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Attempting to make 4M KCL


Gregory Berardi:
I am not a chemist! I am trying to make 4M KCL for storing my pH probe. I took ~ 800 ml of RO water (not double distilled) and added 4M of KCL. The solution is supposed to have a pH of ~ 4, but it is ~ 8 pH.

I used 298.2 g of KCL and added the water until it was 1000ml. I heated the solution to 90°C on a hot stir plate and let it go for about an hour. I was adding water to keep it at the 1000ml level.

What am I doing wrong?

Why do you need to know the pH of the solution? Is your pH meter calibrated? Did you use the right compound, how pure? What water did you use? Tap water may be at a higher pH due to additives.

What was the purpose of heating it? Just to dissolve the solid? At what temperature did you take the pH?

Sorry just noticed you said you used RO water.

Technically the pH should be 7, but addition of salt can cause a lower reading.


--- Quote from: Gregory Berardi on May 30, 2022, 03:52:10 PM ---The solution is supposed to have a pH of ~ 4
--- End quote ---

No, that's not expected pH for KCl solution.

If prepared with distilled water and kept in an inert atmosphere, it should have pH close to neutral (not exactly neutral, K+ reacts with water acidifying the solution a bit, plus there are thermodynamic effects). If allowed to equilibrate with air (containing CO2) pH should go even further down down and stay somewhere between 5-6 (due to the creation of the carbonic acid), but it won't go lower.

No idea why the pH is higher - but there is always a chance water or salt are contaminated.

@OP, "M" is only for molar, not for moles, which uses a lowercase letter.


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