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We have an effluent tank, with a consent to discharge at between 5.5 - 11 pH, the tank holds 100000 liters of dirty water.

When the pH reaches 10.5 we add citric acid to bring the pH down. How many liters of citric acid with a pH of 2.2 should we add to the system to bring the pH down to around 7. We always end up putting too much in then add caustic to bring it back up.

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Gotta start with my favorite formula: $$Q = CV$$

$$C_\textrm{eff}V_\textrm{eff}+C_\textrm{acid}V_\textrm{acid} = C_\textrm{out}V_\textrm{out}$$

$$(10^{-10.5}~\mathrm{mol/L})(10^5~\mathrm{L})+(10^{-2.2}~\mathrm{mol/L})V_\textrm{acid} = (10^{-7}~\mathrm{mol/L})(10^5+V_\textrm{acid})$$

$$V_\textrm{acid} = 1.51~\textrm{L}$$

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  • $\begingroup$ Well, we normally add somewhere in the region of 200 litres of acid, the pH then drops below our bottom end (5.5). So are you saying to add 1.58 litres of acid in total? $\endgroup$
    – Daniel Bridges
    Nov 16, 2015 at 19:27
  • $\begingroup$ Yes, it appears that the amount of acid required is extremely small compared to the total volume (I was honestly expecting something on the order of 100-300L, so it was surprising to get ~1.5L) $\endgroup$
    – costrom
    Nov 16, 2015 at 19:34
  • $\begingroup$ Does it make any difference that this is all being diluted with water? The tank gets filled from the drains within the factory which normally runs at about 6.4pH, the pH of the effluent tank only gets so high when we carry out internal cleaning with the caustic ph of about 14.4. 1.5litres seems incredibly small $\endgroup$
    – Daniel Bridges
    Nov 16, 2015 at 19:38
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    $\begingroup$ Well the "math" does lie in this case. How much citric acid to add depends on what else is in the dirty water. So getting a pH of 10.5 with NaOH is different than getting a pH of 10.5 with NH3. It really seems that you should do a quick test on a sample of the water. You wouldn't need super accuracy. It would seem that you could just count drops of citric acid solution. So amount of citric acid depends on what other "stuff" is in the dirty water, and how much. $\endgroup$
    – MaxW
    Nov 16, 2015 at 22:07
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    $\begingroup$ @Martin-マーチン The highest pKa of citric acid is about 6.4 (per Wik). For solutions with pH above about 7.5 to 8, citric acid will behave essentially as a (triprotic!) strong acid. Beyond that, at the target pH of 7, with a second-highest pka of ~4.8, it will still behave roughly as a diprotic strong acid, though with some buffering coming into play from that 6.4 pKa. $\endgroup$
    – hBy2Py
    Apr 2, 2016 at 12:25

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