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[Experience Sharing] The two days when I "struggled" with the phenolphthalein indicator


Phenolphthalein indicator is my "little partner" in my daily work. I am familiar with its habits and know that it changes from colorless to red at pH 8.2-10.0. It can also help me determine the titration end point of the test. Analysis Out the correct data. But not long ago, we had a little awkwardness.

On that day, the laboratory was going to carry out mixed alkali analysis, and the leader required that the results must be produced within two days. After checking the sample preparation steps, I found that it was only a very simple operation that required my "little partner" to indicate the end point. "So easy" I said to myself . Just say nothing, I immediately took the phenolphthalein indicator and HCl titration solution to the analysis room to carry out the experiment, but when approaching the end of the titration, my little friend was not as obedient as before, and it stayed in pink for a long time. It becomes colorless, but the procedure clearly states that it is titrated to colorless. At this point, the prelude to my struggle with it...


How to judge the accuracy of the experiment results when I check the information online immediately? According to the data, phenolphthalein indicator can be added to the NaHCO3 solution of the same concentration, and the color displayed at this time is the color of the end point of the titration. "Oh my god, the color turned out to be pink!" I deeply despised the person who wrote the question and couldn't help but secretly delight in my intelligence. I was really too smart! I arrived at the result according to my own method of judging the end point. This is the end of the matter? No, far from...

Although the results were obtained, the end color judgment was always different from the requirements, and my heart was always unreliable. In the afternoon, I accidentally saw the video of the mixed alkali titration when I was searching the Internet. I wanted to open it and watch it to make me feel at ease. But after reading it, my heart was ups and downs. The video also dripped to colorless and suddenly started to treat myself. No confidence.

In the end, I thought that the teachers in the university should be the clearest, surely not so, the first day passed in anxious waiting, the next morning the teacher finally sent a message "colorless". It was really going to collapse at that time. Is the titration wrong? ? But when verifying the accuracy of the results by preparing accurate concentrations of NaOH and Na2CO3 solutions, the calculation results when titrated to pink are accurate, so why are others' colorless?


In the end, I carefully checked the reagents required for the experiment and found that the experiment requires 0.1% phenolphthalein indicator, while I used 1.0% phenolphthalein indicator. Is this really the reason for the different endpoints? I immediately prepared a new indicator, and the end point of the titration was really colorless. At that time, I was so angry that I beat my chest and scolded the "little buddy" for being unreliable, but calmed down and thought about it. This was all the result of carelessness, although the result this time was luckily correct.

Through this incident, I deeply realized that basically all tests in the laboratory are conditional tests, which can only be established under specified conditions, and changing test conditions without authorization can only deviate from the correct route. In fact, success is not far away, it just covered a veil...


Laboratory Center Cui Yuhua