USING INSTRUMENTAL TECHNIQUES TO INCREASE THE ACCURACY OF THE GRAVIMETRIC-DETERMINATION OF SULFATE

A gravimetric method for the determination of sulfate in a sulfate solution standard by the precipitation of barium sulfate is coupled with the instrumental determination of trace sulfate and precipitate contaminants to improve the accuracy and precision of the analysis. Sulfate in a solution of potassium sulfate is separated by a reverse precipitation with barium chloride in very dilute hydrochloric acid, Coulometry, ICP-MS, and flame atomic emission spectrometry (FAES) are used to quantify the level of the contamination in the barium sulfate precipitate and the solubility loss of sulfate in the filtrate, from which correction factors are calculated. Coprecipitating contaminants contribute about 0.3% to the total precipitate mass, while the analyte lost to the filtrate contributes 0.4%. Despite the poorer precision and accuracy of instrumental methods, the over-all precision and accuracy of the sulfate determination is actually improved, since the instrumental methods are used to determine only a very small part of the analyte. The expanded uncertainty (k = 2) of the method is below 0.2% relative to the precipitate mass.