Cobalt(II) as a titrant in the direct potentiometric titration of micro-amounts of L-cysteine

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Abstract
A method for the determination of micro-amounts of cysteine by potentiometric titration in aqueous buffered solution with 5 mmol dm–3 of cobalt(II) is described. The end-point of the titration is detected by using a sulfide ion-selective electrode and a double-junction reference electrode. In the automated titration of 0.5–4 mg of cysteine, results accurate to within ±0.70% are obtained with appropriate standardization of the titrant; the results are reproducible to within ±0.60% and the relative standard deviation in the titration of 1 mg of cysteine is 0.28%. The use of cobalt(II) as a titrant has proved to be a valuable alternative to previously described methods. Cystine and methionine do not interfere.

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