December 10, 2023, 05:30:29 AM
Forum Rules: Read This Before Posting

### Topic: Why is the pH of these two ammonia solutions different?  (Read 1357 times)

0 Members and 1 Guest are viewing this topic.

#### ikkjkhhgs

• Regular Member
• Posts: 9
• Mole Snacks: +0/-0
##### 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)

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?

« Last Edit: August 15, 2022, 11:45:29 AM by ikkjkhhgs »

#### Hunter2

• Sr. Member
• Posts: 2023
• Mole Snacks: +157/-45
• Gender:
• Vena Lausa moris pax drux bis totis
##### 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.

#### ikkjkhhgs

• Regular Member
• Posts: 9
• Mole Snacks: +0/-0
##### 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?

#### Hunter2

• Sr. Member
• Posts: 2023
• Mole Snacks: +157/-45
• Gender:
• Vena Lausa moris pax drux bis totis
##### 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%.

#### ikkjkhhgs

• Regular Member
• Posts: 9
• Mole Snacks: +0/-0
##### 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?

#### Hunter2

• Sr. Member
• Posts: 2023
• Mole Snacks: +157/-45
• Gender:
• Vena Lausa moris pax drux bis totis
##### 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.

#### rolnor

• Chemist
• Sr. Member
• Posts: 2139
• Mole Snacks: +145/-10
##### 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.