# Obtaining pH curves for acid/alkali titrations.

## Obtaining pH curves for acid/alkali titrations

Obtaining pH curves for acid/alkali titrations

Planning

In order to obtain a pH curve, I must first devise a way in which to measure both the pH and the volume of alkali being added to the weak acid in order to draw a graph from my results and then from this graph find Ka.

## Method

I will set up the equipment as shown in the diagram below:

I will use a pipette and pipette filer to take 25cm3 exactly of the weak acid (0.1M ethanoic acid, CH3COOH) and place it in a beaker. I will then take a burette and place in it 50cm3 of 0.1M sodium hydroxide (NaOH). I will then measure the exact amount of NaOH added to the acid whilst using an accurate pH meter rather than indicator to measure the change in pH. Before using the pH meter, I will first dip it into two buffer solutions of pH 4 and pH 7 and calibrate it so that it reads accurately. I will record the values for the pH and the volume of NaOH added. Theoretically, this should give me a pH curve from which to work. In order to obtain a value for Ka, I shall find the equivalence point (at around 25cm3 where the graph goes up vertically) and find the pH at this point, I shall then divide this by two to give me the ½ equivalence point. Having obtained this value, I can assume that:

[CH3COOH] = [CH3COO-] = [H3+O]

And as:

Ka = [CH3COO-] [H3+O]

[CH3COOH]

Therefore:

Ka = [H3+O]

Or:

pKa = pH (at ½ equivalence point)

Or:

pH = -Log10 Ka

## Prediction

I know that the curve for a weak acid/strong alkali should start higher up the axis than a strong acid/strong base curve. This results in the vertical section of the graph being shorter and due to salt hydrolysis, the curve will rise to 14 far quicker than it should. This is due to the following mechanism:

CH3COOH + NaOH → CH3COONa + H2O

CH3COONa → CH3COO- + Na+

H2O→ H+ + OH-

The ...