Titration testing for CO2 concentration

Oreo

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May 6, 2010
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There is a titration method for determining CO2 concentration in water. I'm curious to know if anyone thinks anything in typical aquarium water would interfere with this test. (Carbonate or other buffers, other acids?)

The full summary of method can be found here:
http://www.hach.com/fmmimghach?/CODE:1690008_24ED-210509|1

In a nut-shell:
Acidity due to carbon dioxide in a sample is titrated with sodium hydroxide to a phenolphthalein end point. Strong acids are assumed to be absent or of insignificant concentration [since] other acid components in the sample will be titrated and interfere directly in this determination
 

Biollante

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Jun 21, 2009
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Basically, You Got It!

Hi Oreo,

Only acids will affect the results. What you are measuring is carbonic acid. That is why an assumption, is that there are no strong acids present. :)

I suppose a sudden addition of carbonate hardness would change the result. Therefore, I would not test the water immediately after dosing anything. :p

I notice page 57 of your manual under “Interferences” covers this.

  • Other acid components in the sample will be titrated and interfere directly in this determination.
  • Sodium hydroxide standard solutions tend to lose strength slowly with age and should be checked periodically by titrating a known standard. Check the solution frequently (monthly) by titrating 50 mL of Potassium Acid Phthalate Standard Solution, 100 mg/L CO2, using Phenolphthalein Indicator Solution. The titration should require 5.00 mL of titrant. If the volume required for this titration is greater than 5.25 mL, discard the sodium hydroxide and replace it with a fresh supply.

Careful, accurate measurement and handling are especially important when dealing with low concentrations.

I would encourage you to run the “Accuracy Check, Standard Additions Method” on page 56. This provides a level of confidence. :cool:

Biollante
 

Oreo

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May 6, 2010
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I guess what I was wondering is how the alkalinity of the water will affect this test. If carbonic acid reacts with the sodium hydroxide titrant, what about the carbonic acid that may have already reacted with other bases / buffers in the water? (carbonate hardness, or those other things that also interfere with the regular pH testing method?)
 

Biollante

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Pretty Much Conclusive I Would Say

Oreo;54787 said:
I guess what I was wondering is how the alkalinity of the water will affect this test. If carbonic acid reacts with the sodium hydroxide titrant, what about the carbonic acid that may have already reacted with other bases / buffers in the water? (carbonate hardness, or those other things that also interfere with the regular pH testing method?)

Hi Oreo,

Assuming a state of equilibrium with the KH, the “buffering” is accounted for. Within aquarium ranges, I doubt significant problems. :)

According to Hach Water Analysis Handbook Procedures, Carbon Dioxide, DOC316.53.01167, for water and seawater, the only interferences are the presence of acids other than carbonic acid, color and turbidity.

Should color and/or turbidity be a problem, titrate to pH 8.3. (This is what I used to do in the days before I got all this newfangled equipment, as I never trusted my color judgments.)

A far greater problem is sampling. Same problem as drop checkers. I recommend robust sampling, many data points.

Volume and handling errors are more likely culprits. I have found process to be the bigger problem.

Just checked my Bottled Water Handbook, same story whether using Sodium Hydroxide or Buret Titration Method as well as Standard Methods for the Examination of Water and Wastewater, all agree on the point of interferences with CO2 tests. :cool:

Biollante
 

Oreo

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May 6, 2010
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I understand & agree about the same sampling / dead-zone issues as with drop checkers. I doubt that will be a problem for my purposes. I'm still working on that NDIR CO2 meter for GerryD and while not absolutely necessary it will be helpful to be able to corroborate the meter's output with another trusted measuring technique.

That and I just got this fancy new titrator on ebay and I'm noticing all these cool uses for it. For instance, I can now make up my own super-accurate 4dkh drop checker solution. (Not that it will matter if I get the NDIR meter working.)