Determination of sulfuric acid in mixed solutions of sulfuric and nitric acid containing hydrofluoric acid

Abstract

A rapid and simple turbidimetric procedure of barium sulfate for the determination of sulfuric acid in waste acid solutions is described.
The solution was composed of sulfuric and nitric acid, and contained hydrofluoric acid and several colored metal ions; the conventional procedure could not be employed because of co-precipitation of barium fluoride with barium sulfate and adsorptions of metal ions on the precipitate. It was found that the following procedure was very effective to take away the interruption of the above ions.
(1) Saturated boric acid solution was added to the sample solution for the masking of hydrofluoric acid. Fluoride ion reacted with boric acid to form BF₄^- and did not co-precipitate with barium sulfate. When the concentration of hydrofluoric acid in the sample solution was higher than 3.5%, the masking with boric acid became less effective. However, since the hydrofluoric acid concentration in the waste acid solutions was lower than this, boric acid was found to be an effective masking agent for this purpose (See Fig. 8).
(2) The wavelength at 560 nm was used in the offered method. This wavelength was obtained from the absorption curves in Figs. 1 and 2. At this wavelength, the blank absorption was minimized although it was not zero. A blank solution containing no barium chloride, was used to eliminate the interference with the colored metal ions.
The recommended procedures are as follows.
(1) 10 ml of the waste acid solution containing less than 7 mg of sulfate ions was taken into a 1000 ml measuring flask and diluted to the mark with distilled water. 5 ml of the diluted solution was taken in two beakers A and B, respectively.
(2) 10 ml of saturated boric acid solution, 25 ml of water, 10 ml of a glycerine (1+1) solution, and 5 ml of 24% sodium chloride solution were added into the beakers A, and B.
(3) 0.8 g of barium chloride was added to the beaker A by stirring with a magnetic stirrer.
(4) The solution in the beakers A and B, were stirred for about 1 minute, lest standing for 4 minutes, and stirred again for 15 seconds.
(5) The amount of sulfate ion was determined from the absorption at 560 nm against the blank solution in the beaker B that has not been added barium chloride.
(6) The concentration of sulfuric acid (g/l) was calculated by the following equation;
H₂SO₄(g/l)
=SO₄^2-(mg)×98.1/96.1×1000/5×1000/10
The time required for the analysis was about 15 minutes for one sample and the results were accurate and reproducible as shown in Table I.