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I have a pH 10 NaOH solution. I need to bring down the pH to 9. So I used the $C_1V_1 = C_2V_2$ approach.

$C_1 = \mathrm{0.0001~mol/L}$

$V1$ is the necessary amount to produce 10 L of pH9

$C_2 = \mathrm{0.00001~mol/L}$

$V_2 = \mathrm{10~L}$

\begin{align} C_1V_1 &= C_2V_2\\ 0.0001~\mathrm{mol/L}\cdot V_1 &= 0.00001~\mathrm{mol/L}\cdot 10\mathrm{L}\\ V_1 &= \frac{(0.00001)(10)}{(0.0001)}\\ V_1 &= 1\mathrm{L}\\ \end{align}

I Applied this calculation in reality and used a pH probe and got a pH of 6.5. I think the drop is too sudden in this type of titration. What should I do?

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  • $\begingroup$ Double check the pH meter calibration. It is impossible to dilute NaOH and get an acidic solution. $\endgroup$
    – MaxW
    Commented Feb 3, 2016 at 22:13
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    $\begingroup$ And who says water is neutral? There's always some CO2 in it, unless it wouldn't have any contact with air. $\endgroup$
    – Mithoron
    Commented Feb 3, 2016 at 22:28
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    $\begingroup$ When you dilute anything down to homeopathic concentrations like 0.00001, expect all kinds of weird effects. Any impurity ($\ce{CO2}$ from air, like Mithoron said) could suddenly become more abundant than your compound. $\endgroup$
    – Ivan Neretin
    Commented Feb 4, 2016 at 5:49
  • $\begingroup$ Even with distilled water? What would be a new approach I should take? My problem is I need to make a pH10 solution which I need to creata pH9, pH8 and pH7 from the pH10 solution . $\endgroup$
    – user25260
    Commented Feb 4, 2016 at 13:08
  • $\begingroup$ Yes, even with distilled water. The distilled water condenses out of the air, and voila! Carbon dioxide! $\endgroup$
    – Oscar Lanzi
    Commented May 25, 2016 at 2:16

2 Answers 2

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Generally, you don't make pH of $7\pm2$ by dilution alone. The required dilution is so great that it would leave very little of your compound, and then any tiny impurity would affect the result greatly. Think of a droplet on the pH meter probe left from the previous measurement. Distilled water does not help much. All the same, it contains $\ce{CO2}$ from the air, so its pH is usually about 5.5, not 7 as one might expect.

When people want to have certain pH level more or less reliably, they use buffer solutions. Come to think of it, that's what nature itself does, because our blood is a buffer solution as well.

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the dilution increases the pH because the base will be completely sorrounded by water molecules.therefore more of the base will ionise Into constituent ions to produce fewer hydroxide ions than before dilution which increase the PH.

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  • $\begingroup$ A dilution should bring a pH towards seven. Adding distilled water to a basic solution should result in a decrease in pH. I think you are trying to use Le Chatelier's principle to explain that the bases should have a larger effect when the amount of water increases. While it is true that adding water will shift the base towards acting as a base, it will not do so quicker than the increase in concentration of water. $\endgroup$
    – Niels Kornerup
    Commented May 24, 2016 at 23:03

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