Method by Cline

Cline s reagent No. 4 for sulphide concentrations between 250 and 1000 /xmol/L is used. Mixed diamine solution Dissolve 20 g of A,fV-dimethyl-p-phenylenediamine dihydrochloride and 30 g of iron(///) chloride, FeCls, in 500 mL of 6mol/L HCl (cool, a. g.). The reagent is stable for several months. 

Samples are drawn as described in Section 5.3.1.3, but 50 mL calibrated flasks are used instead of oxygen bottles. The rubber or plastic tubing of the sampler should touch the bottom of the flask, while the flask is filled with sample water allowing at least one flask volume to overflow. The tube then is withdrawn carefully and the flask filled to the mark. Note that the calibrated flasks must have space for at least 4 mL of reagent above the mark. 

A 4mL aliquot of the mixed reagent is added to the calibrated flasks with a piston pipette or a dispenser. The flasks are stoppered, turned upside down and shaken to mix the reagent with the sample. The samples are stored for at least 1 h before analysis. 

The samples are diluted 1 25 with distilled water no sooner than 1 h after addition of the reagent and are measured at 670 nm with the spectrophotometer using a 5 mm cuvette. 

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