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Need to derive total carbonate with total alkalinity as the analytical input parameter


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We have a  situation with having a need to derive the total carbonate concentration in a closed system from laboratory analytical input parameters.  The measurement we often have is total alkalinity, as defined by strong acid titration to the carbonic acid equivalence point.  In drinking water context, there often are other dissociated weak acids, like phosphoric acid, hypochlorite ion, NH3, silicic acid, present in solution, which are titratable. We cannot analytically separate carbonate alkalinity from total alkalinity, in the absence of a direct coulometric or other analysis of total inorganic carbon in the water (which is only sometimes available).  With solution pH and the rest of the major constituent water analysis, for example, total concentrations of Ca, Na, Mg, SO4, PO4, SiO2, free chlorine residual (hypochlorous acid/hypochlorite ion), total NH3, etc. we can get ionic strength, ion pairing and the other weak acids that would be caught in the total alkalinity measurement.  This issue is described nicely in the documentation of the original USGS WATEQ series of computer programs, as well as in many other water chemistry texts.  Is it possible to implement this calculation in GWB, particularly in the GSS spreadsheet and SPECE8?  Assuming carbonate alkalinity = total alkalinity is very often a good approximation.  But in waters of low carbonate concentration (less than 5 mg/L as C), typical water treatment practice results in concentrations of other weak acids and bases that are significant relative to the carbonate alkalinity.

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Hello,

If you have the total alkalinity based on an acid titration, one way to calculate the total carbonate concentration is to use a reaction model to reproduce the titration. Set up your initial fluid with the component concentration from your lab analysis and include an estimated amount of dissolved carbonate in the fluid. Setup to titrate in the amount of acid used to reach the pH at the carbonic acid equivalence point. You can keep changing the initial concentration of carbonates in your fluid until the model reaches the endpoint pH after titrating in all the acid. An example of this is provided in chapter 15.1 of Biogeochemical and Geochemical Reaction Modeling text. You can find the input file for this example in the GWB -> Script -> GBRM -> Ch15_alkalinity.rea. 

If you have stored your samples in a GSS spreadsheet, you can use them as the starting fluid composition in React by going to Analysis -> Launch -> React -> select the sample. You would need to add in the carbonate component manually since it is not included on your spreadsheet. 

Hope this is helps,
Jia Wang

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  • 4 weeks later...

This is a clever way to approach it, but we wouldn't know the pH of the equivalence point exactly.  However, it would make sense that as an approximation you could figure out from the total sulfate, total phosphate (assuming orthophosphate), total silica, hypochlorous acid/hypochlorite (would have to add as weak base but it's problematic for redox because of frequent disequilibrium conditions), ammonia, etc. to approximately the range of the carbonic acid equivalence point.  This could be figured out in principle (the way the USGS programs used to do it) by subtracting the meq/L of the acid/base components neutralized from the total alkalinity observed?

Where is that script file example?  I haven't been able to find it yet.

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Hello,

If your lab analysis reported the pH at the end-point for the total alkalinity titration, you should use that value for the titration simulation. However, if you don't have that information, you might want to figure out if the difference between titrating to the endpoint of pH = 4.5 vs 4.8 is significant for the amount of total carbonates. If the titration ends when the change in pH is greatest, Gtplot can show the results in pH vs. acid added so you can match up your simulation end point to point  It might be good to check the carbonate concentration from your titration simulation against another method, such as charge balancing or conductivity calculation, to see if the values makes sense. If there are any samples that have a measured total inorganic carbon concentration, you can perform the same titration simulation for the fluid and check that value estimated against that of the measured TIC. 

The example file should be installed with your GWB installation in GWB (folder) -> Script (folder) -> GBRM (folder) -> Ch15_alkalinity.rea. The commands for setting up the system in React can also be found in the chapter 15.1 of the Geochemical and Biogeochemical Reaction textbook. 

Hope this helps,
Jia

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Got it, thanks.  I didn't think to look on the hard drive for the installation folder.  The lab reports total alkalinity as mg/L CaCO3, from which we can derive meq/L.  In fact, many titration systems don't even get close pH calibration, since technically you can get the result simply following the mV.  We have often  done exactly as you suggested,  computing DIC and comparing to a directly analyzed value, but when doing field studies we usually don't have the luxury of either totally complete analyses, or DIC.  Thanks for your help! 

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