Titration Endpoint? - The Planted Tank Forum
 
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post #1 of 5 (permalink) Old 11-07-2006, 03:35 PM Thread Starter
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Titration Endpoint?

Ughh.....I'm struggling terribly trying to find the formula, or more information regarding titrating to the carbonic acid equivalence point in freshwater. So far all I can find is that it depends strongly on the ionic strength and alkalinity. Hach tests reportedly titrate to pH 5.1 for freshwater. Is there anyone out there doing there doing their own alkalinity titrations that could help...please!


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post #2 of 5 (permalink) Old 11-07-2006, 09:13 PM
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Well, I think you've confused me, and I'm a chemist . . .

Alkalinity is usually due to bicarbonate ions in freshwater - the deprotonated form of carbonic acid. To measure it, you titrate with a standard acid to about a pH of 4.5 (5.1 is close enough) at which point an indicator changes color (you could use a pH meter and graph the data if you wanted to be really accurate). The acid converts the bicarbonate to carbonic acid (CO2 + water).

The ionic strength can be safely ignored.

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post #3 of 5 (permalink) Old 11-07-2006, 11:36 PM Thread Starter
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Thanks for the information. I seem to be getting really good at confusing people lately. I think it is because I myself am confused.

It seems the confusion is where the endpoint is. If I titrate with HCL acid to an endpoint of 4.5 using methyl orange as an indicator, the alkalinity will be N meq/L. Correct? My confusion is, I'm not finding any information that gaurantees that all of the H2CO3- and HCO3- has been converted at that point, (or maybe 4.5 titrates past that point). The meq/L will of course be different at differing endpoints. Is there a way to calculate what the precise endpoint should be? Help is much appreciated!


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post #4 of 5 (permalink) Old 11-08-2006, 02:26 PM
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Well, the endpoint pH depends on the concentration of the bicarbonate . . . Without going into too much of the chemistry, carbonic acid is a weak acid, so the pH of a solution of carbonic acid (which is what you have at the equivalence point) depends on both the Ka (equilibrium constant) and the concentration of the acid. Here is a basic equation (assuming C is greater than about 0.0001M):

[H+] = [-Ka + SQRT((Ka^2) + 4(Ka)(C))]/2
pH = -log[H+]

Ka = equilibrium constant. For H2CO3, Ka=4.46*10^-7
C = concentration of H2CO3 (for this special case, this is CO2 + H2CO3).

If you put this into Excel and plug in values of C between 0.1M and 0.0001M you will find pH ranges from 3.7 to 5.2. Since aquarium water usually has a relatively low kH (<0.008M), the equivalence point should be between 4.2 and 5. Since the titration curve is steep at that point, there won't be much error in using an indicator that changes color at 5 or 4.5.

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post #5 of 5 (permalink) Old 11-08-2006, 04:10 PM Thread Starter
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Thanks very much for taking the time to explain this. Graphing this out, and not using an indicator but rather a pH meter, the endpoint ends up the same as the second point of inflection. Since, as you've said, the curve at the second point of inflection is pretty steep, the meq/L of acid needed to titrate to that endpoint differs very little. Wow, I think I'm getting it!


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