An analytical technique was developed to directly determine the concentration of aqueous sulphide in exotic scale studies. The technique is based on measuring the absorbance of sulphide samples in a copper reagent using an ultra-violet visible (UV-Vis) spectrophotometer. Calibration curves, generated using stock sodium sulphide solutions, were compared and validated against earlier reports in the literature.
The instantaneous reaction of a stirred (1000 rpm) sulphide solution and a copper reagent produces copper sulphide nano-particles, which turn the solution colour from light blue to brown. Increasing concentrations of sulphides give more intense shades of brown, resulting in increased absorbance values when analyzed by UV-VIS at a wavelength of λ = 480 nm. The absorbance readings are proportional to the sulphide concentrations in the samples, thus generating a calibration curve.
The repeatability of these absorbance measurements has an average standard deviation of 1.9% and a correlation coefficient (R2) of 0.9998 over a range of 7-386 mg/L sulphide species in synthetic brines. The determination of sulphide concentrations from absorbance readings using a 2nd order polynomial equation reveals average variations of ±1.2 and ±9.67 mg/L over the ranges of 7-70 and 105-386 mg/L, respectively. Furthermore, this technique was able to determine excess sulphide concentrations in solutions containing sulphide scale particles, as long as only clear filtered samples were analyzed. An alternative copper chloride reagent was used to prevent the formation of sulphate scale when barium or strontium ions were present in the brine being examined.
Comparing the results from the new detection method against the currently used technique reveals higher accuracy and practicality for the straightforward UV-VIS technique over the ICP analysis of quenched solutions. The new detection method allowed for the determination of any oxidative effects on the sulphide solutions, which is not identifiable using ICP.