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mschock

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  1. 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!
  2. 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/ba
  3. 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 inorga
  4. As an originator of the second of the diagrams (which was done before GWB, actually), I can point to some important strategies and concepts for generating similar diagrams. First, know what solids and aqueous species you want to include in the model, and then make sure they are in the thermodynamic data base called by Act2. It is quite possible that some of the solids and aqueous species are not contained in any or all of the default databases that come with GWB. In our research lab we have been building our own as we need it for different metals, drawing aqueous complexes and solid phases
  5. We are modeling the solubility and speciation of metals released from drinking water/plumbing interactions. We directly analyze for total inorganic carbon in the water by coulometric titration. We also analyze what would be termed "titration alkalinity" to the carbonic acid equivalence point by sulfuric acid titration. Drinking waters can have weak acid contributions from such anions as aqueous ammonia, hypochlorite ion, or various phosphoric acid anions, depending on the pH of the water. So it would not be proper to assume this titration alkalinity to be the system parameter carbonate a
  6. Greetings; I don't have experience with the plot overlay files, but I have a lot of experience with researching and doing plots for lead and copper solubility and mobility in potable water systems. The first two places I'd look at are that you need to assume a total lead(II) activity that's reaslistic for the diagram (log Pb++ = 0 in your .ac2 script) and also make sure the thermodynamic database has the right suite of aqueous and solid species in it for your system. We've had to make huge changes in the thermo data bases to get what we find are both thermodynamically and kinetically pla
  7. I have not run into this specific problem in my own modeling of copper with Act2, but with other software and when trying to do Pourbaix diagrams manually, I have run into similar problems when the Gibbs free energies used for the different species are taken from various different articles and compilations, and they not internally-consistent. The phase stability polygons then do not properly close.
  8. Thank you. That helps clarify what's going on. I guess I need to work through this in some detail and try to figure out exactly how to decouple and work with a high Eh, and also to learn better how to add appropriate species to the thermodynamic database. What is really confusing to me is the co-existence of identical species but having different different names and using different basis species. From your explanation, I still am not quite sure why this is necessary, and whether or not there is a potential conflict in the calculation. For the time being, I seem to get logical and acceptab
  9. Hi, Tom; Thank you. I figured out it might have something to do with the typo with the mass for vanadinite shortly after I posted the question, and your answer confirmed it. So that problem is solved. I have two remaining questions. First, for this current modeling exercise, I am very confused by the different ways the different thermodynamic databases handle the vanadium aqueous speciation, and how I should combine that with the lead. In this case, I have a system where the Eh is lower, and the likely coexisting valence states are Pb(II), V(V) and P(V)--orthophosphate. The
  10. Hi; I'm trying to develop an upper-bounds estimate for the solubility constant and/or Gibbs free energy of formation of vanadinite (as found in lead pipe corrosion scales), from some drinking water monitoring data and some [hopefully wise] estimations of dissolved lead and vanadium levels in places that can't be directly measured. My strategy is to use SpecE8 with known background water chemistry (which varies somewhat for a few constituents) to derive the activities of Pb+2, VO4-3 and Cl- for some expected ends of chemistry ranges, to get an estimated log Ksp. From that, I can use Act
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