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I made two similar ammonia solutions in different ways, and they resulted in different pH, but i'm not sure why.

Solution 1

Added 5.84 g NaCl to a 500 mL flask. Then added 10 mL 25% ammonia solution. Filled to 500 mL with distilled water to make a 500 mL 0.5% ammonia solution (0.267 M) with 0.2 M NaCl.

pH: 11.75

Assumed chemistry: NH3 + H2O + NaCl --> NH4 (but mostly NH3) + OH- (but mostly H2O) + Na + Cl-

Solution 2

Added 14.8504 g NH4Cl to 722 mL distilled water. Added 278 mL 1 M NaOH. This leaves me with a 1 L solution of 0.5% ammonia (0.278 M) and 0.278 M NaCl

pH: 10.01

Assumed chemistry:

~ NH4Cl solution~

NH4Cl + H2O --> NH4 + Cl (pH.5?)

NH4 + Cl + H2O --> NH3 + H3O+ + Cl (pH 5?)

~NaOH solution~

NaOH + H2O --> Na + OH- (pH 10)

~Adding NaOH to NH4Cl solution~

Na + OH- + NH3 + H3O+ + Cl --> NaCl + NH3 + H2O

Both solutions contain roughly similar the molar amount of ammonia (~0.2 M) and NaCl. Yet, the pH of the solutions greatly differ.

Is it that in solution 2, ammonia is existing mainly as NH4, or perhaps there are remaining hydronium ions?

Any advice would be appreciated.

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  • $\begingroup$ The solution 2 is essentially a buffer solution. $\endgroup$
    – Mathew Mahindaratne
    Commented Aug 15, 2022 at 15:45
  • $\begingroup$ @MathewMahindaratne Thanks Mathew! What part of it makes a buffer? I thought adding a strong acid and a strong base will produce salt and water (as above). But I suppose NH3 is a weak acid? $\endgroup$
    – ammonia_help
    Commented Aug 15, 2022 at 15:48
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    $\begingroup$ NH3 is not a weak acid. It is a weak base. And the final solution is not really a buffer solution, as the amount of ammonium ion $\ce{NH4^+}$ is equal to the amount of $\ce{OH-}$ ions. $\endgroup$
    – Maurice
    Commented Aug 15, 2022 at 16:53
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    $\begingroup$ The theoretical pH value should be $11.34$, if the concentrations are equal to the activities. So there is something odd in your second solution, where the pH was $10.01$. This pH is too low to be credible. $\endgroup$
    – Maurice
    Commented Aug 15, 2022 at 16:57
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    $\begingroup$ Your solutions 1 and 2 have the same final composition, if I read correctly your descriptions. This is why I don't understand the pH difference. There must be some unknown impurity, or some mistake somewhere... $\endgroup$
    – Maurice
    Commented Aug 15, 2022 at 19:20

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