Preparation of bromocresol green-methyl red mixed indicator
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According to GB603, the preparation method is as follows: Preparation method: 1. Solution I: Accurately weigh 0.1 g of bromocresol green, dissolve it in 95% ethanol, and dilute to 100 mL with 95% ethanol. 2. Solution II: Accurately weigh 0.2 g of methyl red, dissolve it in 95% ethanol, and dilute to 100 mL with 95% ethanol. 3. Take 30 mL of Solution I and 10 mL of Solution II, and mix them together. If I want to prepare 100 mL of this indicator, can I simply weigh 0.075 g of bromocresol green and 0.05 g of methyl red, dissolve them in ethanol, and then bring the volume up to 100 mL?
We use this indicator to detect the residual amount of sodium carbonate. We can directly weigh 0.075 g of bromocresol green and 0.05 g of methyl red, dissolve them in ethanol to make up a total volume of 100 mL; this method works well in practice. Since the indicator is not easily dissolved completely, care must be taken during preparation. However, even if the proportions are not entirely accurate, it should still be usable in practice? I’m not sure what others think about this issue; feel free to leave comments to share your views~
We directly weighed the two indicators and dissolved them together; since there were no issues in actual use, it was considered acceptable to do so. When preparing reagents, I always require the operators to follow the methods specified in the standards as much as possible. However, when using this mixed indicator, the two dissolution steps are relatively troublesome, so I personally think the procedure can be simplified. (Please ensure accurate weighing and complete dissolution during the operation.) ) But I’m worried that it might mislead others, so I’m posting this to ask if other users have different opinions.
The direct preparation mentioned by the poster is feasible, but the dissolution time may be longer; due to alcohol evaporation and adhesion to the walls during the dissolution process, the standard procedure does not allow this approach. The method specified in the standard procedure is to first prepare a stock solution of the exact concentration, and then carry out mixing and dilution. There are no absolute prohibitions regarding this type of physical mixing; the only differences lie in efficiency and potential accuracy, with the difference in accuracy being very difficult to detect.
Thank you, Teacher Q. Hearing you say that puts my mind at ease. Next time, if there’s time and those in the lab are interested, they can compare and check for any differences; if the differences are minimal and do not affect process control, that’s fine. Generally, the difference should not be significant.
The standard method requires rigor, whereas actual routine analytical testing is merely intended to provide reference conditions for process operations. Adapting and modifying certain methods according to actual circumstances is a way to improve efficiency. However, for quality control analysis, it is best to follow standard methods.
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Simply weigh 0.075 g of bromocresol green and 0.05 g of methyl red, dissolve them in ethanol, and then bring the volume to 100 mL. It’s simple and practical; that’s how I prepare it. This method also makes it easy to calibrate sulfuric acid