Calculation of dissolved total inorganic carbon

[EnvEng]

Homework Statement

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

Homework Equations

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??

 

[nate808]

Thank you for your post. it is important to have accurate and reliable methods for calculating parameters such as Total Inorganic Carbon (TIC). I can understand your confusion and frustration with getting two different answers from different methods.

In order to determine which method is correct, it would be helpful to have more information about your experiment. For example, what is the source of the headspace CO2? Is it from a chemical reaction or from a biological process? This could affect the accuracy of your calculations.

In general, it is important to consider all factors and assumptions when calculating TIC. The first method you described, using Henry's law, assumes that the CO2 is in equilibrium between the headspace and dissolved phases. However, this may not be the case if the source of CO2 is actively producing it. The second method takes into account the dissociation of CO2 into different forms, but it also relies on the assumption of equilibrium and may not be accurate if the reaction is not at equilibrium.

It would also be helpful to compare your results with literature values for TIC in similar experiments. This can give you an idea of the expected range of values and help you determine which method is more accurate.

In conclusion, it is important to carefully consider the assumptions and limitations of each method and to compare your results with literature values. If possible, it may also be helpful to consult with other scientists or experts in the field for their insights and recommendations. I hope this helps and good luck with your calculations.

 

[Blueshift5]

it is important to use the correct method for calculating dissolved total inorganic carbon (TIC) in order to accurately interpret your experimental results. Both methods that you have attempted have their own limitations and it is important to understand the underlying assumptions and factors that may affect the results.

In the first method, using Henry's law constant to estimate the dissolved CO2 concentration assumes that the gas is in equilibrium with the liquid phase. However, this may not be the case in your experiment as the pH of the solution is 7.3, which is not the optimal pH for CO2 dissolution. Additionally, this method does not take into account the other forms of inorganic carbon (H2CO3, HCO3-, and CO32-) that may be present in the solution.

The second method takes into account the equilbrium constants for the different forms of inorganic carbon and the partial pressure of CO2 in the headspace. However, this method also has its limitations as it assumes that the solution is in thermodynamic equilibrium, which may not be the case in your experiment. The equilibrium constants used in this method may also vary depending on the temperature and salinity of the solution.

To determine which method is more appropriate for your experiment, it would be helpful to compare your results with literature values or to perform additional experiments with known concentrations of TIC. This will help validate your chosen method and provide a better understanding of the factors that may affect your results.

In terms of ballpark figures, it is difficult to provide an exact range without knowing the specific conditions of your experiment. However, typical TIC concentrations in natural waters can range from 1-10 mM, and can vary significantly depending on factors such as temperature, pH, and salinity.

In conclusion, it is important to carefully consider the assumptions and limitations of different methods for calculating TIC and to validate your results through additional experiments or comparison with literature values. This will ensure the accuracy and reliability of your experimental data.