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I collect water quality samples used for irrigation. The samples are sent to a lab to measure $\ce{Ca^{2+}, Mg^{2+}, CO3^{2-}, HCO3^-}$, pH, and total dissolved solids. I need to use acid to prevent scaling on my water emitters, so I want to calculate, based on the results of each sample, how much HCl I need to add to lower the pH to 6 and to 4. I understand that the target pH gives the amount of H+ but how do I add to that the buffer capacity of alkalinity? Thanks

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    $\begingroup$ The pH value would be running due progressive escaping of CO2. To what point should the pH value refer ? Immediately after neutralization or after excess of CO2 escapes ( may take long time ). BTW, is not pH 4 too low for irrigation? $\endgroup$
    – Poutnik
    Commented Dec 4, 2022 at 13:33
  • $\begingroup$ Lets start with immediately after neutralization. When CO2 escapes the pH rises? The soil buffers the low pH as long as we don't spray it directly on the plants $\endgroup$
    – sdabach
    Commented Dec 4, 2022 at 13:36
  • $\begingroup$ Initial pH or CO2 content would help, as I do not suppose there would be much carbonates, as pKa of $\ce{HCO3^2-}$ is 10.32. $\endgroup$
    – Poutnik
    Commented Dec 4, 2022 at 13:56
  • $\begingroup$ But, nothing beats the operational tests on site and acting on it in the real time. Usual and cheap titration procedures to determine alkality usually use methyl orange ( 4.4 yellow -> 3.1 red ), heating the sample up to get rid of CO2, cooling down and then fine turn the acid volume, that temporarily raised up. For pH6, methyl red could be used ( 4.4 red - 6.2 yellow) or bromthymol blue ( 6.0 yellow - 7.6 blue ) // pH meter is advantage. $\endgroup$
    – Poutnik
    Commented Dec 4, 2022 at 14:04
  • $\begingroup$ For example: Ca - 101 mg/l, Mg - 37 mg/l, HCO3 - 205 mg/l, CO3 - 0, pH - 8.01, TDS - 754 mg/l. We usually rely on previous years analysis so not possible to do the titration test itself. $\endgroup$
    – sdabach
    Commented Dec 4, 2022 at 14:13

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