Volu

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A very useful method of chemical analysis is based upon determining the volume ofa solution of known concentration that is needed to react exactly with the substancebeing analysed. This is called volumetric analysis. It uses molarity and calculationsbased upon that concept to work out answers (as was discussed on pp. 259–63 ofChemistry in Use Book 1).The process of determining the volume of one reactant solution which exactlyreacts with a given amount of another reactant is called titration.Before discussing volumetric analysis, we need first to consider somepreliminary matters, such as equivalence point and indicators.

The commonest indicators are acid–base indicators. They change colour as pHchanges. The fi rst indicator was litmus, a dye that was extracted from lichens whichare mixed algae–fungi organisms that grow on rocks and trees. Litmus is red inacid solution (pH < 6) and blue in alkaline solution (pH > 8). Between pH 6 and 8it gradually changes colour from red to blue. We say that litmus has a pH range of6 to 8, meaning that this is the range of pH over which it changes colour.Many other acid–base indicators have been isolated or synthesised. They areactually weak acids with the conjugate base having a different colour from thatof the acid itself. Different indicators have different acid strengths (Sections 1.16(pp. 224–5) and 2.9 (pp. 254–6)); and so they change colours over differentpH ranges. Figure 3.1 shows the colours of four indicators in acidic, neutral andalkaline solutions. Common indicators are shown in Table 3.1 along with theircolour changes and their pH ranges. Figure 3.2 shows t

3.3 pH METERSpH meters are instruments that measure pH directly. The sensing device, shown inFigure 3.3, consists of a pair of electrodes (often mounted concentrically in the oneunit). The measuring electrode, called a glass electrode, consists of a very thin andfragile membrane made out of a special kind of glass. This develops an electricalpotential which depends upon pH. The other electrode is a reference electrode.When this sensing unit is immersed in the solution to be tested, it forms a galvaniccell; the accompanying electronics unit simply measures the voltage (EMF) of thiscell and converts it into a pH reading. A big advantage of a pH meter is that inmeasuring the pH of a solution, it does not alter the pH of the solution in any way;it does not disturb any chemical equilibria involved.