NBSIR 73-254

A Mercury Vapor Generation and

Dilution System

E. P. Scheide. R. Alvarez. B. Greifer, E. E. Hughes, J. 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 73-254

A MERCURY VAPOR GENERATION AND

DILUTION SYSTEM

E. P. Scheide, R. Alvarez, B. Greifer, E. E. Hughes, J. K. Taylor

National Bureau of Standards

Departmen* 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

*"»r, u o«

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

A Mercury Vapor Generation and Dilution System

ABSTRACT

This report describes a system capable of producing

well-defined test atmospheres of mercury in air or other

diluent gas at concentrations between 0.005 and 0.5 ug/1

and an analytical system for the analysis of these gas

mixtures. Various parameters that affect the generator

and analytical system and their interactions are discussed.

This gas delivery system provides a means of calibration of

the various analytical systems for mercury now in use. The

analytical unit of the system can also be used for the

determination of mercury in industrial atmospheres by

collecting the mercury on a silver wool collector, and

then desorbing it by heat into a flameless atomic

absorption spectrometer.

1 . INTRODUCTION

This report describes a system capable of producing welldefined test atmospheres containing concentrations of mercury

for the calibration of analytical instruments and evaluation

of analytical methodology. The system developed and described

was designed to produce concentrations in the industrially

important range of 0.005 to 0.5 ug/1 but other concentrations

could be achieved by suitable adjustment of the operational

parameters

.

2 . EXPERIMENTAL

2 . 1 Apparatus

Mercury lends itself readily to the evaporation method

for generating test atmospheres. Basically, this entails

saturating the gas of interest with mercury and diluting the

subsequent test gas to achieve the concentrations desired.

A schematic of the mercury generation system is shown in

Figure 1. Two streams of air pass through silica gel/charcoal

driers and filters to remove impurities. One stream goes

through a differential flow controller (DFC-1) , a regulating

valve (V-2), and a high flow rotameter (FM-1), to the mixing

chamber. This stream is subsequently used as the dilution

air. The other stream flows through a differential flow

controller (DFC-2), an on-off valve (V-3) , a regulating

valve (V-4) , through a low flow rotameter (FM-2) and into the
saturation unit. The saturation unit is composed of two

parts; a heater- vapori zer and a condenser. The air stream,

upon passing through the heater- vapori zer unit, becomes

saturated with mercury at a relatively high temperature. It

then passes into a condenser unit where it cools rapidly and

any excess mercury collects on the mercury pool present in

the condenser. The temperature of the condenser determines

the concentration of the mercury-in-air stream. This stream

then passes into the mixing chamber where it is diluted with

the other air stream, and then into a manifold where samples

can be withdrawn. The condenser is connected to a constanttemperature water-circulating bath, containing anti-freeze

effective to -

0 C . The bath maintains temperatures between

0°C and 26°C to at least ±0.1°C.

2 . 2 Analysis

The method used for analysis of the mercury concentrations

was similar to that described by Long, Scott, and Thompson [1]

Analyses were made by withdrawing samples from the manifold

at 200 cc/min and collecting the mercury on silver wool. The

mercury was then desorbed by a controlled heating cycle and

measured using a flameless atomic absorption spectrometer.

The spectrometer was calibrated by injecting a known volume

of saturated mercury vapor onto the silver wool, using a "headspace" technique from standards kept at a very constant and

well known temperature. A block diagram of the mercury generation and analysis system is shown in Figure 2.

3. PERFORMANCE EVALUATION

3 . 1 Calibrat ion

As stated previously, the mercury generation system was

calibrated by injection of known volumes of saturated air.

A comparison of the atomic absorption signals given by these

standards and samples taken from the generation system is

shown in Figure 3. The non-linearity of the line is due to

the non-linearity of the atomic absorption instrument and not

to the generation system.

3 . 2 Sampling

When samples of mercury are absorbed onto the silver

wool collector and then heated to desorb the mercury for

analysis, recorder traces similar to those shown in Curve A

of Figure 4 are obtained. The peak shape, though not

symmetrical, is very reproducible so that the peak height can

be measured and compared with standards to determine the

amount of mercury in the sample.

2
An alternative to collecting the mercury samples on

silver wool and then heating to desorb them would be to

connect the atomic absorption instrument directly to a sampling port of the generation system and monitor the mercury

output continuously. Curve B of Figure 4 shows the type of

graph obtained. The sensitivity is not as great as in the

first method, but this is not serious, since a mercury concentration of 0.005 ug/1 will still give a recorder deflection.

However, the analyzer must be previously calibrated for this

mode of operation by some other technique.

3 . 3 Response Time

As indicated in Curve B of Figure 4, the response time

in producing a steady-state concentration of mercury vapor

upon going from one concentration to another is about 5 minutes at concentrations not too different from each other,

and about 10 minutes when large concentration changes are made.

The response time is about the same in going from high to

low and from low to high concentrations. It is recommended

that 10 minutes elapse after changing concentrations before

taking samples.

3 . 4 Temperature Effects

Initially, upon comparing the calculated and observed

mercury concentrations from the generation system, a difference

of about 7 percent was noticed, with the output higher than

that calculated. It was determined that the cause of this

discrepancy was a temperature difference between the water

bath-circulator and the condenser. To correct this problem,

the condenser and the water lines were insulated and a copper - constantan thermocouple was installed at the condenser in

order to accurately measure its temperature. The temperature

difference using the modified arrangement is about 0.3°C at

0.0°C, which will cause results that are 3 percent higher

without a temperature correction. If the temperature is

measured and corrected for, the calculated and observed values

are the same. When the condenser is operated at room temperature, its temperature may be considered that of the bath.

The optimum condenser temperature for the concentration

range of interest was determined to be 0°C for concentrations

between 0.005 and 0.200 ug/1 and 26°C for concentrations

between 0.100 and 0.500 ug/1.

3
Experiments were also conducted in order to determine

the effect of the heater saturator temperature upon the

generator output. The optimum temperature difference between

saturator and condenser is about 25°C. At higher temperature

differences, complete condensation in the condenser does not

occur and the output is higher than calculated, whereas, at

lower temperature differences, complete saturation is not

achieved and the output is lower than calculated. The

temperature difference should be controlled at 25°C ±5°C

for best results

It should be noted that a correction to the volume of

mercury saturated air must be made when there is a difference

between the temperature of the condenser and of room air.

3 . 5 Condenser Design

Figure 5 shows the final condenser design. The condenser

is imbedded in styrofoam and placed in an aluminum box which

is attached to the chassis. The water inlets and outlets are

connected to bulkheads at the rear of the chassis to which

the water bath-circulator is connected. The water lines

are insulated to minimize temperature changes of the flowing

stream. The mercury pool and stainless steel wool that are

placed inside the condenser are to facilitate the condensation

of mercury vapor.

4 . CONCLUSIONS

The system described in this report is capable of producing well-defined test atmospheres of mercury vapor in air

and an analytical system is described for the analysis of these

test mixtures and/or the industrial atmospheres under investigation directly. Figure 6 shows the relationship between the

observed and calculated values for the output of the mercury

generation system. As can be seen, the output is theoretical

within the accuracy of the analysis of ±2 percent.

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U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OK REPORT NO. NBSIR 73-254

2. Gov 't Accession

No.

3. Kec lpient 's Accession No.

4. I l l LE AND SUB 1TT1.E

A M^TrnTv A/m^nT flpTiPTaf i on ;p n rl Pi lirtion ^ at

- q "f~ m

5. Publication Date October 1973

6. Performing Organization Code

7. author(S)E . P . Scheide, R. Alvarez, B. Greifer,

E.E. Hughes, and J.K. Taylor

8. Performing Ortrin. Report No. NBSIR 73-254

9 PI' R FOR Ml N ( ; OT)C, AN 1/ AXIOM NAME AND ADHK l-'^S NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projcct/Task/W0 rk Unit No.

11. Contract/Grant No.

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

National Institute for Occupational Safety and

Health

Cincinnati, Ohio

13. lype of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

s This report describes a system capable of producing well-defined

test atmospheres of mercury in air or other diluent gas at concentrations between 0 . 005 and 0 . yg/1 and an analytical system for

the analysis of these gas mixtures. Various parameters that affect

the generate*** and analytical system and their interactions are

discussed. This/system provides a means of calibration of the

•> various analytical systems for mercury now in use. The analytical

unit of the system can also be used for the determination of mercury

in industrial atmospheres by collecting the mercury on a silver wool

collector, and then desorbing it by heat into a flameless atomic

absorption spectrometer.

17. KEY WORDS (six to twelve entries; alphabetical order; capital i ze only the first letter of the first key word unless a proper name; separated by semicolons) Atomic absorption; gas generation system; mercury; occupational safety.

18. AVAILABILITY \%_ Unlimited

f f or Official Distribution. Do Not Release to NTIS

[ Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CI 3

[" Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

14

20. SECURITY CLASS

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UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74