III.6.1. Analysis by chemical treatment of water sample for detection with liquid scintillation counter

There are several procedures for sample treatment, one of which is described below. The flow chart of this analytical procedure is shown in Fig. 93.

(1) The samples (2 L) are delivered in plastic bottles and are weighed to determine their volumes. A known quantity of inactive I — (5 mg) is added to act as a carrier, as well as to ensure that the final precipitate is of sufficient mass (about 10 mg) to be reliably filtered and weighed. The samples are then filtered if inspection reveals any debris or cloudiness, and NaOH is added to make the samples slightly alkaline (~pH9).

(2) The I — is then oxidized to IO — with KMnO4 and allowed to stand for about 20 min. At the same time, any sulphide present (S2-, HS- or H2S, which would form an Ag2S precipitate in competition with Agl) is oxidized to SO4-. A longer time might be used if organic matter is present or if there is a high sulphide concentration.

(3) The IO — (including the carrier) is then reduced back to I — by addition of an acid mixture (HNO3 and HF) followed by Na2SO3 solution. The HF is included to inhibit formation of silica (Si(OH)x), which would clog filters and interfere with the weight of the final precipitate. The SO4- is unaffected by this step, thus effectively removing sulphide interference. After standing, the solution is filtered to remove any traces of Si(OH)x which might have formed.

(4) An excess of AgNO3 solution is added soon after the filtration to form a precipitate of AgI. Because AgI is much less soluble than AgCl, it is precipitated preferentially despite the approximately thousand-fold excess of chloride ions. However, small quantities of AgCl (and AgBr) are formed. After standing in the dark, the precipitate is filtered through cellulose acetate paper under vacuum. The AgCl and AgBr are then removed by washing with ammonia. The precipitate, now pure AgI, is then passed quickly through a further oxidation-reduction cycle for purification purposes before being dried and weighed.

(5) The precipitate is then dissolved in the liquid scintillation cocktail. This is done by inserting the rolled filter paper into the cocktail vial, adding about 20 mg of acidified thiourea and then immersing the vial in an ultrasonic bath to disperse the AgI into the cocktail. The AgI dissolves to form the silver complex Ag[SC(NH2)2]+. The paper is translucent and should be left in the vial (20 mL). The precipitates are dried and weighed.

(6) The analytical yield is calculated by dividing the mass of iodide in the AgI precipitate by the quantity of iodide added plus that known from prior analysis to be in the sample, typically 0.1-0.2 mg/L.