NBSIR 73-260

A Gas Dilution System for

Arsine-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 73-260

A GAS DILUTION SYSTEM FOR

ARSINE-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

I

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

A Gas Dilution System for

Arsine-Air Mixtures

ABSTRACT

A gas -blending system originally designed for

chlorine-air mixtures was modified for producing

arsine-air mixtures in the concentration range from

0.02 to 0.25 ppm. This system has been tested in

order to provide accurately known concentrations of

arsine- in-air for calibration of analytical monitoring devices. An analytical method has been developed

for checking the concentration of arsine in the working standard and consists of the spectrophotometric

measurement of an arsine-diethyldithiocarbamate

complex in solution.

1. INTRODUCTION

The system originally designed for preparing chlorineair mixtures [1] has been adapted for producing arsine-air

mixtures in the range from 0.02 to 0.25 ppm. The system

combines a gas blending unit and an analytical unit. The

gas blending unit produces concentrations of arsine by dilution of a relatively high concentration of arsine in nitrogen

(i.e. the bulk mixture) with a stream of clean air (i.e.

the diluent). The concentration of arsine is measured by

absorption of a known volume in a fritted glass absorption

tube containing a 0.5 percent solution of the silver salt of

diethyldi thiocarbamate in pyridine (DDC) . The intensity of

the color produced is measured spectrophotometr ically at 540 nm.

2. EXPERIMENTAL

2.1 Description of Gas Dilution System

A schematic diagram of the system is shown in Figure 1
where the midget impingers of the chlorine system have been

replaced with a fritted glass absorption tube. This is an

important modification to ensure efficient collection of arsine

in the DDC solution. Photographs and additional diagrams of

the system are included in NBS Report NBSIR 73-292 and in the

operational manual,

2 . 2 Description of the Analytical System

The analysis of arsine is based on the formation of a coordination complex with the silver salt of diethyldithiocarbamate in pyridine (2,3). The arsine is collected quantitatively in 5-ml of a 0.5 percent solution of this salt in

pyridine. The intensity of the color formed is measured

spectrophotometr ically at 540 nm and the concentration is

obtained by comparison with a calibration curve. The sensitivity

was found to be adequate for measuring arsine concentrations in

the range from 0.02 to 2.0 ppm. As little as 1 to 2 liters of

gas sample is required at the 2.0 ppm level to produce a visible

red color change,

2 . 3 Preparation of a Calibration Curve via the Gutzeit Method

A known concentration of arsine was generated and collected

in the DDC reagent to prepare a calibration curve. The arsine

was generated via the Gutzeit method which depends on the

following reaction:

As"*"^ + 4 Zn + 5H"^ = AsH^ + 4 Zn^"^ +

The arsine is produced and absorbed quantitatively. Figure 2

is a schematic diagram of the arsine generating system.
2 , 4 Preparation of Calibration Curve

A solution o£ NBS SRM 83C AS2O2 was prepared by dissolving

0.1328 g of AS2O2 in 25-ml of 10 percent NaOH solution followed

by dilution to 1.0 liter in a volumetric flask. This solution

contained 25.1 yg As^*^/ml. A 1:10 dilution of this solution

was made to provide the stock solution for use in preparing the

calibration curve.

From 1 to 10-ml aliquots of the stock As^"^ solution con-

+ 3 taining 2.5 yg As /ml were added to a glass vial as shown in

Figure 2. The vial was capped immediately and the arsine was

allowed to bubble through a fine capillary tube for one hour

into 5-ml of a 0.5 percent solution of the DDC reagent in pyridine.

It was found that the arsine gas must be bubbled through

a fine capillary tube at a relatively slow rate to ensure

quantitative absorption in the DDC solution. The DDC-arsine

solution was then transferred to a 1-cm cuvette and the absorbance

was measured at 540 nm vs. the DDC reagent as a blank. A

typical calibration curve is shown in Figure 3.

2 . 5 Preparation and Analysis of Arsine Bulk Mixtures

The initial attempt to prepare an arsine bulk mixture consisted

of adding arsine gas at a partial pressure of 5-10 mm Hg to a cylinder and filling the cylinder with air to 1000 psi. Analysis

of the arsine concentration was performed within several days

after initial preparation. The results indicated that the^ arsine

concentration had decreased considerably after initial preparation. Another arsine- in-air mixture was prepared containing

3
3000 ppm arsine according to the respective partial pressures

o£ arsine and air added. Subsequent analysis after two weeks

indicated that the arsine concentration was reduced to less than

0.5 ppm. As a result of these measurements, it was concluded

that arsine-in-air mixtures in cylinders were not suitable as

working standards. The decrease in arsine concentration is

believed to be caused by the oxidation of AsH^ by O2 in the

added air as follows:

4AsH^ + 30^ = 4As + 6U^0,

2 • 6 Preparation of a Bulk Mixture of Arsine in Q

2 -Free Nitrogen

; Efforts were then directed to the preparation of a mixture

of arsine with 02-free nitrogen to provide a working standard.

The procedure consisted of first preparing a relatively high

concentration of arsine in 02-free nitrogen (< 5 ppm O2) . Arsine

was added at a partial pressure of approximately 15 mm and filled

to 200 psi with 02-free nitrogen. The mixture was allowed to

stand for 24 hours. Then the cylinder pressure was reduced to

50 psi and refilled to 1200 psi with 02-free nitrogen. Subsequent

analysis of this mixture yielded a result of 75 ppm. To prepare

arsine mixtures via previously described dynamic dilution systems

in the concentration range from 0.02 to 0,25 ppm, it was desirable

to prepare a bulk mixture having an arsine concentration of about

2.0 ppm. This was accomplished by successive volumetric dilution

of the 75 ppm arsine bulk mixture with 02-free nitrogen.

After each dilution the arsine concentration was analyzed
and the results tabulated in Table 1. The good agreement

between the calculated arsine values obtained by dilution and the

observed measured values indicates no detectable effect of added

oxygen- free nitrogen on arsTne,

Table 1

Dilution of Bulk Mixture from 75 to

2 ppm with 02-Free Nitrogen

• Cone, of Calc'd Cone, of Bulk Meas. Cone, of Bulk

Bulk Mixture Mixture after Dil. Mixture after Dil.

(arsmep-pin-nitrogen) ppm (mean, ppm)

75 35.0 34.1

35 6.9 7.8

6.9 4.2 4.4

4.2 2.0 2.0

3. RESULTS

3 . 1 Stability of Arsine- in-Nitrogen Bulk Mixtures

An important requirement of the bulk mixture is that

it remain stable over a reasonable length of time. For this

reason the arsine concentration was monitored over a period of

several months to determine any change in concentration. The

results shown in Figure 4 indicate a decrease in arsine concentration of 56 percent over a period of 4 1/2 months (i.e.

2.0 to 0.88 ppm). In view of the limited stability, it is

recommended that bulk mixtures of arsine-nitrogen be analyzed

each time they are used to prepare mixtures by the gas dilution

procedure.

3 . 2 Evaluation of the Performance of the Dilution System

To prepare arsine concentrations in the range from 0.02

to 0.25 ppm, the bulk mixture was diluted with air using the

5
previously described dynamic dilution system. The arsine

concentrations were monitored to determine any change in the

arsine concentration due to oxidation by the oxygen present

in the diluent air. The concentration of arsine at the sample

port outlet, C , was calculated as follows:

X

r = r bulk

^ bulk F^^^k * ^air

where

C = concentration o£ arsine.

X

= concentration of bulk mixture.

bulk

F, -,1 = flow rate of bulk mixture, bulk

V . = flow rate of diluent air. air

Calculated' values were compared with the measured values of

the concentration levels chosen. The results are tabulated

in Table 2 and plotted in Figure 5.

Table 2 Comparison of Calculated and Observed Arsine

Concentrations from Sampling Manifold

Cone, of Bulk Mixture 2.04 ± ,02 ppm

AsHj Air

flow rate flow rate Dilution Calc'd cone. Obs. conc

g./min £/min factor ppm ppm

0.053 0.718 0.07290 0.149 0.148

0.153 2.83 0.05129 0.105 0.103

0.053 2.83 0.01840 0.038 0.040

0.030 2.83 0.01049 0.021 0.025

6
4. CONCLUSION

It can be concluded from these results that an arsine-innitrogen bulk mixture can be diluted with air in the previously

described dynamic system with sufficient accuracy to produce

arsine concentrations in the sampling manifold in the range

from 0.02 to 0.25 ppm. The overall error in concentration will

depend upon the accuracy of the concentration of the bulk mixture

and the flow measurements.

7
5. REFERENCES

E. E. Hughes, W. D. Dorko, E. Scheide, J. K. Taylor,

Gas Generating Systems for the Evaluation of Gas

Detecting Devices, NBS Report NBSIR 73-292, October 1973.

0. P. Bhargava, J. F. Donavan, and W. G. Hines, Anal.

Chem. 44 , 2402- (1972) .

T. Kaneshige, M. Takizawa, and H. Nagai , Jap Anal. 13 , 780 - (1964)

.

8
e

<D

4-) W

COo

•H

-p3

H
c

as"

PLUS 1:1

HCL SOLN.

\

TYGON TUBING

MIDGET

IMPINGER

FINE BORE

CAPILLARY TUBE

ICP

0.5% DDC

REAGENT

2N METAL

Figure 2. Arsine Generation System


CALCULATED CONC..PPM

Figure 5. Performance of Arsine Dilution System

NBS-l 14A (REV. 7-73)

U.S. DfiPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. !'l M.K A VION OR RI' I'OR I' NO. NBSIR 73-260

2. Gov't Acccs.sion No.3. Kfc ipii-nt 's AcctsMon .\'o.

4. in i.i- AM) SI H i l l i.i: A Gas Dilution System for Arsine-Air Mixtures

5. Publ iL ation Date

6. Performing Organi/at icjn t.oJi7. AUTnoR(S)p.A. Pella, E.E. Hughes, and J.K. Taylor 8. Performing Organ. K< pi"'" ^•>J-

| NBSIR 73-260 | 9. PERFOKMING ORGANIZATION NA.ML: AND ADDRHSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Pro|ett/Ta.sk/Work I n;! No.

11. Contract/Grant No.

12. Sponsoring Organization Wimf and Compiete AJJrcss (Street, City, State, ZIP)

National Institute for Occupational

Safety and Health

Cincinnati, Ohio 45202

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPi.l-.MIiNTARY NOThS

16. APSIR.AC, I (A 200-word or lesx factual summary of most tii^iificant information. If document includes a significant \ bibliography or literature survey, mention it here.)

{ A gas-blending system originally designed for chlorine-air mixtures I was modified for producing arsine-air mixtures in the concentration ; range from 0.02 to 0.25 ppm. This system has been tested in order

to provide accurately kno\vn concentrations of arsine - in- air for

calibration of analytical monitoring devices. An analytical method has been developed for checking the concentration of arsine

in the working standard and consists of the spectrophotome trie

measurement of an arsine-diethyldithiocarbamate complex in solution.

——-, 17. KE\ UORDS (six to twelve entries; alphabetical order, capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Arsine; gas -blending ; gutzeit method; spectrophotometry.

18. AVAILABILITY ' Unlimited

[Z2 For Official Distribution. Do Not Release to NTIS

C2 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20i02. SD Cat. No. Cl'i Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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