pH Curves

\text{pH} curves can provide very important information about what is happening in an acid-base titration. Two important things that can be found from \text{pH} curves are the equivalence and half-equivalence points.

Equivalence point: The equivalence point is the point where the titrant has completely neutralised the analyte (the substance in the conical flask, also known as the titrate). The equivalence point lies at the midpoint of the vertical/ steep part of the curve.

Half-equivalence point: This is the point where half of the volume needed for neutralisation has been added to the analyte. At this point, the \text{pH} of the solution is equal to the \text{pK}_a of the acid.

There are four main types of \text{pH} curves can be sketched.

1. Strong acid + strong base
2. Strong acid + weak base
3. Weak acid + strong base
4. Weak acid + weak base

By carrying out an acid-base titration, we can monitor pH changes. These changes are usually represented by pH/ titration curves.

Analysis: Sketching a \text{pH} curve from experimental data.

Analyzing the data from \text{pH} titrations is not as straight forward as simply plotting the data and drawing a line of best fit. During the experiment, the \text{pH} will begin to change very rapidly when close to the equivalence point. This means that its difficult to collect enough data to create a detailed curve for this period of the experiment. To get around this problem there are three steps when drawing out a \text{pH} curve from experimental data.

1. Plot the data collected during the experiment on a graph of \text{pH} against volume of base added.
2. Draw two curves on the data. One curve should be drawn for the acidic region of the data (i.e. the data at low \text{pH}). The second should e drawn over the alkaline region of the data (i.e. the data at high \text{pH}).
3. Extrapolate between the two curves approximate the value of \text{pH} around the equivalence point.

Some important indicators to know a little about are methyl orange and phenolphthalein.

Methyl Orange (represented below) can be used in titrations with strong acids but not weak acids. This is because methyl orange undergoes its colour change roughly between \text{pH: } 3.1 - 4.4. Methyl orange will show red in acidic conditions and yellow in alkaline conditions.

Phenolphthalein can be used in titrations with strong bases but not weak bases. This is because phenolphthalein undergoes its colour change roughly between \text{pH: }8.3 - 10.0. Phenolphthalein will have no colour in acidic conditions but will turn pink in alkaline conditions.