NBSIR 75-977

Development of Solid State

Samplers for Work Atmospheres:

Phosphine

Bernard Greifer and John K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

January 1976

Final Report

Prepared for

National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

NBSIR 75-977

DEVELOPMENT OF SOLID STATE

SAMPLERS FOR WORK ATMOSPHERES:

PHOSPHINE

Bernard Greifer and John K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

January 1976

Final Report

Prepared for

National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

it

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson. Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

CONTENTS

Page

1. INTRODUCTION 1

1.1 Review of Phosphine Sorption 1

1.2 Review of Phosphorous Determinations 2

2. EXPERIMENTAL PROCEDURES 3

2.1 Gas Regulation and Sampling 3

2.2 Solid Sorbent 4

2.2.1 Sampling Tube 5

2.2.2 Breakthrough Experiments 5

2.2.3 Results of Breakthrough Experiments 7

2.2.4 Discussion of Sorption Results 10

3. QUANTITATIVE DETERMINATION OF PHOSPHORUS ....... 10

3.1 General 10

3.2 Phosphates on KMnO^-Silica Gel Sorber 11

3.3 Phosphine Desorption from Charcoal 15

3.3.1 Aeration Experiment 15

3.3.2 Furnace Experiment 15

3.4 Effect of Charcoal on AgDDC Color Reaction .... 17

3.5 Phosphorous Oxyacids on Charcoal 17

3.6 Silver Nitrate Silica Gel Sorber 17

3.7 Ascorbic Acid Procedure 18

4. RECOMMENDATIONS FOR FUTURE WORK 18

5. SUMMARY 19

6. REFERENCES 21

7. APPENDICES 24

7.1 Appendix A. Preparation of AgNO 3 - Impregnated. . . 24

Silica Gel

7.2 Appendix B. Modification of ASTM D515-72, .... 26

Standard Methods of Test for Phosphorous in

Water, Method A (Colorimetric Ascorbic Acid

Reduction Method)

7.3 Appendix C. Charcoal Treatments 2 7

i
TABLES

Page

- 1. Volume of Reactive Gas (Liters) 4

2. Weight of Phosphorus Collected 4

3. Phosphine Sorption on Solid Materials 8

4. Phosphine Sorption on Solid Materials from Reference (1). 9

5. Oxalic Acid Interference in Phosphate Determination as. . 13

Phosphomolybdate

6. Furnace Elution of Phosphine 16

FIGURES

1. Diagram of Gas Sampling System 6

2. Oxalic Acid Interference with Phosphate Determination . . 14

ii
DEVELOPMENT OF SOLID STATE SAMPLERS FOR WORK ATMOSPHERES:

PHOSPHINE

Summary Report, July 1974 - June 1975

ABSTRACT

Investigations to find suitable solid

sorber materials for phosphine in work atmospheres

at the ppm level are described. The best sorber to

date is silver nitrate- impregnated silica gel. Some

difficulties remain to be overcome before a procedure for determining the amount of phosphine

adsorbed can be recommended.

1. INTRODUCTION

This report describes work on the development of a

solid-state sampler for phosphine in industrial atmospheres.

It is a continuation of previously reported investigations

in which the methodology, contaminant generation system, and

gas sampling systems were discussed (1).

The program's objective is to find suitable solid-state

sorber materials for phosphine in air at ppm levels, and to

quantitatively measure the efficacy of these sorber materials

by chemical analysis in the laboratory. Attention has been

directed toward sampling phosphine reproducibly and analyzing

the sorber materials to determine the quantity of phosphine

sorbed. Various procedures for determining phosphine adsorbed

on a solid surface have been tested on two candidate sorbers:

charcoal, and silver nitrate- impregnated silica gel. Difficulties have been encountered with all the analytical procedures, and these have not yet been completely resolved. The

most promising system to date appears to be sorption on

silver nitrate- impregnated silica gel, elution of sorbed

material with dilute acid, and quantitative determination by

a spectrophotometr ic measurement of the phosphomolybdate

according to the ascorbic acid method, ASTM D-515.

1 . 1 Review of Phosphine Sorption

Liquid and solid sorbers have been described for the

direct reaction of phosphine with color-forming reagents.

Dechant et al (2) have collected phosphine in air by bubbling

the air through a pyridine solution of silver diethyldithiocarbamate and measuring the absorption of the stable complex
at 465 nm. The collection efficiency of their bubbler was

only 86 percent. Solid sorbers for phosphine have included

impregnated papers and impregnated silica gel. Hughes and

Jones (3) described silver nitrate impregnated filter paper;

they compared the visible spot with standard stains, and

dissolved it in hot HN0 3 and reacted it with sodium sulfide

and boric acid for spectrophotometr ic determination at 470 nm.

Muthu et al (4) described mercuric chloride papers containing

methyl yellow indicator which turned from yellow to red upon

liberation of HC1 in the presence of phosphine. Nelson and

Milun (5) determined phosphine at ppb levels in air by using

detector tubes packed with silver nitrate impregnated silica

gel, and measuring the length of the black stains as a measure

of phosphine concentration.

In recent years, attention has been directed toward

phosphine residues remaining after fumigation of cereal

grains. Originally thought to leave no residues at all (6,7),

recent investigations have indicated that there may be both

water-soluble and insoluble phosphorus residues on porous

materials exposed to ppm concentrations of phosphine. The

studies of Berck (8) , Robinson and Bond (9) , Robinson (10)

,

Awasthi et al (11) , Hilton and Mee (12) , Tkachuk (13)

,

Vardell et al (14,15) and Sato and Suwanai (16) point to the

existence of non-phosphine residues which cannot be removed

by aeration or vacuum, and which are throught to be watersoluble lower oxy-acids of phosphorus. Tkachuk (13) could

not remove phosphine residues by aeration for two days in an

air flow of 400 linear feet/min, but he was able to extract

64 percent of the phosphorus with water, and so determined

that 88 percent of the extract was hypophosphite (H 3 P0 2 )

and 12 percent of it was pyrophosphate (Hi^O?) . Robinson

(9,10) measured phosphine residues on filter paper by neutron

activation analysis and found that they consisted mainly of

the lower oxyacids hypophosphite (H 3 P0 2 ) and phosphite

(H3PO3). Robinson found that spectrophotometric analysis of

a 24-hr extract into 5 ml dilute H2SCU was unsuccessful

because the 0.25 ug phosphorus content (N.A.A. measurement)

was below the workable spectrophotometric limit. Robinson

(10) postulated that the primary step in the residue deposition mechanism may be the sorption of phosphine as a monomolecular layer on the substrate surface, followed subsequently by fixation through oxidation. Bastick (17-19) and

Zorin et al (20) have measured adsorption isotherms for

phosphine on charcoal and silica gel.

1 . 2 Review of Phosphine Determinations

Phosphine has been determined by gas chromatography

(21-25), colorimetric methods (2,11,16,26), and radioactive

2
labeling techniques using 32 P (8,9,10,27). Other methods

for phosphine determination have included titration, fluorescence, potent iometric and conductometr ic methods, turbidimetry

,

and gravimetry; these were mentioned in the previous literature review (1) . Special mention must be made of the specific

ion electrode for phosphate determination (28) which could

have been of great value except that its sensitivity was too

low to permit measurements to be made at the ppm level.

2. EXPERIMENTAL PROCEDURES

2 . 1 Gas Regulation and Sampling

To carry out the screening of solid sorbents, Matheson*

phosphine gas standard certified to contain 4.5 ppm in

nitrogen was used undiluted for rapid screening of candidate

materials, and diluted with air to obtain gas concentrations

ranging between 1/5 and five times the threshold limit value

(TLV) for quantitative measurements. The gas dilution

system and contaminant sampling system have been described

previously (1) . Figure 1 is a diagram of the sampling

system. Since the mixing system used air to dilute the

compressed gas standard, the composition of the matrix gas

was effectively air rather than nitrogen except during the

sorber screening tests when a 4.5 ppm mixture of phosphine

in nitrogen was used as supplied.

In use, the flow rate was set by adjusting a flow controller needle valve to the desired flow rate as read on a

rotameter flowmeter, and the pressure gauge was read to

assure that the pressure drop across the sampling tube did

not exceed 25.4 mm Hg at a flow rate of one liter/min. The

system maintained the preset flow rate to within ±2 percent

for sampling periods of minutes to hours. The contaminant

gas was sampled by loosely inserting the sampling tube into

the sampling manifold. For calibration purposes, to determine

the gas concentration supplied to the sampling tube, the gas

from the manifold was passed through an empty sampling tube

and collected in two liquid bubblers in series for subsequent

analysis

The use of trade names is for identification only, and does

not imply any recommendation or endorsement by the National

Bureau of Standards.

3
2.2 Solid Sorbent

The gas volumes and flow rates required for the prescribed

sampling conditions are shown in Tables 1 and 2. The sorber

materials had to be capable of capturing between one and 200 yg phosphine, retain it for as long as required for storage and

transport to a laboratory, and then permit quantitative

desorption of the phosphine or stable phosphorus oxidation

products during chemical analysis.

Table 1. Volume of Reactive Gas (Liters)

Time Flow Rate

(hr) (ml/min)

50 200 1,000

1/4 --- 15

4 12 48

8 24 96

Table 2. Weight of Phosphorus Collected (ug) 1

Gas Cone. 2 Volume of Gas (Liters)

(ppm) 12 15 24 48 96

0.06 0.91 1.14 1.82 3.65 7.30

1.5 22.8 28.5 45.6 91.3 183.

l \ig = ppm x liters x M/24.45

where M = molecular weight of phosphine = 31

2 Phosphine cone, range = 1/5 to 5 times the TLV of 0.3 ppm

Additional requirements that the solid state sorber must meet are:

* The resistance to air flow should not exceed 25.4 mm Hg

at a flow rate of one liter/min (as specified by NIOSH).

* The device should retain the contaminant of interest

in the presence of other substances in the air, and

release it for subsequent chemical analysis.

* The device should indicate if breakthrough has occurred.

* The device should be stable to the effects of temperature,

humidity, and storage.

4
2.2.1 Sampling Tube

The sampling tube is a straight 100 mm length of borosilicate glass tubing, 9 mm O.D. x 7 mm I.D. with three

identations 20 mm from one end to act as retainers for a wad

of quartz wool or polyester fabric which supports the sorber

material. The tube is packed with silver nitrate impregnated

silica gel or with the sorber material under test, using a

5-10 mm layer of AgN0 3 /silica gel as a breakthrough indicator.

The latter turns yellow at the first traces of phosphine,

eventually turning brown and then black upon continued exposure

to phosphine. The sorber column is secured with a second wad

of quartz wool or polyester fabric, and the tube is capped

with plastic end caps (e.g. Caplugs* B-715).

In use, the sampling tube is supported in the manifold

by a stainless steel Cajon* high-vacuum tubing coupling

(where it is held by an "0" ring) , and the coupling is

connected to the gas sampling system with 1/4-inch Teflon*

tubing

2.2.2 Breakthrough Experiments

Compressed gas (4.5 ppm phosphine in nitrogen) was

flowed through a rotameter flowmeter into a mixing manifold,

from which it was sampled into the solid state sampling tube

at atmospheric pressure by aspiration through a constantflow regulator. A subatmospher ic reference pressure for

flow regulation control was supplied by a rotary vane pump

(Figure 1) . (Some preliminary screening experiments were

carried out by passing 4.5 ppm phosphine through the rotameter directly into the sampling tube.)

The phosphine was flowed into the samping tube until a

yellow coloration in the detector section signaled the first

traces of breakthrough, or until 80-100 liters of 4.5 ppm

gas had been sorbed without breakthrough. For speed and

convenience in screening, the 4.5 ppm standard was used

without separate analytical confirmation of the concentration. The strongly reducing nature of the phosphine imposed

no limitations on materials of construction, which were

brass, glass, Teflon* and neoprene.

Trade names are used for identification only and do not

imply endorsement or recommendation by the National Bureau

of Standards.

5
Ul

6
Breakthrough experiments reported previously (1) showed

that the best sorbers for phosphine at the ppm level were

chemisorbers with high surface areas, such as AgN0 3 impregnated

silica gel, or KMnOi, impregnated silica gel. Physical

sorbers such as charcoal were less effective but still

satisfactory if used in sufficient quantity to accommodate

the expected quantity of phosphine.

Breakthrough experiments were carried out on the following additional candidate sorber materials: ascorbic acid,

hydroquinone , chromium chloride hexahydrate, sodium dichromate

dihydrate, water-washed charcoal (coal based charcoal with

the fines flotated off), acid-washed calcined charcoal

(calcined at 400 °C for 1 h) , aluminum oxide, silver nitrate

impregnated silica gel, and manganese dioxide impregnated

silica gel. The solid sorbers included water-soluble material,

which would simplify the quantitative determination of sorbed

phosphine by eliminating the desorption step in the analytical

procedure. An attempt to prepare KMnCU -coated sodium acetate,

(taking advantage of the acetone- solubility of KMnOO was

unsuccessful because the KMnCU was converted to Mn0 2 during

evaporation of the solvent.

2.2.3 Results of Breakthrough Experiments

The results of the current series of breakthrough

experiments on candidate sorber materials are shown in Table

3. For comparison, previous experiments from reference (1)

are reproduced in Table 4. The experiments showed that

neither strong oxidants nor reducing agents retained PH 3 effectively at a concentration of 4.5 ppm (nominal), nor did

highly porous materials alone (e.g. silica gel and acidwashed charcoal). The combination of oxidizing material

plus porous substrate gave the most effective sorbers.

Water-washed charcoal, thought to be an effective

sorber for phosphine, had a high natural phosphorus content

which interfered with subsequent phosphorus determinations.

When the charcoal was acid-washed to remove phosphorus

impurities, the charcoal was no longer effective as a sorber.

Either the activated charcoal was deactivated by the acid

treatment, or sorption on charcoal depended on the trace

content of oils and metals removed by the acid. Calcining

at 400 °C (i.e. heating in a closed container to limit

access of air) was attempted to reactivate the charcoal

that might have been deactivated by acid treatment but the

calcined charcoal was also an unsatisfactory sorber for

phosphine.

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The sorber KMn0 4 /SiGel could not be prepared in quantity

because the KMnO^ reduced to Mn0 2 . Hence, the only remaining

effective sorber for phosphine is AgN0 3 /SiGel. The preparation

of this material is outlined in Appendix A.

2.2.4 Discussion of Sorption Results

Adsorption isotherms for phosphine on charcoal and

silica gel have been measured (17-20) but the sorption

capacity at flows of about 1 liter/min may have little if

any relation to capacity under equilibrium conditions.

Zorin et al (20) found that phosphine sorption on silica gel

followed the Langmuir isotherm for physical adsorption

between +20 °C and 80 °C, with sorption capacities in the

range 0.11-0.33 x 10" 3 mole/g of silica gel (317-11.2 mg/g)

Bastick (17) found charcoal adsorption of phosphine to be

0.3 x 10" 3 mole/g of charcoal (10.2 mg/g). However, Table 4

shows that 4.5 ppm phosphine (0.00626 mg/liter) broke through

3 g of silica gel after 0.3 liter at a flow rate of 1 liter/min

so that the apparent sorption was not 3.7-11.2 mg/g but 0.0006 m

of silica gel. Even AgN0 3 /SiGel, the best sorber material,

retained 0.64 mg phosphine when the experiment was concluded

at the end of 127 min, with about 1 g out of 1.68 g of

sorber material darkened (Table 3), which equals about 0.64 mg/

considerably less than the literature values for silica gel

alone

3. QUANTITATIVE DETERMINATION OF PHOSPHORUS

3 . 1 General

Two independent analytical determinations are needed:

(1) determination of the quantity of phosphine applied to

the sampling tube, and (2) determination of the quantity of

phosphorus retained on the solid state sorber. A third

analysis, useful but not strictly necessary, might be the

determination of phosphine concentration in the manifold gas

mixture; strictly speaking, this is not the same as analysis

#1 because it is independent of sampling time and sampling

flow rate.

Gas chromatographic procedures would be suitable for

determining phosphine gas concentrations in the manifold,

but they would require standardization with phosphine mixtures

of known concentration, so that these procedures would be

referred back to standard colorimetric analyses anyway.

Since analysis #2 requires solvent elution of (probably)

nonvolatile species, gas chromatography of the solutions

would not be effective. Because the analyzed species is

probably nonvolatile, the colorimetric measurement of

phosphorus in solution was chosen.

10
To determine the quantity of phosphine applied to the

sampling tube, the gas mixture from the manifold is passed

through an empty sampling tube into two bubblers in series,

containing either an oxidant to convert PH 3 to phosphate for

subsequent analysis, or a color-forming complexant such as

silver diethyldithiocarbamate for direct spectrophotometr ic

measurement after calibration with known phosphine mixtures.

Analysis #2 involves solvent elution of adsorbed phosphorus

and subsequent colorimetric determination of dissolved

phosphine or oxyacids of phosphorus as the case may be. The

investigations of analytical procedures for phosphorus were

directed toward determining (a) whether phosphine could be

desorbed from the solid sorber as a gas, or whether the

phosphorus was present as a nonvolatile phosphorus oxyacid

or soluble phosphate; and (2) whether 100 percent of the

sorbed phosphorus could be eluted from the chosen sorber

material. The experiments included examination of:

1. Desorption of phosphine as phosphine gas from

charcoal

2. Solvent elution of phosphate from KMnCK impregnated

silica gel

3. Solvent elution of phosphine from charcoal

4. Solvent elution of phosphate from charcoal

5. Solvent elution of phosphate from silver nitrate

impregnated silica gel

The following paragraphs describe experiments carried out in

these areas, and present results which, in each instance,

caused the individual analytical procedures to be rejected.

3 . 2 Phosphates on KMnCU -Silica Gel Sorber

Preparation of sorber material, described in Appendix

A, involved soaking silica gel in a dilute aqueous solution

of KMnOn, pouring off the excess liquid, and drying. Since

elution of the sorber material would yield a KMnOi* solution

of soluble phosphates, the analytical procedure of Muller

(29) as described by Dechant et al (2) was used, that is,

bleaching of KMn0 4 with oxalic acid followed by determination

of the phosphate as phosphomolybdate by a standard method.

Phosphate standards in acidified 0.01 N KMnOi* solutions were

determined by the standard APHA stannous chloride procedure

(30) which required that spectrophotometr ic measurements be

11
made during a very brief time interval, not less than 10 min

nor more than 12 min after addition of SnCl 2 reagent. The

time period proved too short to permit accurate measurements

to be made , because samples, blanks, and standards had to be

treated one at a time so that the measurements could be made

at exactly the same number of minutes after addition of

reagent. Identical handling of the standards, blanks, and

samples was not assured because they were not prepared all

at the same time, in the same series of laboratory manipulations.

The hydrazine sulfate procedure of Kingsley and Schaffert

(31) was tested and found to be more satisfactory because it

permitted a longer working time, up to 24 hr according to

these authors; this was the procedure employed by

Dechant et al (2) . However, the oxalic acid used to decolorize

the KMnC\ interfered with the hydrazine reduction procedure,

causing a reduction of absorbance of samples, blanks, and

standards which could not be compensated for in the calibrations .

Table 5 presents some data on the magnitude of oxalic

acid interference with the phosphate determination, and Figure 2

is a plot of these data. To three standard phosphate solutions

containing 10, 50, and 100 yg P per 100 ml, were added 20 ml

0.01 N KMnOi* (the composition of the bubbler solution that

would be used to determine phosphine concentrations in the

gas mixtures, and also the estimated concentration of KMnC\

in the eluted solutions from the solid sorbers) and 10 drops

of 10 percent oxalic acid solution. Figure 2 shows the

nonlinearity of phosphate standards containing KMnC\ and

oxalic acid. The odd point at 50 yg P is the absorbance of

a 50 yg P/100 ml solution to which only 10 drops of oxalic

acid were added, and no KMnOi,. The oxalic acid completely

prevented the formation of the molybdenum blue color and the

resulting yellow solution had very low absorbance.

The interference of oxalic acid with the determination

of phosphate as the phosphomolybdate was studied by Chalmers

and Sinclair (32) who found that oxalic acid and tartaric

acid decreased the absorbance of phosphomolybdate solution

when added in large amounts because of a shift in the equilibrium of the reaction:

phosphomolybdate + oxalic acid phosphate + oxalatomolybdate

Since the oxalic acid had such a profound effect on

phosphate determination, it was apparent that in effect KMnO^

was interfering with the phosphate analysis.

The subject of quantitative determination of phosphates

in the presence of KMnCs became moot when the initial (40 g)

12
Table 5. Oxalic Acid Interference in Phosphate

Determination as Phosphomolybdate 1

Solution

(yg P/lOOml) Addition 2 Absorbance

10 0. 089

100 0.891

50 A 0. 039

10 B 0.087

50 B 0.387

100 B 0. 589

Absorbance at 825 nm, 1-cm cell, IR phototube of

Beckman DU spectrometer.

A = 10 drops oxalic acid.

B = 10 drops oxalic acid plus 20 ml 0.01 N KMnO„.

13
>
quantity of sorber material was exhausted, and attempts to

prepare it in quantity were unsuccessful. Impregnation of

200 g of silica gel with KMnC\ gave Mn0 2 instead of KMnC\ due

to the extended drying time required by the larger quantity

of material. The Mn0 2 impregnated silica gel was not a

satisfactory sorber for phosphine.

3 . 3 Phosphine Desorption from Charcoal

If phosphine is adsorbed physically onto charcoal as

maintained by Bastick (17, 18) and Zorin et al (20), then it

should be removed by aeration or by heating in a furnace if it

is unchanged as phosphine.

3.3.1 Aeration Experiment

Phosphine-containing charcoal was aerated to see whether

phosphine could be removed from the charcoal column onto a

silver nitrate impregnated silica gel indicator section. A

6 mm I.D. glass sampling tube containing a 10 mm indicator

section AgN0 3 on SiGel was filled with 130 mm of waterwashed charcoal (2.44 g) , and exposed to 4.5 ppm phosphine

in nitrogen at a flow of 0.7 liter/min for 25 min corresponding

to 17.5 liters of gas containing 110 ug of phosphorus. No

breakthrough was expected, nor was any observed. Room air

was substituted for phosphine, flowing in the same direction

at a rate of 1.4 liters/min. After 24 hrs the AgN0 3 /SiGel

indicator section was still white, demonstrating that the

phosphine was converted to a nonvolatile species; for if it

was still present as a physically adsorbed gas it would

surely have been flushed into the indicator section in 24 hrs.

3.3.2 Furnace Experiment

Phosphine at a concentration of 1.2 ppm (about 4 times

TLV) was sampled into a 7 mm I.D. x 130 mm long charcoal

tube at a flow rate of 1 liter/min for 15 min. The sampling

tube was transferred to a tube furnace where it was heated

at 300 °C for 22 min under a helium flow of 25 ml/min. The

helium was led into two bubblers in series, each containing

15 ml silver diethyldithiocarbamate (AgDDC) in pyridine.

The resulting solutions were measured spectrophotometr ically

in 2 cm cells at 465 nm against fresh AgDDC in pyridine.

For calibration, 1.2 ppm phosphine was flowed directly into

2 bubblers in series, at 1 liter/ min for 15 min. (An

AgN0 3 /SiGel indicating tube downstream of the second bubbler

signaled phosphine breakthrough as soon as the calibration

flow started, showing that the bubblers were not 100 percent

efficient) . Table 6 presents the results of the spectrophotometry measurements.

15
Table 6. Furnace Elution of Phosphine

Sample Absorbance*

Bubbler #1, 15 ml reagent containing

15 liters of 1.2 ppm phosphine 0.127

Bubbler #2, 15 ml reagent containing

overflow from Bubbler #1 0.110

Bubbler #1, 15ml reagent containing

evolved gas from 300 °C heating of

charcoal 0.009

it Measured at 465 nm in a 2 cm cell

In the absence of a complete calibration curve of

AgDDC absorbance vs phosphine standards, for these initial

experiments, one can still draw several rough (and unfavorable)

conclusions about the efficiency of furnace elution of

phosphine from charcoal, and the efficiency of midget bubblers

containing 15 ml AgDDC in pyridine for measuring phosphine

concentrations in the gases applied to the sampling tubes.

Assuming a linear response and taking the total absorbance

of bubblers #1 and #2 as representing the total phosphine

(even though the AgN0 3 /SiGel indicator showed that collection

was not 100 percent complete) , then the furnace eluted only

about 3.8 percent of the sorbed phosphine [i.e. 0.009/0.237].

The collection efficiency of the first bubbler of the calibration experiment was 54 percent [i.e. 0.127/0.237], much

less than the 75.1 percent efficiency claimed by Dechant et al

(2) with their tall form bubblers. It was concluded from these

tests that (1) midget bubblers containing only 15 ml of AgDDC in

pyridine were not efficient enough for use in measuring

phosphine gas concentrations, and (2) that phosphine sorbed

on charcoal was either changed to a nonvolatile species as

previously suspected, or else was decomposed at 300 °C into

P and H 2 which would not have reacted with AgDDC. Support for

the latter view is given by Mellow (33) who writes the equilibrium constant for the reaction

as K

p

=l x 10 atm at a temperature of 627 °K (354 °C) , where

K,-. is defined as

4 PH 3 (g)^=±P,t (g) + 6 H 2 (2)

i i

P

16
[P.]x[H 2 ]

6

3 . 4 Effect of Charcoal on AgDDC Color Reaction

Charcoal tended to decolorize solut ions of AgDDC in

pyridine by variable amounts depending on contact time. The

magnitude of this effect was demonstrated by adding charcoal

(from the furnace- treated sample tube) to 15 ml of phosphinecontaining AgDDC/pyridine from Bubbler #1 (absorbance = 0.127

at 465 nm) . After 5 min contact and filtration through Whatman

42 paper, an absorbance of 0.105 was measured, down about

21 percent. This effect, plus the consideration that phosphine

might no longer be present as phosphine in the sampling tubes,

led to the decision not to pursue the AgDDC method any further.

3 . 5 Phosphorus Oxyacids on Charcoal

Evidence in the literature that phosphine residues on

cereal grains consisted of the lower oxyacids of phosphorus,

e.g. hypophosphite H 3 P0 2 , phosphite H 3 P0 3 , and ortho -phosphate

HsPOtt (9,10) introduced the possibility that these compounds

might be present in the solid state sorbers, and be acid

extractable. Contact with air would oxidize these substances

to phosphate, so that a standard method could be used for

analysis. The Ascorbic Acid method, ASTM D-515 was chosen;

the modifications from the standard analytical procedure are

summarized in Appendix B.

It was discovered that water washed charcoal had a high

phosphate blank. This was to be expected because the charcoal

was of plant origin. Acid washing (Appendix C) reduced the

blanks to near zero, however the absorptivity of the acidwashed charcoal was reduced drastically (Table 2) . Since

the choice was between prohibitively high phosphate blanks

and poor absorptivity, the use of charcoal as a solid sorber

was discontinued.

3 . 6 Silver Nitrate Silica Gel Sorber

Although this was the best sorber for phosphine, difficulties were anticipated due to the possible interfence of silica

with the phosphate measurements, since the s il icomolybdate and

the phosphomolybdate complexes absorb at the same wavelength.

Considerable investigation into silica interference with

17
phosphate analysis has been documented (27-32), and some

investigators have concluded that only automated stoppedflow kinetic methods which take advantage of the different

rates of formation of silicomolybdate and phosphomolybdate

can remove silica interference completely. However, rapid

elution of sorber tubes with dilue acid (2 N H 2 S0O gave

satisfactorily low blanks because the silica did not elute

from the silica gel in the expected quantity. Thus, silver

nitrate impregnated silica gel could be analyzed for phosphate

content despite the expected silica interference.

3 . 7 Ascorbic Acid Procedure

Phosphates were eluted from AgN0 3 /SiGel tubes with

2 N HaSdt, the solutions neutralized with NaOH, acidified

slightly with one drop of acid, and determined by the Ascorbic

Acid Procedure, ASTM D 515 as summarized in Appendix B. The

silver interfered in two ways: (a) precipitating as the

solution became momentarily alkaline during neutralization,

and (b) precipitating slowly after addition of the ascorbic

acid molybdate combined reagent. The precipitation during

neutralization could be avoided by changing from a phenolphthalein end point to instrumental pH measurement, where

the solution would not fyave to become alkaline to determine

neutral pH. The precipitation during molybdate color development has not yet been satisfactorily solved. Work on the

reduction of silver interference ought to be continued, for

this approach appears to offer the most promising chance of

success. When the procedure for determining sorbed phosphorus

on AgN0 3 impregnated silica gel is brought under control,

the phosphine in the gas entering the sampling tubes may be

measured by the method of Bruce et al (6) , using saturated

bromine water to oxidize phosphine to phosphate, destroying

the oxidant by boiling it out of solution, then determining

the phosphate in solution by a standard method, viz. ASTM

D515, the Ascorbic Acid Method. The Bruce et al method has

the advantage that the bromine water is destroyed thermally,

an advantage over the use of oxalic acid to destroy KMnCU , qv.

4. RECOMMENDATIONS FOR FUTURE WORK

The proposed analytical procedure for determining phosphorus

after sorption on AgN0 3 /SiGel, is as follows: Two measurements are required for its evaluation, (1) determination of

phosphine applied to the solid state sorbers by passing the

gas through bubblers containing saturated bromine water to

oxidize phosphine to phosphate, then boiling to drive off

bromine, and determining phosphorus as the phosphomolybdate;

and (2) introducing phosphine gas mixtures into the AgN0 3 /SiGel

18
solid state sorbers at the same flow rates and sampling times

as the calibration runs above, and determining sorbed acidsoluble materials by eluting with 2.2 N H2SO4 neutralizing,

and determining phosphomolybdate spectrophotometrically by the

Ascorbic Acid Method, D-515.

The interference of silver with phosphate determination

as the phosphomolybdate may be controllable by a suitable choice

of neutralizing the dilute acid extract. If the interfering

silver can be complexed, and the AgN0 3 /SiGel sorber can be

analyzed for phosphorus content, then a sampling and analysis

procedure will be available.

The solubility of phosphorus oxyacids in organic solvents

for possible gas chromatographic determination has not been

investigated. Although pyridine extraction of phosphine

collected on charcoal was not successful for determining

phosphorus as the colored AgDDC complex, it is possible that

silated compounds may form volatile adducts with phosphites,

hypophosphites , phosphates, etc. to permit gas chromatographic

methods of analysis to be used. Such an instrumental approach

would have considerable advantage over the difficult colorimetric procedures now employed to determine phosphorus in

aqueous solutions.

5 . SUMMARY

The sorption of phosphine at ppm levels by physical

sorbers such as silica gel and charcoal, under non-equilibrium

conditions of active gas flow and in the presence of air and

moisture was considerably less than the air- and moisture-free

equilibrium values reported in the literature. Physical

adsorbers depending on porosity alone (e.g. silica gel, acidwashed charcoal) were not effective sorbers for 4.5 ppm

phosphine at flow rates up to 1 liter/min. Sorbers depending

on reactivity alone (e.g. KMnOi* , Na 2 Cr 2 07 , AgN0 3 ) also were

ineffective; only those providing both porosity and reactive

sites AgN0 3 impregnated silica gel, KMnOi* impregnated silica

gel, water-washed charcoal) retained phosphine in sufficient

quantity to be considered useful for solid sampling tubes for

work atmospheres.

Difficulties were encountered with various analytical

methods for determining phosphorus collected on solid sorbers.

At this writing, only AgN0 3 /SiGel appears to be capable of

being analyzed for sorbed phosphorus, but all the problems

have not been solved for this material, either.

19
Phosphine appears to be oxidized to a non-volatile,

pyridine-insoluble form on the solid sorber, so that it is

not removable by aeration, furnace heating, or elution with

pyridine; hence an instrumental method of gas analysis (such

as gas chromatography) does not appear to be usable for

sorbed phosphorus.

In summary, the following approaches toward developing a

solid sorber system for samping and determining phosphine in

work atmospheres were shown to be unsatisfactory:

1. Charcoal as a sorber, because of its high natural

phosphorus content.

2. AgDDC colorimetric analysis for phosphine, because

the sampled gas did not remain as phosphine.

3. KMnCU impregnated silica gel as a sorber, because it

was difficult to prepare in quantity, and because the

oxalic acid destruction of KMnCK interfered with the

colorimetric determination of phosphate.

4. Instrumental methods (e.g. gas chromatography) for

phosphine because phosphates eluted from the solid

sorbers in aqueous acids were not volatile.

20
6. REFERENCES

1. Greifer, B. , Cadoff, B.C., Wing, J., and Taylor, J.K.,

Development of Solid State Samplers for Work Atmospheres,

NBSIR 74-527, June 1974, prepared for the National Institute

for Occupational Safety and Health, Cincinnati, Ohio 45202

2. Dechant, R. , Sanders, G., and Graul, R. , Determination of

Phosphine in Air, Am. Ind. Hyg. Assoc. J., 27(1), 75 (1966).

(CA 64, 16520e)

3. Hughes, J. G. and Jones, A. T., The Estimation of Phosphine

in Air, Am. Ind. Hyg. Assoc. J., 2_4(2) , 164 (1963). (Air

Pollution Abstr. 23415).

4. Muthu, M. , Majumder, S. K. , and Parpia, H. A. B., Detector

for Phosphine at Permissible Levels in Air, J. Agr. Food

Chem. 21(2), 184 (1973). (Air Pollution Abstr. 29372).

5. Nelson, J. P. and Milun, A. J., Rapid Determination of

Phosphine in Air, Anal. Chem. 29, 1665-6 (1957).

6. Bruce, R. B., Robbins, A. J. , and Tuft, T. 0., Phosphine

Residues from Phostoxin Treated Grain, Jour. Agric. Food

Chem. , 10(1) , 18-21, (1962)

7. Dieterich, W. H. , Mayr, G. , Hild, K. , Sullivan, J. B., and

Murphy, J., Hydrogen Phosphide as a Fumigant for Foods,

Feeds, and Processed Food Products, Pesticide Reviews 19 , 135 (1967).

8. Berck, B., Sorption of Phosphine by Cereal Products, J. Agr.

Food Chem., 16(3), 419-425 (1968).

9. Robinson, J. R. , and Bond 32 E. J., The Toxic Action of

Phosphine - Studies with PH 3 ; Terminal Residues in

Biological Materials, J. Stored Prod. Res., b_> 133-146

(1970) .

10. Robinson, J. R. , Residues Containing Phosphorus Following

Phosphine Treatment: Measurement by Neutron Activation,

J. Stored Prod. Res., 8, 19-26 (1972).

11. Awasthi, M. D. , Misra, S. S., Verma,S., Handa, S. K. , and

Dewan, R. S., Phosphine Residues from "Celphos" Fumigated

Wheat Grains, Bull. Grain Technology, 9(1), 36-39 (1971).

12. Hilton, H. W. , and.Mee, J. M. , Studies with Radioactive

Phosphine- 3

P in Sugarcane, J. Agr. Food Chem., 20(2) , 334-336 (1972).

21
13. Tkachuk, R. , Phosphorus Residues in Wheat Due to

Phosphine Fumigation, Cereal Chem. £9(3), 258-267 (1972).

14. Vardell, H. H. , Cagle, A., and Cooper, E . , Fumigation

with Phosphine Against Stored-Product Insects in Bagged

Flour in Plywood Overpacks, Jour. Economic Entomology,

66(5), 1209-10 (1973).

15. Vardell, H. H. , Cagle, A., and Cooper, E., Phosphine

Residues on Soybeans Fumigated with Aluminum Phosphide,

Jour. Economic Entomology 66(3), 800-1 (1973).

16. Sato K. and Suwanai, M. , Adsorption of Hydrogen Phosphide

to Cereal Products, Appl. Ent. Zool. 9(3), 127-132 (1974).

17. Bastick, J. ; Sur les Chaleurs d'Adsorption de l'Ammoniac,

de l'Hydroglene Phosphor^, et de l'Hydrogene Arsenie" sur le

Charbon Actif, Comptes Rend. 230, 1163-5 (1950).

18. Bastick, J., Note de Laboratoire sur l'Adsorption de NH , PH3, AsH3 sur le Charbon Actif, Bull. Soc. Chim. (France),

(1952) , pp 169-171.

19. Bastick, J., Sur les Chaleurs d'Adsorption de NH 3 , PH 3 , AsH 3 sur le Gel de Silice, Compt. Rend. 234, 1279-81, (1952).

20. Zorin, A. D., Dudorov, V. Ya., Rogozhnikova, T. S. , and

Ryabenko, E. A., Adsorption of Arsine and Phosphine on

Silica Gel, Russian Jour. Phys . Chem. 44(3), 398-400 (1970),

English translation of Zh. Fiz. Khim. JT(3) , 717-9 (1970).

21. Dumas, T., Determination of Phosphine in Air by Gas

Chromatography, Jour. Agr. Food Chem. 12^(3), 257 (1964).

22. Dumas, T., Microdetermination of Phosphine in Air by Gas

Chromatography, Jour. Agr. Food Chem. 17_ (6) , 1164-5 (1969).

23. Berck, B., Westlake, W. E., and Gunther, F. A., Microdetermination of Phosphine by Gas Liquid Chromatography

with Microcoulometric , Thermionic, and Flame Photometric

Detection, Agr. and Food Chem. 18/(1), 143-7 (1970).

24. Bohl, C. D., and Kaelble, E. F., Determination of

Elemental Phosphorus in Air by Gas Chromatography, Jour.

Am. Industr. Hygiene Assoc., 3_4(7), 306-9 (1973).

25. Kaelble, E. F., Gas Chromatography Detectors for Phosphorus

and Its Compounds, in "Environmental Phosphorus Handbook,"

Griffith, E. W. , Beeton, A., Spencer, J. M. , Mitchell,

D. T., Eds., Wiley, N.Y. (1973), pp 351-353.

26. Dhaliwal, G. S., Recent Advances in the Methodology of

Phosphine Analysis, Bull. Grain Technol. 11 (3-4), 214-219

(1973)

22
27. Hilton, H. W. and Mee, J. M. L., Studies with Radioactive

Phosphine 32 P in Sugarcane, J. Agr. Food Chem. 20(2), 334-6

(1972)

28. Sensorex Phosphoric 5 Electrode S800P5, advertising

literature, Sensorex, 17502 Armstrong Ave., Irvine, CA 92705.

29. Muller, W. , A Specific Method for the Determination of PH 3 in Small Concentrations, Arch Hyg. Bakt. 129 , 286-292 (1943).

CA 38, 3216 (1944).

30. "Standard Methods for the Examination of Water and Wastewater,

13th Ed., A.P.H.A., Washington, D.C. (1971). Method 223-E,

Stannous Chloride Method for Phosphate, p 530-532.

31. Kingsley, G. R. and Schaffert, R. R. , Microdeterminat ion of

Arsenic and Its Application to Biological Material, Anal.

Chem. 23(6) , 914-919 (1951).

32. Chalmers, R. A. and Sinclair, A. G., Analytical Applications

of Beta-Heteropoly Acids, Part II: Influence of Complexing

Agents, Anal. Chim. Acta. 34, 412-418 (1966).

33. Mellor, J. W. , "Supplement to Mellor's Comprehensive Treatise

on Inorganic and Theoretical Chemistry, Vol. VIII, Supplement

III, Wiley-Interscience, New York (1972), pp 272-283.

34. Levine, H. , Rowe, J. J., and Grimaldi, F. S., Molybdenum

Blue Reaction and Determination of Phosphorus in Waters

Containing Arsenic, Silicon, and Germanium, Anal. Chem. 27(2) , 258-262 (1955).

35. Shen, C. Y. and Dyroff, D. R. , Determination of Phosphate

in Presence of Silicates by Molybdenum Blue Method, Anal.

Chem. 34(11), 1367-70 (1962).

36. Crouch, S. R. and Malmstadt, H. V., A Mechanistic Investigation of Molybdenum Blue Method for Determination of

Phosphate, Anal. Chem. 39(10), 1084-9 (1967).

37. Crouch, S. R. and Malmstadt, H. V., An Automatic Reaction

Rate Method for Determination of Phosphate, Anal. Chem. 39(10)

,

1090-1093, (1967).

38. Javier, A. C, Crouch, S. R. , and Malmstadt, H. V., An

Automated Fast-Reaction-Rate System for Quantitative

Phosphate Determinations in the Millisecond Range, Anal.

Chem. 41(2), 239-243 (1969).

39. Ingle, J. D., Jr. and Crouch, S. R. , Simultaneous Determination

of Silicate and Phosphate by an Automated Differential

Kinetic Procedure, Anal. Chem. 43(1), 7-10 (1971).

23
7. APPENDICES

7.1 Appendix A. Preparation of AgN0 3 - Impregnated Silica Gel

1 . Calculation of Required AgNQ 3 Concentration

PH 3 + 6AgN0 3 = Ag 3 P.3AgN0 3 + 3HN0 3

Ag 3 P.3AgN0 3 + 3H 2 0 = 6Ag° + 3HN0 3 + 3H 3 P0 3

Require 6 moles AgN0 3 for each mole P. To contain 200 yg P,

use at least

x = (1020) (200 yg)/(31)

= 6.6 mg AgN0 3

Sampling tube filled to 5 cm contains about 1.5 g silica gel.

Thus, 6.6 mg AgN0 3 /g silica gel would occupy 2/3 of the

sampling tube.

To prepare 200 g impregnated silica gel containing 6.6 mg/g

AgN0 3 , takes about 1.3 g AgN0 3 . Use 1.4 g to compensate for

losses.

2. Dissolve 1.4 g AgN0 3 in 300 ml H 2 0 and pour 200 g

silica gel (a) into it slowly, with constant stirring.

3. Stir vigorously for 10 min while air is being displaced from the silica gel, then once every 5 min for about

40 min.

4. Pour off excess solution (b) and dry for 4 h at 90 °C,

with occasional stirring of the mass of crystals.

5. Cool in a vacuum desiccator away from light (to protect

it from ambient air and moisture)

6. Sieve through a 16-mesh screen to remove fines caused

by decrepitation of the silica gel during drying, and store

in a foil-wrapped polyethylene bottle, (c)

(a) W. R. Grace silica gel, grade 05, 6-16 mesh, non- indicating

,

available from laboratory suppliers under various trade

names

24
Appendix A (contd)

Although some AgN0 3 is undoubtedly lost when excess

solution is poured off, this is preferable to attempting

to evaporate off the excess solution, with risk of

obtaining a non-uniform product.

KMnOi, - impregnated silica gel was prepared in the same

manner, using 0.8 g KMnOit in 50 ml H 2 0, adding silica gel

until the total volume was 100 ml (about 40 g silica gel).

25
7.2 Appendix B. Modification of ASTM D515-72, Standard

Methods of Test for Phosphorus in Water,

Method A (Colorimet ric Ascorbic Acid

Reduction Method)

Extraction Procedure

Suspend the solid sorption tube vertically over a 50 ml

Erlenmeyer flask. Add 20 ml of 2.2 N H 2 SCU dropwise (D515

reagent 11.6 diluted 1:4) to the top of the tube, allowing

the acid to drain into the flask. Neutralize to phenolphthalein with sat. NaOH (but NH 4 0H may be better if silver is

present) and immediately acidify with 1 drop of 2.2 N H 2 S0 4 .

Add 10 ml combined ascorbic acid reagent (D515 reagent 11.2),

dilute to 50 ml, wait 10-30 min, and read the absorbance at

880 nm using a 20 mm cell.

Calibrate against phosphate standards and blanks prepared in

the same manner (i.e. percolated through an unused solid

state sorber tube and eluting with acid as described)

These modifications to ASTM D515 are designed to extract

phosphate from the solid sorber material, and include the

effects of the sorber material in the calibration curves.

26
7.3 Appendix C. Charcoal Treatments

A. WATER WASHING TO REMOVE FINES

1. Soak activated charcoal* in water for 10 min. with stirring.

Pour off fines. Repeat 3-4 times, pouring off fines after

each soaking.

2. Pour off excess water, dry at 150 °C for 3 h.

3. Cool in vacuum desiccator.

B. ACID WASHING TO REMOVE PHOSPHORUS IMPURITY

1. About 80 g of charcoal* which had been dried at 150 °C for

3 h was rinsed twice in 2.2 N H 2 S0 4 , six times with H 2 0,

and left overnight in 2.2 N H 2 S0 4 .

2. Next day the charcoal was heated at 90 °C for 1 h in 200 ml

2.2 N HzSO^.

3. The acid was poured off, and the heating repeated twice

more, 1 h each time.

4. The charcoal was rinsed three times in 200 ml portions of

water.

5. The charcoal was boiled 1 h in water.

6. The charcoal was rinsed three times in room temp, water,

until the water from the charcoal was neutral to phenolphthalein.

7. The charcoal was dried for 3 h at 146-160 °C and cooled in

a vacuum desiccator.

8. Acid elution of this treated charcoal gave no detectable

phosphorus blank.

Pittsburgh Activated Carbon Div. , Calgon Corp., Pittsburgh, Pa.,

Type BPL, 20-50 mesh (coal-based vapor adsorption grade) was

used.

USCOMM-NBS-DC 27
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U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR RF.PORT NO.

NBSIR 75-977

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLF. AND SUBTITLE

Development of Solid State Samplers for Work

Atmospheres : Phosphine

5. Publication Oate

January 19 7 6

6. Performing Organization (-ode

7;-AUTHOR(S)

Bernard Greifer and John K. Taylor

8. Performing Organ. Report No.

9 PFR FORMING ORG AN 17 ATION NIAMF AND ADDRFSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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

National Institute for Occupational Safety 5 Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

13. Type of Report & Period

Covered

Summary Report

7/1974-6/1975

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

Investigations to find suitable solid-state sorber materials for

phosphine in work atmospheres at the ppm level are described. The

best sorber to date is silver nitrate- impregnated silica gel. Snme

difficulties remain to be overcome before a procedure for determining

the amount of phosphine adsorbed can be recommended.

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; industrial hygiene; phosphine; sorber; work atmosphere.

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