Regarding the alcohol solution configuration of potassium hydroxide

When preparing the alcohol solution of potassium hydroxide, aldehyde-free ethanol should be used, that is, the ethanol needs to be re-distilled, because aldehydes will generate acetals or acetals under strong alkali conditions and produce color.

Several issues in the preparation of potassium hydroxide alcohol standard solution Zhu Shuai (Zhengzhou Paint Factory, Zhengzhou 450016, Henan) Abstract: When preparing the potassium hydroxide alcohol standard solution, it was found that the solution turned yellow and turned turbid. This article analyzes the main reasons for the turbidity and yellowing of the potassium hydroxide alcohol standard solution, and discusses the impact of the aqueous potassium hydroxide ethanol standard solution on the acid value determination. keywords: potassium hydroxide ； Standard solution ； ethanol ； Moisture content CLC classification number: rQ421.31 Document identification code: B article number: 1003-3467(2003)07-OO4O-01 According to GB6743-86 and HG2-569-77, the paint base and value are formulated. The correct preparation of the potassium hydroxide ethanol standard solution is the prerequisite for accurately determining the acid value of the paint. In the actual determination of the acid value of the paint, 0.1 mol/L potassium hydroxide acid value must be used. During work, a 0.1 mol/L potassium hydroxide alcohol (usually methanol or ethanol) standard solution is prepared. However, when a clear standard solution is placed with an ethanol standard solution, a small amount of water is often added first. After testing the potassium hydroxide for a period of time, it will gradually turn yellow, turbid, and sometimes crystallize. After the agent is dissolved, add ethanol. The effect of the amount of water added on the acid value is shown in Table 1. For this reason, this article conducted the B test to eliminate the turbidity and water content of the potassium hydroxide alcohol standard solution in accordance with GB6743-86. The results are shown in Table 1. The effect of alcohol standard solution on acid value was discussed. 1 Causes and solutions for the turbidity of the potassium hydroxide alcohol standard solution. The reason why the potassium hydroxide alcohol standard solution is turbid is that the potassium hydroxide reagent contains carbonate. When potassium hydroxide reagent is added to alcohol, potassium carbonate and sodium carbonate that are insoluble in alcohol are suspended in the alcohol, causing the solution to be turbid. The key to preventing solution turbidity is decarbonation. If you want to remove carbonate from the potassium hydroxide methanol standard solution, just weigh the potassium hydroxide reagent required to prepare the concentration, then add it to the methanol, shake well, and let it stand for about a week. When the solution is clear, filter and discard the precipitate and take the clear solution. However, the above method cannot be used to remove carbonates in the potassium hydroxide ethanol standard solution, because 95% ethanol has a high water content, and some carbonates will be dissolved in the water, making it difficult to separate. At this time, it is necessary to add 10% barium chloride solution (1L requires about 35mL barium chloride solution) to the potassium hydroxide ethanol standard solution, and let it sit for about two weeks. When the solution is clear, filter it, discard the precipitate, and take the clear liquid. 2. Reasons and solutions for the yellowing of the potassium hydroxide ethanol standard solution. The reasons for the yellowing of the potassium hydroxide ethanol standard solution are:: ①There are impurities such as iron, nickel and sulfate in potassium hydroxide reagent ； ②Ethanol reagent contains aldehydes and equivalent impurities ； ③ When these two reagents are mixed together, the iron, nickel and aldehyde react together to form colored substances, causing the solution to gradually turn yellow. In order to eliminate the influence of the purity of the reagent on the color of the standard solution, genuine reagents of analytical purity or above must be used when selecting reagents to eliminate the yellowing of the standard solution. 3 Effect of water content of potassium hydroxide ethanol standard solution on acid value. The concentration and volume of potassium hydroxide ethanol standard solution directly affect the acid value. Table 1 Effect of water addition amount on acid value of sample Note: The amount of water added refers to the amount of water added to 1L potassium hydroxide ethanol standard solution. The acid value unit is mgKOH/g. As can be seen from Table 1, the less water added to the potassium hydroxide ethanol standard solution, the smaller the impact on the acid value. The amount of water added per liter should not exceed 50 mL, because the more water added, the lower the measured acid value. This is because the paint base has poor water solubility and forms floc when exposed to water, surrounding some carboxyl groups. The wrapped carboxyl groups fail to participate in the reaction, which reduces the amount of standard solution, resulting in low acid value data. 4 Conclusion In summary, when preparing the 0.1 mol/L potassium hydroxide ethanol standard solution, genuine reagents of analytical grade or above should be used to remove carbonate. ； When preparing the standard solution, the less water is added, the better. The maximum amount of water added to 1L standard solution should not exceed 50mL.

The two people above explained the causes of turbidity and yellowing very clearly. Let me talk about the treatment: 1. Prepare a saturated solution of potassium hydroxide to reduce the carbonate. 2. Prepare aldehyde-free ethanol according to the preparation method of aldehyde-free ethanol. 3. After mixing, shake well, let it stand for more than a day, and use the supernatant. In addition, the concentration you prepared is too high. It is best to reduce the concentration by half. 4. To reduce errors. In addition to the above three points, you must also ensure that the potassium hydroxide is fully overdosed and the sample is completely saponified.

Thanks to the friends above, I also plan to distill ethanol again to see if the effect is better.