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Author Topic: How do reduce the ionic strength without changing pH?  (Read 157 times)

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AussieKenDoll

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How do reduce the ionic strength without changing pH?
« on: March 15, 2019, 11:37:54 PM »

I have a acetic acid-sodium acetate buffer which has a 1.0 mol dm-3 ionic strength but i want to lower the ionic strength to 0.1 mol dm-3 without changing the pH of the buffer?
pH of the buffer=5
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AWK

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Re: How do reduce the ionic strength without changing pH?
« Reply #1 on: March 16, 2019, 12:33:10 AM »

A solution to this question can be sketched, but relatively precise calculations are rather difficult.
You must have a computer program that allows you to calculate the activity coefficients at ionic strength 1.0. Then you have to count the acetic acid concentrations in both buffers and decide whether to add some acid (easier situation) or to neutralize some of the acid with NaOH (a bit more difficult situation)
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Borek

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Re: How do reduce the ionic strength without changing pH?
« Reply #2 on: March 16, 2019, 05:13:35 AM »

While answer AWK gave is perfectly right, are you sure this question doesn't just check if you understand what the buffer is? What happens to pH when you dilute the buffer solution?
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AussieKenDoll

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Re: How do reduce the ionic strength without changing pH?
« Reply #3 on: March 17, 2019, 08:56:12 PM »

pH remains unchanged when the buffer is diluted slightly and at extreme dilutions, the pH of the buffer reaches 7!
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AWK

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Re: How do reduce the ionic strength without changing pH?
« Reply #4 on: March 17, 2019, 10:09:41 PM »

pH remains unchanged when the buffer is diluted slightly
This is a good approximation for relatively diluted solutions with low ionic strength only, but when you change ionic strength from 1.0 to 0.1 this rule may give even an unpredictable result.
Quote
the pH of the buffer reaches 7
This is not always true. A high dilution of some buffer solutions may result in exceeding the pH of 7.
Working with ionic strength ~0.1 M, a row error of 0.05 pH unit is obtained. Such an error is acceptable at the beginning of studying chemistry.
But using ionic strength 1.0 you are going up to advanced chemistry level.
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Borek

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Re: How do reduce the ionic strength without changing pH?
« Reply #5 on: March 17, 2019, 10:29:47 PM »

Yep. It is hard to tell what kind of answer is expected - OP seems to be at the very beginning of the GenChem course, yet the question is either advanced or ambiguous.
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