Acid-Base Practicals: RP9, Titration Curves, Ka & Buffers (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about Required Practical 9, titration curves, finding Ka and making buffers.

All four come off the same pH curve: Required Practical 9 and the titration curves experiment produce it by measuring pH as base is added to an acid, and that curve is where the equivalence point, the half-equivalence point that gives Ka, and the buffer region are all read off.

Required Practical 9 investigates how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base. The titration curves experiment runs the same pH measurement across four different acid and alkali combinations. Finding Ka takes one point off that curve, the half-equivalence point, and turns it into the acid dissociation constant. Making buffers prepares a solution that sits in the flattest part of the curve, where the pH changes very little when acid or alkali is added. What joins them is that they are all readings from the same graph of pH against volume of base added.

We'll start with Required Practical 9 and how the curve is measured. Then the four characteristic curve shapes. After that the half-equivalence point and Ka, and last making and testing a buffer.

Required Practical 9 investigates how pH changes when a weak acid reacts with a strong base, and when a strong acid reacts with a weak base. Ethanoic acid goes into a beaker on a magnetic stirrer, the pH probe is positioned so it does not interfere with the stirrer bar, and sodium hydroxide is added from a burette. The pH is recorded after each addition, in 2 centimetre cubed portions at first and then 1 centimetre cubed portions once the pH begins to rise rapidly.

Plotting pH against the volume of base added gives the characteristic s-shaped curve, and the midpoint of the inflection is the equivalence point. From that curve you can read the pH of the acid, the pH and the volume of base at the equivalence point, the range of pH over the vertical section, the buffer region, and the half-equivalence point. Everything in this video is read off that one graph.

The titration curves experiment runs the same measurement across four combinations: hydrochloric acid or ethanoic acid, against sodium hydroxide or ammonia solution. The pH probe is calibrated first by a two-point calibration, set in pH 4 buffer and then checked against pH 9 buffer.

25 centimetre cubed of the acid is pipetted into a beaker and its pH recorded, then the alkali is added from a burette 5 centimetre cubed at a time up to 50 centimetre cubed, with the pH measured after each addition. The four graphs that come out are the four characteristic titration curves, and it is often better to add smaller portions nearer the equivalence point, because more data points make the shape easier to draw. These are the same curves as Required Practical 9's, with the strengths of the acid and the base varied.

The Ka of a weak acid is found from the pH at the half-equivalence point, because at that point Ka equals the hydrogen ion concentration. 25 centimetre cubed of ethanoic acid is titrated against sodium hydroxide with phenolphthalein until the indicator just turns pink, and a further 25 centimetre cubed of the acid is then added to the flask and the pH measured.

That leaves an effectively half-neutralised sample of acid, which is the half-equivalence point. Measuring the pH there converts to the hydrogen ion concentration and so gives the Ka, and you can also say directly that the pKa is the pH at the half-equivalence point. This is the same point that gets located on the curve in Required Practical 9.

Buffers can be made by a direct or an indirect method. The direct method measures out the exact quantities needed of a weak acid and its salt, ethanoic acid and sodium ethanoate. The indirect method uses sodium hydroxide to partially neutralise the weak acid, so that a mixture of the acid and its salt is obtained.

A calibrated pH probe measures the pH of the buffer. To test it, 1 centimetre cubed of hydrochloric acid is added to 100 centimetre cubed of the buffer and the pH measured, and the process is repeated with sodium hydroxide. The pH should change very little on adding either. When the ratio of salt to acid is one to one, the pH equals the pKa. Making a buffer with any other pH comes from adjusting that salt to acid ratio. The almost flat section is the buffer region of the titration curve.

Examiners expect you to choose an indicator whose pH range falls within the steep vertical section of the titration curve. When you state an indicator's colour change, both the starting and the finishing colours must be given, in the correct order: stating only the final colour scores zero.

pH values in calculations must always be given to exactly two decimal places, and examiners penalise one decimal place or more than two. Students also commonly forget to square root when calculating the hydrogen ion concentration from Ka, and examiners cap marks when that happens.

Required Practical 9 measures pH against the volume of base added and gives the s-shaped curve, with the equivalence point at the midpoint of the inflection.

The titration curves experiment repeats that measurement for four acid and alkali combinations, giving the four characteristic shapes.

The pH at the half-equivalence point gives the Ka, and is the pKa.

A buffer holds its pH in the flattest section of that same curve. All four are readings off one graph.