulk mixture with diluent air at measured

flow rates. A summary of the measurements made are tabulated in

Table 2.

Table 2 Summary of Calculated and Observed HF

Concentrations

Bulk Mixture

Flow Rate

1/min

0.272

0.272

0. 109

0.105

0.050

0 . 054

Air

Flow Rate

1/min

O.SOO

0.943

0.411

0.905

0.503

2 . 83

Average Deviation

Dilution HF Cone. HF Cone. ObsFactor Calc ' d ,ppm Obs . ,ppm Calc ' d ,ppm

0, 3523

0.2239

0.2096

0. 1040

0.0904

0.01869

21.5

13. 7 12 . 8

6.3

5.5

1.1

19.5(4)

11.6(5)

13.0(15)

5.3(9)

4.7(5)

1.1(4)

-2

-2

+ 0

-1

-0

0

Columns 1 and 2 of Table 2 contain the measured flow rates from

which the dilution factors in column 3 are calculated. The

calculated HF concentrations, obtained by multiplying 61 ppm

(i.e. the bulk mixture concentration) by the respective dilution

factor are given in column 4. The mean observed HF concentrations,

based on the number of fluoride determinations given in the

parenthesis are tabulated in column 5. The last column is the

difference between the observed and calculated HF values, in ppm.

Examination of these results indicates that the measured values

5
are generally 10 to 20 percent lower than the calculated values.

This is also demonstrated in Figure 8 where the slope of the

solid line (measured values) is less than the predicted line

(dotted line in figure)

.

Two factors may contribute to the imprecision of the results

The first involves analytical errors including collection

efficiency. Physical or chemical adsorbtion of the HF in the

dilution system may also be a problem. The latter would also

account for the observed values being generally less than the

calculated values.

The sample collection efficiency was evaluated by using

two collection tubes in series, each containing 15 ml of 0.1 M

NaOfl. Fluoride analysis of each solution indicated that the

HF collection efficiency in the first collection tube was

better than 99 percent. The reproducibility of the analytical

measurement was investigated by rep ated measurements of

fluoride standard solutions, and solutions obtained from the

HF monitor of the dilution system. The relative standard

deviation calculated from these results did not exceed 5 percent.

Therefore, it appears that the analytical system is not the

major cause of the imprecision.

To obtain more definitive information on the cause of the

deviation between the observed and calculated values, experiments were performed in which the HF bulk mixture was analyzed

under different conditions of flow rate. The results are plotted

in Figure 9 and summarized in Table 3.

Another experiment was performed to further characterize

this behavior. This consisted of preparing a bulk mixture in

"super-dry" nitrogen at a 40 ppm HF concentration level and

comparing the HF measurements from the manifold of the dilution

system with measurements made directly from the cylinder as shown in Figure 10. Super-dry nitrogen was used to prepare this

6
TABLE 3

Observed Bulk Mixture Concentrations of l{¥ for

Various Flow Rates and Collection Times

Hxperimcnt Obs. HP Vol. Collected, Flow Rate

No . Cone .

, ppm Liters ml/min . Comments

1 12.3 18.0 400 No prior flushing of

system with HF standard

2 37.7 21.0 300 System flushed with HF

standard for 1-2 min.

then sample 2 collected

3 59.5 21.0 923 Sample 3 collected at

greater flow rate than

sample *2

4 50.5 20.0 400 HF valve (V-l) turned

off and sample #4

collected one hour

after 3 with no prior

flushing of system with

HF standard

5 45.4 16.2 136 Flow rate decreased

from 400 ml/min to 136

ml/min

.

6 57 . 5 20 . 1 775 Flow" rate increased

from 136 ml/r;in. to 775

ml/min.

7 56.0 21.0 540 System flushed for 5 min. at 1.0 £/min

before collecting 7 through 11

8 59.8 12.0 540

9 55.1 12.0 540

10 56.8 12.0 540

11 61.3 12.0 540

7
mixture in order to minimize any moisture in the cylinder (e.g.

breathing air may contain as much as 50 ppm H2O) . A series of

measurements was made as mentioned above and the results are

shown graphically in Figure 11. The HF values indicated by A

in the figure were measured as collected from the manifold of

the dilution system after first flushing the system for five

minutes at 3-4 liters/min. Group B represents the range of

values measured directly from the tank via the Teflon impinger.

Group C represents the range of values measured via the dilution system after first flushing the system for 20 minutes at

3-4 liters/min.

One conclusion that may be drawn from these experiments

is that several hours of flow are necessary to establish a stable concentration in the system. The series of experiments

4, 5, 6 in Table 3 and indicated by a broken line in Figure 9

suggests that low flow rates may result in decreased concentrations in the manifold by an adsorptive or reactive means. A

minimum flow rate of 500 ml/min appears to be required to maintain a stable concentration in the manifold. These results show

that even after flushing the dilution system for 20 minutes (C

in Figure 11) at a high flow rate the mean result is 13 percent

less than the average found when using the Teflon impinger only.

The stability of the HF concentration generated over an

eight hour working day, was investigated. This consisted of

first flushing the dilution system for 20 minutes with the

bulk mixture containing 61 ppm of HF at a flow rate of 0.7 1/min.

The flow rate was then decreased to .109 1/min and diluent air

was added at a flow rate of 0.411 1/min and mixed. The calculated dilution factor was 0.21. The HF concentration was

measured 3 to 10 1/2 hours after the initial adjustment of the

flow rate conditions. These results are plotted in Figure 12.

The average value to be compared with a predicted value of 12.8

ppm was 14.2 ppm with a relative standard deviation of 0.8 ppm.

8
4. SUMMARY

In summary, the results indicate the feasibility of

dilution of bulk mixtures to obtain concentrations in the 1 to

20 ppm range. An empirical formula can be written to correct

for the observed bias between the measured and calculated HF

values as follows:

C , = k Co X D.F.

d B

where = the desired HF concentration level corresponding to

a particular value of the dilution factor, Cg is the concentration of the bulk mixture, and k is the experimentally determined

factor. For this particular dilution system, k = 0.9. However,

it is recommended that users of this system periodically measure

the concentration of a diluted HF sample using the fluorideion selective electrode analytical method to ensure accurate

values. The analytical method requires only a minimum of

experience and expertise to perform the measurements.

9
WORKING

STANDARD

HF

AIR

DILUENT

V-I

%- DFC-I —

R0TAI4ETER

FCV-I

V-2

DFC-2 -6

MIXING

MANIFOLD

ROTAMETER

SAMPLING

MANIFOLD J

irnr

FCV-2

SAMPLE

PORT

TO EXHAUST

I iMONITOR

PORT

NAOH

SOLN. H2)

HF

COLLECTION

TUBE

i Figure 1. HF" Dilution System.

I

10
Figure 2. Front View of Dilution System.

11
Figure 3. Rear View of Dilution System.

12
Figure 4. Bottom View of Dilution System.

13
FLUOR

ELECTRQPE

FLUORTDE

SOLN.-

STIRRING

BAR

UORIDE pLjll CALOMEL

IQN LJ REFEREN

OPE^ ^ ^

( I

J

^

ICE ELECTRODE

50-ML

POLYETHYLENE

BEAKER

MAGNETIC STIRRER

PH MLItk

?iF CONN.

^EF. CONN.

Figure 5. Apparatus for Fluoride Analysis.

14
Figure 6. Calibration Curve for Fluoride Ion.
• • • •

.8

OL

Q.

-

!4 Jn ^

Q

-Kg

• • • 4 •

Wdd' ONOO JH

Figure 7. Observed HF Working Standard Concentration vs. Time.

16


MONE

REG

CI

HF

WORKING

STANDARD

(HF-IN-NITROGEf )

TEFLON TUBE

Ml

^

TO WET TEST METER

^ TEFLON

IMPINGER

NAOH

SOL'N

Figure 10. Measurement of HF Working Standard Via Teflon

Impinger.

19
ro

Q.

Q.ro

I—• iQD

Q.

a.O

ro I—•—1<

Q ^ CVJ 00 ^ ro ro Wdd'ONoo dH aaAHasao

Figure 11. Comparison of HF Measurements.

20
0

#•

5

a

a

5r

<

LU

lO

iX3

Figure 12. Observed HF Concentration vs. Generating

Time

.

21
:£ i

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NBS-n4A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. IJUHLK-A TION OR Kl.l'OR l' NO. NBSIR 73-261

2. Ciov't Actc.s.sion No.

3. Rc c ipic iit '.s Acce.s.siiin No.

4. ri'i LH .w'l) rm.i-; A Gas Dilution System for Hydrogen Fluoride-Air

Mixtures

5. I"^u b 1 1 c a 1 1 on Date October 1973

6. Performing Organization ( oJ(

7. AUTHOR(S) p^^^ Pella, E.E. Hughes, and J.K. Taylor 8. Performing Organ. Report No. NBSIR 73-261

9. PERFORMING ORGANIZATION NAME AND ADORHSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

i u. r^rojtct/ lasK/ work unit ino.

11. Contract/Grant No.

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

National Institute for Occupational

Safety and Health

Cincinnati, Ohio

13. Type of Report & Period Covered

Final

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary 'of most sigriificant information. If document includes a significant

bibliography or literature survey, mention it here.) A gas-blending system was constructed for producing hydrogen fluorideair mixtures in the concentration range from 1 to 2 0 ppm for the

purpose of calibrating analytical monitoring devices. The system

has been tested by measuring these HF concentrations using a fluoride

ion-specific electrode.

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) Vluoride analysis; gas "b lending ; HF-air mijctures • ion-specific electrode. '

18. AVAILABILITY |

' Unlimited

1^ For Official Distribution. Do Not Release to NTIS

1

' Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402. SD Cat. No. CI?

1 ! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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22

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