When performing a titration of a monoprotic acid and a monoprotic base, water can not be added to the titrant but water can be added to the analyte. To prove this statement to myself, I decided to analyze the equation for a hypothetical titration where the acid is the analyte and the base is the titrant:

[Acid] = (Volume of Base) $\cdot$ [Base] / (Volume of Acid)

So, if water is added to the base in the burette, the volume of base would increase but the molarity of the base would decrease and the effects seem to cancel out, leading to the same [Base] calculation. However, when water is added to the acid in the flask, the volume of acid increases which would lead to a decrease in the [Acid] calculation.

This contradicts the common protocol and knowledge when adding water to the burette or flask. Many answers I have seen state that the moles of acid and base stay constant if water is added to the analyte; however, wouldn't this also increase the volume of analyte and skew the calculation of [Acid]?

It would be awesome if someone could explain the effects of adding water to the burette and analyte on the equation above.

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    $\begingroup$ Your title don't match the body. Avoid asking multiple questions once $\endgroup$ – Zenix May 13 at 23:59
  • $\begingroup$ RE: "water can be added to the analyte" - Yes and no. Yes small quantities can be usually be added without a problem. However if you wanted to add a million liters of water to the analyte solution then that would be a problem. // Let's say that you need to add 500 ml of water to get the solid analyte to dissolve. What you'd do is run a blank. No analyte, just 500 ml of water. The blank value would be how much titrant it takes to reach the titration end point with just 500 ml of water. That volume would be subtracted from all the analyte titration volumes. $\endgroup$ – MaxW May 14 at 0:52

Your title and the main body of the post are two different questions. Anyway, it is important to understand the difference in each case.

Let us talk about the analyte. Your analyte can come in two forms: (i) either as a solid or (ii) as a solution.

In the case of a solid analyte you would quantitatively transfer a known exact weight of the sample to a flask, let us say an Erlenmeyer flask. You can now add some water to dissolve it and you do not need to worry because you have recorded the actual weight of the sample. [yes the conc. will change, depending on how much water you are adding].

In the case of a analyte solution you would quantitatively transfer a known volume with the help of a pipette to an Erlenmeyer flask. You can still add some water and need not worry because you know the initial volume. [again the conc. will change with the addition of water].

*What is important in titration calculations are absolute moles of the analyte not its total volume in which it is being titrated*.

Now why can't we add water to a buret? The reason is that by adding water to buret, it will change the concentration of the titrant. As I said earlier, only moles are important in the calculations, and moles of titrant can only be calculated by

[Volume of titrant][Conc. of titrant] = moles of titrant

if the [Conc. of titrant] is an incorrect number, which has changed by adding water, the moles of titrant will be a wrong number.

Eventually, we calculate the concentrations with the initial volume we added with the help of pipette or a known mass.

*The problem is arising because you are using a titration formula taught by those who shouldn't be teaching science. This formula is not only wrong (prone to error), but it will lead to all type of problems in future with redox calculations. I strongly discourage using the formulae given in your query.

[Acid] = (Volume of Base)*[Base] / (Volume of Acid)

This formula is valid for all acid-base and redox titrations iff (if and only if) concentrations are provided in normality. Normality is an ancient 18th century concept. It has stuck in certain education systems like cancer. You quash it here but it appears somewhere else.*

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