Development and application of automated methods for determination of selected sulphur species in the environment
An automated Gas Phase Molecular Absorption Spectrometry (GPMAS) method for the determination of sulphide in solution has been developed. Sulphide ions react with 3M hydrochloric acid and the released hydrogen sulphide is swept into a gas-liquid separator by an air-stream. The absorbance was measured at 200 nm using a deuterium hollow cathode lamp. The method is relatively free from interference with a detection limit of 0.06 ug ml-1 of sulphide and relative standard deviations of 1.4-3.3% for repeated analyses. The calibration graph is linear up to 100 g ml-1 of sulphide and twenty samples can be analysed in one hour. The method has been applied to the determination of sulphate sulphur in plants. An automated procedure for determination of sulphite in aqueous solution by GPMAS was also developed. Using the optimised conditions obtained in this investigation, the method has a detection limit of 0.20 g ml-1 and relative standard deviations of 2.3% and 1.8% for 20 g ml-1 and 10 g ml-1 of sulphite respectively. The calibration graph is linear up to 120 g ml-1 and samples can be analysed at the rate of 20 per hour. The method has been applied to the determination of sulphur dioxide in synthetic samples and in white wines. The gas phase sampling introduction technique was also utilized in emission spectrometry. A sensitive and selective cool flame emission spectrometric method for the determination of sulphide in environmental solution samples was developed. The technique is based on passing the hydrogen sulphide liberated on acidification of samples with 3M hydrochloric acid into a cool, nitrogen-hydrogen diffusion flame and the resulting S2 molecular emission is measured at 384 nm. The method, which is automated, has a detection limit of 0.026 ng ml-1 for sulphide and relative standard deviations of 0.8- 2.5% for repeated analyses. Under the optimised conditions, the proposed method is applicable over the concentration range 0.1 - 8.0 g ml-1 of sulphide. Twenty samples can be analysed in 1 hour and the method has been applied to the determination of sulphide in waste water samples. The conversion of sulphite and sulphur dioxide [fixed as disulphitomercurate (II)] to hydrogen sulphide by reduction with sodium tetrahydroborate (III) has been used to provide a much improved sensitive, cool flame emission spectrometric method for the determination of sulphur dioxide and sulphite. The sample is mixed with NaBH4, acidified with 6M hydrochloric acid, and carried by a continuous-flow stream into a gas-liquid separator where the evolved hydrogen sulphide is swept by nitrogen into a cool, hydrogen- nitrogen-entrained air flame. The intensity of the blue S2 emission generated is measured at 384 nm. The proposed method has a detection limit for sulphite of 0.029 g ml-1 and relative standard deviations of 1.2 and 1.5% for 1 and 5 g ml-1 respectively. The calibration graph is linear up to 24 g m-l of sulphite and samples can be analysed at a rate of about 40 per hour. The method has been applied to the determination of sulphur dioxide in air and sulphite in wines. The proposed method, which is more sensitive than many earlier methods, is simple, versatile and capable of good precision. It is also rapid and convenient. Interferences are known but their effects may be minimized. The method is relatively free from nitrogen dioxide interference. Forty samples can be analysed in 1 hour and the results obtained by the proposed method correlate well with those obtained using an official method. The contribution of dissolved sulphate to total dissolved sulphur in a range of environmental samples was also studied. The results confirmed the importance of non-sulphate sulphur in a range of environmental samples. The importance of sample preparation in soil analysis was also demonstrated because of the effect of drying on sulphur constituents of soils.