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I'm trying to calculate Total Inorganic Carbon (TIC) for an experiment in which I have measured pH and Headspace CO2 concentration (%v/v). Approximate values are:

pH 7.3

Headspace CO2 2.5 %v/v

2. Relevant equations

3. The attempt at a solution

I've tried two approaches:

Firstly I used the CO2 concentration and Henry's law constant to estimate the dissolved CO2 concentration, and then combined this with the headspace concentration to get total concentration.

Secondly, I have tried a more elaborate method, which takes account of dissolution of CO2 in water to H2CO3, HCO3- and CO32-, and uses the partial pressure of CO2 in the heaspace, the equilbrium constants for each reaction and the pH to calculate TIC.

I get two completely differenet answers, and I have no idea which one is correct.

Method 1 gives dissolved concentration that is similar to the headspace concentration, whereas method 2 gives a dissolved concentration that is approximately 10 times greater than the headspace concentration. Any thoughts on which is the correct method, and what kind of ball park figures I should expect??

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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# Homework Help: Calculation of dissolved total inorganic carbon

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