How to measure the concentration of waste sulfuric acid containing potassium sulfate and manganese sulfate

The acid-base titration method can be used to measure the concentration of waste sulfuric acid. First, a certain volume of waste sulfuric acid sample is taken and diluted with deionized water. An appropriate amount of indicator (such as phenolphthalein) is added during the titration process, after which standard sodium hydroxide solution is used for titration until the color change of the solution indicates the endpoint. The concentration of the waste sulfuric acid can be calculated by determining the volume of sodium hydroxide consumed. Be careful when conducting the experiment to avoid exposure to toxic sulfuric acid vapors or splashes. .

It can be titrated and analyzed using a sodium hydroxide standard solution with methyl orange or a mixed indicator.

An amount of waste acid equivalent to 0.2 mL of the original sample can be taken, an appropriate amount of water added, and the pH can be determined using a bromocresol green-methyl red mixed indicator with a 0.1 mol/L sodium hydroxide standard solution until the endpoint is reached. The method and amount of waste acid to be taken are determined based on the preliminary estimated concentration. I mentioned that phenolphthalein should not be used as an indicator because of the presence of manganese sulfate; before reaching the phenolphthalein endpoint, manganese hydroxide may form, leading to higher measured values. I’m not sure if it’s correct; I’m just responding based on intuition. Because I have previously conducted tests on sulfuric acid in copper electrolytes, using bromocresol green-methyl red as the indicator as well; this is different from simply titrating a strong acid with a strong base.

Titrate with a pH meter until the pH value reaches 7

Without referring to the manual, I still am not quite in agreement with setting the pH at 7; if the amount of manganese ions in the waste acid is high, it’s uncertain whether manganese hydroxide will form at a pH of 7. By setting the endpoint at slightly acidic conditions, even if a trace amount of manganese hydroxide is formed, it has little impact on determining the endpoint.

This post was last edited by qqgd on 2024-9-2 at 12:08. It is necessary to estimate the concentration of manganese ions; based on this approximate concentration, the initial pH value at which manganese hydroxide precipitation occurs can be calculated. Using this pH value as the endpoint for acid-base titration, it is possible to accurately determine the acidity level of this mixed system. However, the acidity level should not be too low, as low values will result in large errors. If the manganese ion concentration is below 0.01 M, setting the titration endpoint at pH 7 should be sufficient.

It’s been a long time since we last saw Teacher Q. I only recently realized that there seemed to be some issue with your account; I’m very happy to see you back on the forum! I agree with what Haiyou said before – that you are the true pillar of this forum. It is a privilege for the forum to have such selfless and guiding teachers.

This post was last edited by qqgd on 2024-11-6 08:49. You’re too kind; it’s possible that some inappropriate comments in the post were blocked, so I changed the username and restored the original permissions. I’m proud to be able to help everyone a little bit:lol

It is suggested to add hydrogen peroxide, heat it to oxidize manganese sulfate and increase the valence of manganese, and then proceed with sodium hydroxide titration – is this correct?

I feel that hydrogen peroxide is relatively difficult in oxidizing manganese ions to form divalent manganese. Even if it can be oxidized to divalent manganese to form Mn(SO4)2, wouldn’t this Mn(SO4)2 interfere with the titration? So I still think it’s sufficient to titrate until the pH reaches around 4; even if there is a trace amount of sulfuric acid that hasn’t been completely titrated at this pH level, the error will be only 0.0001 mol/L, which is sufficient to reflect the actual concentration of sulfuric acid in the waste acid.