Solution AA is an aqueous alkali. Solution BB is a dilute acid. In this experiment you follow the pH of the alkali as the acid is added to it. Use a pipette...

This is a pH-monitoring titration: a fixed 25.0 cm³ of an aqueous alkali (AA) has a dilute acid (BB) run into it 5.0 cm³ at a time, and the pH probe follows how the mixture changes from strongly alkaline to strongly acidic. The whole question tests careful recording to a fixed precision, plotting a titration curve and reading the neutralisation point from its steepest part.

(a) Starting pH [1]. Before any acid is added the beaker holds only the alkali, so the reading is high. Record it to one decimal place, the precision of the meter, for example pH 13.0. Any value of 12 or above earns the mark; a whole number such as "13" without a decimal place would lose the precision requirement carried in part (b).

(b) The eight further readings [4]. One mark is for entering all eight further values, one for keeping every value to the same one-decimal-place precision, and the remaining marks reward the correct shape: the pH falls only slowly at first while excess alkali is still present, drops steeply as the last of the alkali is neutralised, then falls slowly again as excess acid builds up, and no reading lies outside the 0 to 14 range. A representative correctly recorded table is:

(c) The graph [4]. Volume goes on the horizontal axis and pH on the vertical axis, each labelled with its quantity and unit; the scales are chosen so the points fill more than half the grid; all nine points are plotted; and a single smooth curve is drawn through them, not straight lines joining point to point. Plotting the values above gives the characteristic titration curve, almost flat at each end with a steep fall in the middle:

(d) Neutralisation volume [2]. Neutralisation is the mid-point of the steep, near-vertical part of the curve, where the pH passes through 7. Reading down from that point on the curve above gives about 25.0 cm³ of solution BB. In an examination the candidate's own value, read correctly from the centre of their own steep section, is accepted.

(e) How the pH changed around this volume [2]. For the portions before and after this volume the pH changed only slightly (from about 12.1 at 15.0 cm³ to 11.4 at 20.0 cm³, and from about 2.2 at 30.0 cm³ to 1.8 at 35.0 cm³) [1], but over the single portion that spans the neutralisation point the pH fell very steeply, by several pH units (from about 11.4 down to about 2.2) [1]. This is because near the end point almost all the remaining alkali is used up by one small addition of acid.

(f) Colour of universal indicator at neutralisation [1]. At pH 7 universal indicator is green.

(g) Why the pH stays above 1 [2]. Although solution BB has pH 1 on its own, the excess acid added past the end point is mixed with, and so diluted by, the 25.0 cm³ of neutralised mixture already in the beaker [1]. Diluting it lowers the concentration of hydrogen ions, so the pH stays higher than the pH 1 of the undiluted acid [1].

(h) Preparing a pure dry salt without an indicator [3]. Because part (d) tells you exactly how much acid neutralises the alkali, you no longer need an indicator to find the end point:

1. Pipette a fresh 25.0 cm³ portion of solution AA into a clean flask and run in exactly the volume of solution BB deduced in (d), giving a neutral salt solution [1].
2. Pour this neutral mixture into an evaporating basin and heat it to the point of crystallisation (until crystals just begin to form at the edge) [1].
3. Leave it to crystallise as it cools, then filter off the crystals and dry them between filter papers [1].