March, 19721 SENSORS FOR ATMOSPHERIC POLLUTION MONITORING 71

Electrochemical Sensors for Atmospheric Pollution Monitoring The following are summaries of the four main papers presented at a Meeting of the

Electroanalytical Group held on October 14th, 1971, and reported in the November issue of Proceedings (p. 227). Dr. I. Bergman (Safety in Mines Research Establishment) gave a brief introduction to the topic and discussed the problems involved in sampling.

Applications of the Dohrmann Microcoulometric Detector in Pollution Analysis BY F. C. A. KILLER

(Esso Research Centye, A bingdon, Berkslaire) DURING the last two or three years, microcoulometry has become widely accepted as a method for the determination of low concentrations of sulphur, chlorine and nitrogen in organic and inorganic media.1,2 I t compares favourbly with other analytical techniques and combines high speed and sensitivity with good accuracy and precision and relative freedom from interferences.

I t has been used to determine sulphur dioxide in combustion gases (with a coulometric iodine cell), and of hydrogen sulphide and thiols in hydrocarbon gases and air (with a silver

Adams, Bamesberger and Robertson4 developed a microcoulometric method for the consecutive determination of hydrogen sulphide, sulphur dioxide, methanethiol and dimethyl sulphide in ambient air. These compounds were sequentially separated with chemically impregnated filters and the compounds remaining in the air stream, emerging from each selective filter, were determined by direct titration with electrogenerated bromine in a cell, modified by Adams et al. by replacing the iodine reference electrode by a bromine electrode. The detection limits for these sulphur compounds in air were 5 to 25 parts per lo9.

Scaringelli and Rehme5 reported the determination of atmospheric concentrations of sulphuric acid aerosol by microcoulometry. In this method, the aerosols are collected by impaction or filtration, which separates the sulphuric acid from sulphur dioxide. The sulphuric acid is then decomposed isothermally to sulphur trioxide in a stream of nitrogen, the sulphur trioxide liberated is converted to sulphur dioxide over a bed of copper a t 500 "C, and the sulphur dioxide is determined microcoulometrically with electrogenerated iodine.

Ammonia in gases can be determined rapidly and conveniently by direct coulometric titration in a hydrogen cell. The sample is injected into a stream of hydrogen and swept through a calcium oxide scrubber at 450 "C to remove acidic gases and into the cell where the ammonia is titrated with electrogenerated hydrogen ions. The ammonia content is calculated directly from the peak area measured on the recorder.

Williams and Umstead6 determined trace amounts of chlorinated and brominated hydro- carbons in air by concentration on a gas-chromatographic column and detection of the con- taminants in the column effluent by microcoulometric determination of halogen. The sample components were collected on a Porapak Q column from relatively large (100 to 500 cm3) air samples at room temperature and subsequently eluted at elevated temperatures by tempera- ture programming. Mixtures of various compounds containing chlorine, bromine and/or

The method can be applied to problems related to air pollution.

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72 SENSORS FOR ATMOSPHERIC POLLUTION MOKITORING [Proc. SOC. Analyt. Chem. fluorine (including Freons) were determined quantitatively down to concentrations of 20 10 parts per lo9.

The major advantages of the microcoulometer as a gas-chromatographic detector are its linearity of response (it covers three orders of magnitude) and specificity. I ts major dis- advantages are a slow response (of the order of 2 to 5 s) and a relatively low sensitivity, which lies between that of a catharometer and a flame-ionisation detector. To obtain good separations, for example of low-boiling sulphur compounds, it is necessary to work with longer columns and lower initial column temperatures than with the use of the catharometer or the flame-ionisation detector.

REFERENCES 1. 2. 3. 4. 5. 6.

Killer, F. C. A., and Underhill, K. E., Analyst , 1970, 95, 505. Drushel, H. V., Prepr. Div. Petrol. Chern. Amer . Chem. SOC., 1969, 14, No. 3, B-232. Fredericks, E. M., and Harlow, G. A., Analyt . Chem., 1964, 36, 263. Adams, D. F., Bamesberger, W. L., and Robertson, T. J. , J . A i r Pollut. Control Ass . , 1968, 18, 145. Scaringelli, F. P., and Rehme, K. A., Analyt . Chem., 1969, 41, 707. Williams, F. W., and Umstead, M. E., Ibid., 1968, 40, 2232.

Measurement of Low Levels of Sulphur Dioxide by Using a Conductivity Detector BY T. NASH

THE sulphur dioxide in the air of central London has been continuously monitored at the Air Pollution Unit, St. Bartholomew’s Hospital, for several years by using a simple con- ductivity ce1l.l The cell has proved adequate for town air but would not be sensitive enough for country air where the sulphur dioxide concentration seldom exceeds 0.02 p.p.m. The two factors that prevent full exploitation of the potential sensitivity of the method are the temperature coefficient of conductivity for the hydrogen ion and the change in volume of the cell contents as a result of evaporation. These two factors can be counteracted by cooling the cell and solution to the dew point, and experiments on these lines have shown that it should be possible to obtain 10-minute readings at a sensitivity of ten times that of the original instrument.

REFERENCE

(Microbiological Research Establishment, Porton Dowjz, Wiltshire)

1. Nash, T., J . Scient. Instrum., 1961, 38, 480.

An Electrochemical Detector for Cyanide BY A. J. W. BROOK

(Ouality Assurance Directovate (Materials), Royal Arsenal, Woolwiclz, London, S.E. 18)

THE requirement for a portable, highly sensitive and robust continuous monitor for hydrogen cyanide in the atmosphere has been met by designing an electrochemical cell that consists of a silver anode and platinum cathode. The cathode is a perforated platinum tube through which the electrolyte (0.1 M sodium hydrogen carbonate solution) is pumped continuously. The outer surface of the tube is covered with a few turns of filter-paper, around which the silver wire anode is coiled. The concentration of hydrogen cyanide is proportional to the current produced by the ce1l.l The cell is suitable for use at a cyanide level of 0.01 pg 1-I.

Firstly, a pyro- lyser has been added to the air inlet so that some organic compounds of nitrogen can be detected.2 The organic compounds in the atmosphere were pyrolysed to cyanide, which was detected by the cell. Secondly, a simplified cell has been used to check that cyanide in a pharmaceutical chemical undergoing storage trials was below a certain level.3 This method replaced a long and tedious procedure involving ion exchange followed by colorimetry. Thirdly, a similar system is used to monitor the effluent from a chemical plant.

REFERENCES

Several modifications have been made to the system to extend its use.

1. 2. 3.

Baker, B. B., and Morrison, J. D., Analyt . Chem., 1955, 27, 1306. Thomas, C. O., and Baker, B. B., Ibid., 1959, 31, 1391. Brook, A. J. W., Moores, V. T., and Palmer, C. W., Chem. & Ind., 1968, 1275.

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March, 19721 PAPERS ACCEPTED FOR THE ANALYST 73

The Use of Brewer-type Bubblers for the Measurement of Ozone Profiles in the Atmosphere and the Total Oxidant Concentration at Ground Level

BY E. L. SIMMONS [The Meteorological Ofice, London Road, Bracknell, Berkshire, RG12 2SZ)

THE bubbler of Brewer and Milfordl has been widely used for vertical sounding, but variations in sensitivity having a standard deviation of 10 per cent.2 and even more have been detected.

At the Meteorological Office, experiments have been performed at surface pressure and at reduced pressure to understand and eliminate these variations. The following conclusions have been reached-

1. At surface pressure not all the ozone is absorbed, the loss depending on the bubbling pattern.

2. Reduced pressure changes both the pump efficiency and the bubbling pattern. 3. The effect of evaporation of the solution is minimised at the iodide concentration used. 4. There is no evidence that even prolonged sampling of the air a t Bracknell produces

any “poisioning” of the bubbler. 5. The greater part of the variation from sonde to sonde can be eliminated by comparing

the sondes with a standard bubbler before launch. The standard bubbler used is based on the design of De~jard ins ,~ which also formed the

basis of those used to sample the outside air. By immersing the bubbler in a l-litre flask of solution with which i t communicates through a small hole,4 the need for continuous replenish- ment of solution is eliminated. A trap to remove reducing agents is used, which is alternately inserted and removed from the sampling line. This allows the concentration of the reducing agents to be estimated. For this to work i t is necessary to displace the zero by electrolysing the solution to maintain a background level of iodine production.

On sunny days when there were signs of an atmospheric inversion, high surface ozone values were obtained, peak values in 1971 being as high as 0.07 p.p.m. by volume. Even higher values were obtained in 1970. These high oxidant values were always accompanied by a reducing agent level of typically 0.01 p.p.m. Ozone sondes launched on similar days both from Bracknell and from Larkhill on Salisbury Plain showed that the high values of the ozone mixing ratio were confined to the region below the inversion, therefore comfirming the impression that there exists an incipient photochemical smog of the Los Angeles type.

REFERENCES 1. 2.

3.

Brewer, A. W., and Milford, J. R., Proc. R. SOG., 1960, A265, 470. Mueller, J . I., “Ozonesonde, Bubbler Type,” Air Force Cambridge Research Laboratories, AFCRL-

68-0409, Bedford, Mass., U.S.A., 1968. Desjardins, R. I., “The Detection of the Ozone of Polluted and Unpolluted Atmospheres,” in

Proceedings of First Canadian Conference on Micrometeorology, Department of Transport of Canada, Meteorological Branch, Toronto, 1967, Part 11, pp. 439-451.

4. Sreedharan, C. R., and Tiwari, V. S., J. Physics, E, 1971, 4, 706.

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