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Sodium thiosulphate solutions
Letters to the editor
Sodium thiosulphate
1065
solutions
SIR, IN a recent standard
textbook,’ the following statement appears, concerning the preparation of sodium thiosulphate solutions. “Solutions of sodium thiosulphate keep very well indeed if made up in carbon dioxide-free boiled demineralized or distilled water. On no account should carbonates be added to the solution to make it keep better as recommended by some authors. The addition of carbonates actually has the reverse effect.” We have habitually made an addition of 0.01% w/v of sodium carbonate in preparing thiosulphate solutions, and it has not been our experience that this is deleterious. For example, a solution prepared in distilled water2 in equilibrium with atmospheric carbon dioxide and containing 0.01% sodium carbonate, which was carefully standardised potentiometrically against potassium iodate and cross checked with potassium bromate, had a factor of 14077 i OGOO2on 6th June 1955. The solution was stored in a polythene-screw-capped brown glass Winchester. Rather more than seven years later this solution was independently restandardised, and had a factor of 1.0068 Z!Z04002 on 1st August 1962. The solution remained perfectly clear and free from sediment. E. BISHOP Washington Singer Laboratories The Universitv Exeter * Devon, England 11 August 1962
J. M. OTTAWAY
REFERENCES 1 C. L. Wilson and D. W. Wilson (Editors), Comprehensive Analytical Elsevier Publishing Co., Amsterdam, 1960. p. 253. B E. Bishop and J. R. B. Sutton, Analyt. Chim. Acta, 1960, 22, 590.
Chemistry,
Volume lB,
The oxygen flask method at low halogen levels Sir, THE use of the Oxygen Flask method for the determination
of halogens in organic substances has been encouraged in numerous publications1 since its re-introduction in 1955. It is noteworthy however, that no systematic study of the limitations of the method has been made. The writer wishes to stimulate critical consideration of the combustion conditions and the blank values in this method, especially with regard to its possible application at halogen levels below 1O-5 g equiv. The very nature of the combustion conditions engenders doubt about their efficiency; and experience of the method in routine use indicates that liquid, refractory, and high-halogen-content substances are not always completely combusted. Various expedients facilitate their correct analysis, but these detract from the value of the determination as a routine method. In particular, liquid substances have no universally satisfactory way of combustion, the probability of the escape of vapour before or during burning being high in all the published methods. The blank value from the substance wrapping material has hitherto either been ignored or been assumed to be small and constant. It is certain, however, that this value is not very reproducible, and is often a serious limiting factor of the accuracy of the method. This is particularly so at the halogen level mentioned above, where the blank value amounts at a minimum to approximately 5 % of this level (see Table). The magnitude and variability of the value are increased if finger stalls are not worn by the analyst. If a halogen-differentiating finish is coupled with the method,2 the blank value makes bromide and iodide analysis possible but chloride analysis impossible. The reagents used in the method contribute a constant blank of the order of 8 x 1O-s g equiv of Cl-, which is of little significance The avoidance of a wrapping material, as in carbon determinations,’ is not recommended on account of the poor combustion conditions, and reduction in the material size introduces serious handling problems.
Sodium thiosulphate
1065
solutions
SIR, IN a recent standard
textbook,’ the following statement appears, concerning the preparation of sodium thiosulphate solutions. “Solutions of sodium thiosulphate keep very well indeed if made up in carbon dioxide-free boiled demineralized or distilled water. On no account should carbonates be added to the solution to make it keep better as recommended by some authors. The addition of carbonates actually has the reverse effect.” We have habitually made an addition of 0.01% w/v of sodium carbonate in preparing thiosulphate solutions, and it has not been our experience that this is deleterious. For example, a solution prepared in distilled water2 in equilibrium with atmospheric carbon dioxide and containing 0.01% sodium carbonate, which was carefully standardised potentiometrically against potassium iodate and cross checked with potassium bromate, had a factor of 14077 i OGOO2on 6th June 1955. The solution was stored in a polythene-screw-capped brown glass Winchester. Rather more than seven years later this solution was independently restandardised, and had a factor of 1.0068 Z!Z04002 on 1st August 1962. The solution remained perfectly clear and free from sediment. E. BISHOP Washington Singer Laboratories The Universitv Exeter * Devon, England 11 August 1962
J. M. OTTAWAY
REFERENCES 1 C. L. Wilson and D. W. Wilson (Editors), Comprehensive Analytical Elsevier Publishing Co., Amsterdam, 1960. p. 253. B E. Bishop and J. R. B. Sutton, Analyt. Chim. Acta, 1960, 22, 590.
Chemistry,
Volume lB,
The oxygen flask method at low halogen levels Sir, THE use of the Oxygen Flask method for the determination
of halogens in organic substances has been encouraged in numerous publications1 since its re-introduction in 1955. It is noteworthy however, that no systematic study of the limitations of the method has been made. The writer wishes to stimulate critical consideration of the combustion conditions and the blank values in this method, especially with regard to its possible application at halogen levels below 1O-5 g equiv. The very nature of the combustion conditions engenders doubt about their efficiency; and experience of the method in routine use indicates that liquid, refractory, and high-halogen-content substances are not always completely combusted. Various expedients facilitate their correct analysis, but these detract from the value of the determination as a routine method. In particular, liquid substances have no universally satisfactory way of combustion, the probability of the escape of vapour before or during burning being high in all the published methods. The blank value from the substance wrapping material has hitherto either been ignored or been assumed to be small and constant. It is certain, however, that this value is not very reproducible, and is often a serious limiting factor of the accuracy of the method. This is particularly so at the halogen level mentioned above, where the blank value amounts at a minimum to approximately 5 % of this level (see Table). The magnitude and variability of the value are increased if finger stalls are not worn by the analyst. If a halogen-differentiating finish is coupled with the method,2 the blank value makes bromide and iodide analysis possible but chloride analysis impossible. The reagents used in the method contribute a constant blank of the order of 8 x 1O-s g equiv of Cl-, which is of little significance The avoidance of a wrapping material, as in carbon determinations,’ is not recommended on account of the poor combustion conditions, and reduction in the material size introduces serious handling problems.