sodium sulfate in saltwater
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Our method involves taking 5 ml of the sample and placing it in a conical flask, adding 50 ml of distilled water, acidifying it with 1–2 drops of hydrochloric acid, then adding exactly 15 ml of a mixture of barium chloride and magnesium chloride. The mixture is heated to a gentle boil on an electric stove for three minutes; after cooling to room temperature, 5 ml of triethanolamine, 10 ml of an ammonium chloride-ammonium hydroxide buffer solution, 3–5 drops of chromium black T indicator, and 10 ml of ethanol are added. The solution is then titrated using a 0.05 mol/L EDTA standard solution until a pure blue color is achieved; a blank test is also conducted simultaneously. The problem is that after it turns pure blue, the color reverses after a while. Why is that? How can this be fixed? Have you encountered this issue?
It might be a problem with the buffer or the Eriochrome Black T indicator; try using a freshly prepared one. I’ve encountered similar situations before, and the issue was resolved by changing the buffer or Eriochrome Black T
We took this issue into consideration, we made changes, but it still doesn’t work
I haven’t encountered it, but could it be that the Black T indicator was interfered with by triethanolamine or an ammonium chloride-ammonium hydroxide buffer solution?
I think it should be separated before titration. It is possible that the barium in barium sulfate was displaced by EDTA, thereby causing the color reversal phenomenon. It’s not necessarily correct; I suggest you give it a try.
Chromotropic blue T indicator is unstable; try using a 1% sodium chloride solid indicator