# Determination of HCl concentration in a solution ?



## Noxx (Jan 24, 2009)

Hi guys,

Is there a way to determine the concentration of H+ ions in concentrated HCl (except titration) ? I know there are some pH meters going down to -1 pH but they are $500+ I want to measure to concentration (pH) of HCl in concentration from 0.5M to 1.75M (pH 0.3 to -0.24)

Thanks in advance.


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## goldsilverpro (Jan 24, 2009)

Why don't you want to use titration? I would titrate a pipetted sample with a standardized 1N solution of NaOH from a buret, using an indicator such as bromphenol blue (pH=3.0-4.2, yellow to blue). 

What is the makeup of the solution you're analyzing? HCl only? Does it contain base metals or anything else that could react with the NaOH above?

Depending what else is in there, you may be able to titrate the chloride with .1N AgNO3 , using a sodium chromate indicator.

If it's only HCl + water, a very simple way would be to pipet out a sample, with a large pipet (I would use 25ml or bigger, if you have one), weigh it accurately (I think I remember you have a three decimal place gram scale), compute the specific gravity, and determine the weight % from a chart.

http://www.pioneercompanies.com/customers/tech/data/hcl/SG%20VS%20C.pdf

Then, calculate the molarity from the weight % on the chart. Check the bottom of this page.

http://www.madsci.org/posts/archives/1997-08/867968894.Ch.r.html


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## Noxx (Jan 24, 2009)

My solutions will contain HCl with HAuCl4 (dissolved gold). Exact amount of dissolved gold will often be unknown. And I don't want to titrate because I will certainly loose gold from titration.

P.S. Yes, my scale has 3 decimals and I have 10mL and 25mL pipets.

Thanks


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## goldsilverpro (Jan 24, 2009)

As long as you haven't thrown it away, you haven't lost it. You can always recover it, probably by just putting it back in solution by adding HCl.

You may not precipitate any gold using bromphenol blue since it changes at a fairly low pH (3.0 - 4.2). Of course, the HAuCl4 is also an acid.

Someone recently mentioned a patent (3856507) that raised the pH to 2 to 4 with NaOH and then dropped the gold with sulfite. Evidently, the gold doesn't drop, in that case, with the addition of the NaOH.

I just reread the patent and it says that a pH above 4 favors the production of gold oxide and hydroxide. If the bromphenol blue doesn't work, there are probably other indicators that change at a slightly lower pH.

I read somewhere that, with strong acids, pH meters read too high, due to the fact that the H and the Cl are not completely disassociated.


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## Lou (Jan 24, 2009)

Noxx, why do you want or need to do this? 


If you have only HCl in solution, it is a simple matter--Chris gave sage advice as usual! However, since you have gold in solution it will be more difficult to ascertain the actual concentration of HCl in solution. In this case, you can safely assume that this is what you have going on(if it is just gold and HCl):
HCl ----> H3O+ and Cl- K is large
AuCl3 + H3O+ + Cl- <---> HAuCl4 + H2O This K is also large.

If you know the starting concentration of your HCl, then you can calculate how much acid is used in dissolving the gold, depending on the redox reaction and then you'll know how much free acid is left. This means you know how much volume of solution, and how concentrated your acid was before you diluted it/reacted it. My advice is to keep track of how much metal you dissolved with a particular quantity of HCl, and also make sure you know what (if any) dilutions you performed.

pH meters do not measure [H+] in solution, nor do they measure [OH-] but rather measure oxidation and/or reduction potentials. Still, what Chris said is true--if the acid is too concentrated, then not much dissociates into ions and it is very difficult to get an accurate reading. This is territory that falls off into deep dark chemistry---most of the useful equations for solution chemistry do NOT accurately model very concentrated solutions. However, if you're working around 2M you ought to be OK. It's when it's 12M or 18M or whatever that things break down.

Chris, as far as that patent goes...it's a well-used and clever trick if you work with relatively pure gold solutions. In my opinion, I do not see it as being hugely useful over all. Gold oxide/hydroxide are easily decomposed--collecting them versus gold powder is insignificant if you plan to melt them.


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## Noxx (Jan 24, 2009)

Well, I was reading a patent and they are saying to adjust your gold bearing solution to 1.0N of HCl. Considering the fact that the solution is in higher concentration first, I was wondering how they perform this...

Thanks for your replies.


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## Lou (Jan 24, 2009)

Noxx said:


> Well, I was reading a patent and they are saying to adjust your gold bearing solution to 1.0N of HCl. Considering the fact that the solution is in higher concentration first, I was wondering how they perform this...
> 
> Thanks for your replies.




Dilution.


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## goldsilverpro (Jan 24, 2009)

Noxx,

I agree with Lou. If you're truly interested in this, keep track of what goes in and adjust accordingly. You're complicating the problem

I did learn something by this discussion. It is mathematically possible to have a negative pH. I've always known the equation but haven't thought of it in terms of very strong acids.


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## Lou (Jan 24, 2009)

goldsilverpro said:


> Noxx,
> 
> I agree with Lou. If you're truly interested in this, keep track of what goes in and adjust accordingly. You're complicating the problem
> 
> I did learn something by this discussion. It is mathematically possible to have a negative pH. I've always known the equation but haven't thought of it in terms of very strong acids.



Indeed yes it is possible to have a negative pH. H2SO4 is one such, its pH is something negative, like 4 or so; it's pKa is -3. Triflic acid is a very strong acid and is commonly used in organic reaction. It is related to fluorsulfonic acid (FSO3H) which is of comparable strength--both are about 1000 X stronger than sulfuric acid.


pH isn't really a useful term in my opinion as it is solvent dependent. HF is a good example--in water it is a weak acid that doesn't dissociate to any great extent. Put it in glacial acetic acid and it becomes a super acid. For exceedingly strong acids, the -log[H3O+] (the common definition for pH) is largely irrelevant. Instead, chemists use the _Hammett acidity function_ to describe extremely strong acids.

Ka and Pka are more useful!


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## goldsilverpro (Jan 24, 2009)

Very interesting stuff, Lou.

I can see that pH is only a mathematical construct and doesn't hold true on the low end. Sort of like Newtonian physics.

What about on the other end? Are there superbases?

Would any of the superacids have any application in precious metals refining.


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