pH measurement of ammonia solution
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The company uses ammonia water with a concentration of around 10%; yet when measured with a pH meter, the value is above 13, indicating it is a strong base. Under normal circumstances, ammonia water should be a weak base, and its basicity increases as the concentration rises. What could be causing this interference, according to those with experience in this field? Or perhaps using a pH meter itself is not a suitable method; then what method should be used to measure the pH value of ammonia water?
Personally, I believe that for ammonia solutions with a concentration of around 10%, it is not appropriate to use a pH meter to determine the concentration; instead, a simple acid-base titration method can be employed for quantitative analysis.
This post was last edited by qugd on 2022-9-30 09:11. This 10% ammonia solution is equivalent to approximately 6 M; based on the ionization of ammonia in water, the hydroxide concentration is about 0.015 M, so the pH value should be around 13, which isn’t a problem at all. It’s not possible to determine whether a base is strong or weak based solely on the name of the medium; there are also two important parameters to consider: concentration and degree of ionization. Of course, the basicity of ammonia water is roughly equivalent to that of 0.1 M sodium hydroxide, with a concentration difference of nearly a hundred times – this factor should also be taken into account. Returning to your original question, if you really want to measure the pH of ammonia solution, then you should use a pH meter. However, if you attempt to use the pH of ammonia solution to indicate its degree of basicity, that is a conceptual misunderstanding; concentration and degree of ionization are also parameters that need to be taken into account. If you aim to determine the concentration of ammonia solution using pH, then the testing method you have chosen is not appropriate. An accurate determination of the ammonia concentration requires precise volumetric analysis. Although there are changes in pH during this testing process, the use of pH to measure the concentration of acids or bases is only reliable for very dilute solutions of strong acids and strong bases (such as 0.001 M). It is completely unsuitable for weak acids and weak bases, as their degree of ionization is low and does not reach an ideal level; pH can only provide a rough indication of the partial ionization of weak acids or weak bases.
Thank you to the teacher at qugd for always patiently and thoroughly answering my questions, which has been of great help to me. Over these many years at Haichuan, you have served as a rock of stability in the analysis and testing section!
You’re too kind. It’s a matter of fate that we can all discuss technology here in Haichuan.
Strong bases and weak bases are not classified based on pH value, nor on metrics such as alkalinity; rather, they are classified according to their degree of ionization. It is generally believed that strong bases dissociate completely (in reality, on a microscopic level, it is only nearly complete), while weak bases dissociate partially. What the landlord wants to express is the degree of alkalinity of the solution, which can be indicated using pH or alkalinity. However, the alkalinity of a low-concentration strong alkali solution can very well be lower than that of a high-concentration weak alkali solution. If it is alkalinity, due to the existence of the ionization equilibrium, high-concentration weak bases exhibit higher values compared to pH
In this case, using an online refractometer yields better results than using pH or conductivity
This post was last edited by hach123456 on 2023-2-3 at 22:10. Personally, I think 10% ammonia can be measured directly using a pH electrode; we have used the pH electrode of a multi-parameter water quality analyzer to measure ammonia, and I believe the values obtained are reliable. As for the issue of strong bases versus weak bases, I understood it only after reading the answer from the teacher above; I was confused before too, but now I’ve learned it!
In fact, for 10% ammonia solution, if there are no other components besides water, its concentration can be accurately measured using methods such as density, refractive index, and conductivity; however, it is preferable not to use pH to represent its concentration. Cognitively, it may be easier to think of the pH value as a measure of intensity rather than as a quantity with capacity properties, which can help in understanding the relationship between the intensity and concentration of acids or bases.