NBSIR 73-257

A System for Producing Test

Atmospheres Containing Hydrogen

Cyanide

Eugene P. Scheide, Ernest E. Hughes, and John K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

October 1973

Final Report

Prepared for Wational Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202
4

I'
NBSIR 73-257

A SYSTEM FOR PRODUCING TEST

ATMOSPHERES CONTAINING HYDROGEN

CYANIDE

Eugene P. Scheide, 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

or

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretarv NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

A System for Producing Test Atmospheres

Containing Hydrogen Cyanide

ABSTRACT

A system capable of producing well-defined test

atmospheres of HCN in air (or any other desired diluent)

and an analytical system for the analysis of these gas

mixtures is described. This system provides a means of

calibration of the various analytical systems for HCN

now in use. The analytical unit of the system can also

be used for the determination of hydrogen cyanide in

industrial atmospheres. By collecting the HCN in a sodium hydroxide solution, and measuring the cyanide

content by the use of a cyanide-ion selective electrode,

concentrations of HCN in air between 5 and 500 ppm were

measured. This method is rapid and convenient and can

easily be performed by a technician or adapted to

automation.

1. INTRODUCTION

This report describes a system capable of producing well

defined test atmospheres containing concentrations of hydrogen

cyanide for the calibration of analytical instruments and

evaluation of analytical methodology. The system developed

and described was designed to produce concentrations in the

industrial hygiene range of 5 to 500 ppm but other concentrations could be achieved by suitable adjustment of the operational parameters.

2. EXPERIMENTAL

2 . 1 Apparatus

I'his system produces mixtures of hydrogen cyanide in

air over a wide concentration range. It consists of two units;

a gas mixing unit and an analytical unit. The gas mixing

unit produces low concentrations of HCN by combining a relatively high concentration of HCN in nitrogen (the bulk

mixture) with a stream of clean, dry air (the diluent) . The

analytical unit consists of a system which directs the bulk

mixture through an impinger where the HCN is trapped and

subsequently determined by use of a cyanide ion selective

electrode. Reanalysis of the bulk mixture prior to use

serves to confirm that it has not deteriorated between analyses

or serves to assess the extent of deterioration.
The gas mixing unit and the analytical unit are combined

in one system. Figure 1 shows a diagram of the gas flow of

this system. The bulk mixture and the diluent air are mixed

in known proportions and are directed to the sampling manifold

from which a sample, or samples, may be withdrawn.

2.1.1 Gas Mixing Unit

The flow of diluent air is controlled by (V-1) and the

flow of the bulk mixture of HCN in nitrogen is controlled by

(V-2). The flow rates are measured by means of (FM-1) and

(FM-2), respectively. The differential flow controllers, (DFC-1)

and (DFC-2), maintain a constant flow in each leg of the system

after the initial setting of the flow control valves and will

maintain this constant flow until these valves are readjusted.

Valves 4 and 5 are shut-off valves and Isolate the unit when

itisnotinuse.

The "dryer" shown in Figure 1, contains both activated

charcoal and silica gel and is intended only as a final dryer

for the diluent air. The streams of diluent and bulk mixtures

combine in the mixing chamber and pass to the sampling manifold.

The concentration of HCN in the air in the manifold is calculated from the observed flow rates and from the measured

concentration of HCN in the bulk mixture. The concentration

of HCN in the bulk mixture or dilutions is determined using

the analytical unit.

2.1.2 Analytical Unit

A 3-way valve, (V-9), is located downstream from FM-2

in the bulk-mixture stream. In the "mixing" position the valve

directs the flow to the mixing chamber. In the "analysis"

position, the sample is directed to a 3-way solenoid valve (V-7).

In the normal position (V-7) vents the sample to the exhaust.

Wlien the solenoid is energized, the sample is directed to the

glass socket joint marked (A). A midget impinger containing

a measured volume of base is connected to this socket. A

timer is connected in parallel with the circuit which energizes

(V-7). When the switch which controls (V-7) is in the "on"

position, the bulk gas mixture flows through the impinger at the

flow rate indicated by (FM-2) and the timer operates. When the

collection is complete, the solenoid is de-energized by opening

the switch so that both the flow of bulk mixture through the

impinger and the timer are stopped. The time and the observed

flow rate allow a calculation of the total volume of gas which

passed through the impinger. This total volume is required

for calculation of the amount of HCN in the measured volume of

bulk mixture.

2
Samples may also be drawn at a controlled flow rate from

the sampling manifold. In this case, the midget impinger is

connected at (B) and (C) of Figure 1 and the vacuum system

draws the sample from the manifold as indicated. The flow

rate is controlled with (V-3) and measured with (FM-3). With

solenoid valve, (V-8}, in the normally open position, the flow

rate through this portion of the system is adjusted by drawing

air through the vent. When the solenoid is energized, the flow

is directed from the manifold at the preset rate, through the

impinger and then through the control system to the vacuum line.

In both of the above cases, the cyanide concentration is

measured using a cyanide-ion selective electrode and a digital

research pH meter.

2.1.3 GaseSy Gas Mixtures

A pressurized cylinder containing a bulk mixture of HCN

(99.5 percent) in nitrogen was prepared with a nominal HCN

concentration of 500 ppm. The cylinder was pressurized to a total pressure of 1200 psi corresponding to approximately 3600

liters of the 500 ppm mixture at ambient conditions. The source

of diluent air can be either a pressurized cylinder of "breathing"

air or "house" air from the bench. In either case the air is

passed through a silica gel-charcoal dryer to remove major

contaminants.

2.1.4 Calibration

The solution contained in the impinger consists of 25.0 ml^

(0. 025^1) of 0.1 M NaOH. The KCN standards in the range of lO"''

to 10 M were made using reagent grade KCN and stored in

polyethylene bottles. A 0.1 M NaOH solution was used as the

diluent instead of distilled water to simulate the impinger

solution.

2.1.5 Procedure

Using the system described in Figure 1, the sample of HCN

in air is bubbled through a solution of 0.1 M NaOH in the midget

impinger where the HCN quantitatively is trapped. The cyanide

ion concentration of the solution is determined by use of a cyanide ion selective electrode and comparison of the electrode

potential with a calibration curve. The concentration of HCN

in the sample is calculated by use of the following equation.

2.45 X lO'^ • M ' V

HCN V

3
whore,

^'hCN

^ ^^^^ concentration (ppm) .

M = CN concentration in the impinger solution.

\) ^ volume of liquid in the impinger (liters).

V = volume of gas sample (liters)

.

The concentration of HCN, C , in the dilute mixtures

produced when the bulk mixture is blended with air is calculated

using the following equation.

C

= ^HCN • ^BULK

^ ^HCN + ^AIR

where

,

= dilute HCN concentration (ppm).

3

^HcN ^ bulk ilCN flow (cm /min) .

^BULK ^ bulk MCN concentration (ppm).

F.-pp = air flow (cm^/min) .

The dilute mixtures of HCN can also be analyzed directly

using the same procedure as outlined above for the bulk mixture.

3. PERFORMANCE EVALUATION

3 . 1 Electrode Performance

The cyanide electrode exhibits a 58 mv change in potential

for each tenfold change in cyanide ion activity over the concentration range of 5x10"^ to lxlO"^M. Accordingly, a sufficient

volume of gas should be collected to produce cyanide concentrations in this range.

3. 2 liffect of pH

Hydrogen ion complexes cyanide ion to form the weak acid

HCN. The greater the hydrogen ion activity, the greater the

amount of cyanide ion which is complexed. Figure 2 shows the

4
fraction of free cyanide ion as a function of pH. At pH 8,

less than 10 percent of the cyanide is free; 50 percent is free . at pll 9.2; and at pH 12 virtually all the cyanide is in the

form CN . For this reason, the collection medium in the impinger

is 0.1 M NaOH.

3 . 5 (Collection Efficiency

In order to determine the collection efficiency of the

system, two impingers containing identical solutions were

connected in series and samples were collected. These solutions

were analyzed and it was found that more than 99 percent of the

flCN in the sample was absorbed in the first impinger at flow

rates up to 1 liter per minute.

3 . 4 Flow Rate

In order to determine the effect of flow rate upon

collection efficiency, samples of HCN in air of the same

concentration and same sample size were collected at various

flow rates. Measurements showed that all of the HCN was absorbed

at flow rates up to 1 liter per minute which is the practical

limit for the midget impingers used.

3 . 5 Dilutions

Dilutions of the HCN bulk mixture were made by mixing it

with clean, dry air as outlined earlier in the report. Figure 3

shows a plot of the observed concentration of HCN (as determined

by analysis of the dilute mixture) versus the calculated concentration of HCN (calculated from the bulk concentration and the

respective flow rates of the bulk stream and the diluent air

stream). A linear relation with a slope of 1.00 is expected.

The observed values were always lower than the calculated

values indicating a loss of HCN in the dilution system. Since

[R'N is a reactive gas, this loss could be due to reaction or

adsorption of the HCN within the dilution system. However, the

mechanism was not investigated. Since it is not possible to

reliably predict the concentration, it is necessary to analyze

the gas in the manifold each time a mixture is prepared with the

dilution system.

3 . 6 Stabil ity

Figure 4 shows the results of a series of analyses

performed on samples from the bulk mixture cylinder. After an

initial rapid decrease in concentration, the HCN mixture in the

cylinder appears to be fairly stable. For this reason, the

mixture should not be used until it is 1 week old.

5
4. CONCLUSIONS

The system described in this report is capable o£ producing

well-defined test atmospheres of HCN in air in the concentration

range of industrial hygiene interest. The system described for

verifying the composition of these mixtures is also useful for

direct analysis of industrial atmospheres.

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1. i I ; I A Systoiii for ProJiic j iig Test Atiiiosphc rc s Containing

Hydrogen Cyanide

7. M: I lli.'K E.P. Schcide, E. li . Hughes, and J.K. Taylor

9. IM-:Ul->'KMINv. e,x. ".\N :/.\ VU >\ NAMi! A.\p .\l ''iiiv !:>.< NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. :0234

10. l'ro|. t t i ask '.Vort: l\ui \...

11. Contrat t ('rant No.

12. Sr>oii<or;;:j Ti- _ r; / .! : : ,i \ ,':;.- ; .^Mirlc'.r '>lrcvt. Cilv. St. id- ZIP)

National Insritute for Occupational Safety and Health

Cincinnati, Ohio

13. I'ypf of Report l^rrio.-l C civcrc

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14. Spotisorins; Aj;(.ncv CoJi

15. SLFPLKMi:.\"TAKV NOThS

16. AP>1RAC! r (A 200-\.vord or Icxs tactual iu.Tr.Ti^ri- ol most sigr) i ticant intormation. If bib! loi^ruphy or litorjrjre .surve>-, muntion it ht rt-. j document includes a significant

3. K.'. I pu 111 s A> I t . I.

5. '. 'ul.l iv ai inn Pate October 1973

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8. I'l rlornuni; Ori;an. Ki.po.-i NBSIH 73 -"2 5 7

A system capable of producing well-defined test atmospheres of HCM in

air (or any other desired diluent) and an analytical system for the

analysis of these gas mixtures is described. This system provides a means of calibration of the various analytical systems for HCN now in

use. Tne analytical unit of the system can also be used for the deter- minationof hydrogen cyanide in industrial atmospheres. By collecting

the HCX in a sodium hydroxide solution, and measuring the cyanide

content by the use of a cyanide-ion selective electrode, concentrations

of HCN in air between 5 and dOO ppm were measured. This method is rapid

and convenient and can easily be performed by a technician or adapted' to automation

.

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

j

Gas blending system; hydrogen cyanide; ion selective electrode;

occupational safety.

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For Official Distribution. Do Not Release to NTIS

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Vl'ashin.;ton. D.C. 20 i02. Sli Cat. \' Tn

1 Order [• rorr, National Technical Information Service (.N'TIS) Sprin^^field, Vir.-.nia .2J!1'^1

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