Ferrous sulphate dosimetry by the thiocyanate method

Abstract

The use of ammonium thiocyanate to estimate ferric ions in the ferrous sulphate dosimeter has been investigated. The spontaneous oxidation rate of ferrous ions increased with increase in thiocyanate ion or ferrous ion concentration and with decrease in ferric ion concentrations. The spontaneous oxidation rate was too fast at the usual ferrous ion concentrations to permit accurate measurements at low doses. However it was slowed down to an acceptable level by using a dosimeter solution of 1 × 10 −4 M FeSO 4/1 × 10 −3 M NaCl/0.4 M H 2 SO 4 in which the G(Fe 3+) value was found to be 15.6 and independent of dose up to 5 krad. Two procedures involving different proportions of thiocyanate were used to form the ferric thiocyanate complex. Optical absorbance measurements were made at 477 nm. Details of the method are given for the estimation of doses in the range 200–2600 rad. Only 1 ml of dosimeter solution is required when 10 cm path length spectrophotometer micro-cells which are commercially available are used. A standard error of less than ±1.0% was obtained.