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Topic: How much water to add to get pH 5.5?  (Read 3918 times)

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Offline Rocky143

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How much water to add to get pH 5.5?
« on: October 08, 2015, 09:08:45 AM »
At work, I have to dilute 300 mL of 6 N HCl to a pH of 5.5.  How much water should I add?

Thanks.

(It has been a long time since I had chemistry.)

Offline Corribus

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Re: How much water to add to get pH 5.5?
« Reply #1 on: October 08, 2015, 09:49:15 AM »
Determining how to make a dilution starts with knowing what your initial and final concentrations are supposed to be. pH is just a short hand way to indicate free proton concentration. So, as a first step you need to first determine what your concentration of acid is before and after your dilution.
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Offline Borek

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Re: How much water to add to get pH 5.5?
« Reply #2 on: October 08, 2015, 09:52:07 AM »
It may look as a simple dilution - but in reality just dilution won't work. Either use a pH meter, or - better yet - think whether it really is a HCl solution that will do the job best. If what you need is a pH 5.5 solution I would suggest some buffer, not diluted acid.

Edit: Corribus was slightly faster.
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Offline Corribus

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Re: How much water to add to get pH 5.5?
« Reply #3 on: October 08, 2015, 10:04:50 AM »
Well, ultimately I agree with you Borek. Probably just easier to use a pH meter and slowly add water until you get to your desired pH.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Rocky143

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Re: How much water to add to get pH 5.5?
« Reply #4 on: October 13, 2015, 09:04:18 AM »
At work, I have to dilute 300 mL of 6 N HCl to a pH of 5.5.  How much water should I add?

Thanks.

(It has been a long time since I had chemistry.)

I work in a laboratory that tests limestone for use in building roads.  In some circumstances, the limestone must contain 10% quartz.  We test for this by dissolving the limestone in acid, then weighing the residue and estimating the amount of quartz in it.  This is not a general chemistry laboratory.  We only do specific tests and only have the equipment for those tests.  I do not have a pH meter.

According to ASTM Procedure D 3042, which must be followed, some naturally occurring carbonate minerals will not dissolve in acid at room temperature.  To dissolve these, the Procdure requires that, after dissolving as much as possible at room temperature, 300 mL of 6 N HCl must be added to the residue and that this must be heated to 95-deg C for 90 minutes. 

Since the amount of these low-solubility carbonates is small, I estimate that at the end of the 90 minutes the concentration of the acid is still 6 N.  I know that some amount of HCl gas is lost during heating.  I don't have a way of measuring this, so I assume the concentration after heating is 6 N.  This assumption will result in overestimating the amount of water to add and therefore result in a pH higher than 5.5.  This is an acceptable error.

After heating for 90 minutes, the acid must be decanted through a metal sieve.  The purpose of the sieve is to retain any +200 mesh (75 micron) material that goes out with the supernatent liquid.  Since this is a quantitative test, the sieve must be accurate, but concentrated acid will dissolve the metal from which the screen is made.  The Procedure directs to dilute the acid to pH 5.5 for the sake of reducing the corrosion of the sieve.  It does not tell how much water to add to achieve this.

The necessary water must only be determined once because the test must be performed identically according to the Procedure each time.  Once I know how much water to use, I will use that amount every time.

The man who worked this lab for 23 years died suddenly and I am filling in for him while we hire someone new.  No one else knows how he did it and we have found no written records.  I would appreciate it if someone who is familiar with these calculations would tell me how much water to use.  As I recall from my long-ago chemistry class, the calculation is a simple one, but I just do not know how to do it.

Thanks for your attention.

Offline Borek

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Re: How much water to add to get pH 5.5?
« Reply #5 on: October 13, 2015, 10:01:53 AM »
The only thing you are wrong about is that the simple dilution calculation will give a reasonable result. That's the problem with your situation - it doesn't.

I don't want to sound negative, but things you wrote just don't add up. To dilute 300 mL of 6 N HCl to pH 5.5 you would need about 570 m3 of water (that's the result of the simple calculation that you asked for, convert pH to the concentration of H+, then use C1V1=C2V2 to find out the final volume required). Several hundreds cubic meters of water is a pool, not a beaker.
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Offline Rocky143

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Re: How much water to add to get pH 5.5?
« Reply #6 on: October 13, 2015, 10:52:19 AM »
Thank you Borek.

I did not know there was more involved.

Offline Arkcon

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Re: How much water to add to get pH 5.5?
« Reply #7 on: October 13, 2015, 12:36:45 PM »
  The Procedure directs to dilute the acid to pH 5.5 for the sake of reducing the corrosion of the sieve.  It

This is the only part of your missive that I believe is pertinent. YMMV.  You have something of a solution, check it with a pH meter until it passes, or possibly, pH indicating paper.  Some of the acid has been consumed by the carbonate, it may already be close enough.

If you had a method, instead of just letting the oldest guy there do his thing, you'd know what to do.  Now you're just guessing.  Maybe the old guy never cared for damage to the sieve.  Maybe he secretly had a stack of them hidden in your lab and just claimed it was neutralized.  Borek: said, you can't get 6N HCl, a strong acid, down to pH 5.5 by dilution easily.

Go ahead and try it.  Use 1 ml of HCl and let us know what volume is needed.  If its 3000 ml, apply the factor to your volume, and see if its still feasible.  If its not, you have to admit, your written procedure was never followed.

Old people do this.  They know they're retiring soon, and just ignore rules.  If you're not planning on retiring soon, you still have to follow rules.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

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