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Topic: Why is the pH of these two ammonia solutions different?  (Read 1668 times)

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Offline ikkjkhhgs

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Why is the pH of these two ammonia solutions different?
« on: August 15, 2022, 11:22:39 AM »
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, and should result in a final composition of NaCl + NH3 + H2O. 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.
« Last Edit: August 15, 2022, 11:45:29 AM by ikkjkhhgs »

Offline Hunter2

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Re: Why is the pH of these two ammonia solutions different?
« Reply #1 on: August 15, 2022, 12:20:33 PM »
Ammonia/ ammonium chloride is a buffer. Think about that. NaCl is a neutral salt and dont change anything.Probably some inaccuracy in the make up.

Offline ikkjkhhgs

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Re: Why is the pH of these two ammonia solutions different?
« Reply #2 on: August 15, 2022, 12:28:14 PM »
Thank you! Yes I suppose I have essentially made a buffer. So as to why it's different - in solution 1, there was little opportunity for ammonia to be NH4, thus limited mixing of NH3 + NH4 to form a buffer solution.

In comparison, in solution 2, I directly mixed an acidic solution that was majority NH4 with a strong base, causing large amounts of NH3 to also form. This then equilibrated to a buffer-like solution?

Offline Hunter2

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Re: Why is the pH of these two ammonia solutions different?
« Reply #3 on: August 15, 2022, 12:31:16 PM »
Normaly both pH should be the same. I suppose  25% 10 ml NH3 solution contain not the expected  amount. Repeat with Standard solutions. Like 1 M NH3 freshly opened bottle or do a titration to confirm the molarity of 10 ml 25%.

Offline ikkjkhhgs

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Re: Why is the pH of these two ammonia solutions different?
« Reply #4 on: August 15, 2022, 01:14:17 PM »
Thank you! Yes I will have to repeat. Have I at least got the chemistry correct, and that theoretically both solutions should be a similar pH, and at the end have the same chemical composition?

Offline Hunter2

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Re: Why is the pH of these two ammonia solutions different?
« Reply #5 on: August 15, 2022, 01:49:13 PM »
Yes I checked the calculation. Its correct.
Problem as I said is 25% realy  25% .  Better is  to use a diluted solution to 1 M and confirm with titration first.The second is to do the right weight. It's sensitiv.

Offline rolnor

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Re: Why is the pH of these two ammonia solutions different?
« Reply #6 on: August 16, 2022, 12:02:35 AM »
NaOH is also partially Na2CO3 and this can vary from pellet to pellet.
If you have titrated exact 1M solution of your ingedienses you will get same pH.

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