Neutralisation curves

Fig. 10.2 Neutralisation curves of 100 mL of HCI with NaOH of same concentration (calculated).

Fig. 104 Neutralisation curves of lOOmL of 0.14/ acetic acid (Ke= 1.82 x 10 5) and of 0.14/ acid (AT = 1 x 10-7) with 0.14/sodium hydroxide (calculated).

With a knowledge of the pH at the stoichiometric point and also of the course of the neutralisation curve, it should be an easy matter to select the appropriate indicator for the titration of any diprotic acid for which K1/K2 is at least 104. For many diprotic acids, however, the two dissociation constants are too close together and it is not possible to differentiate between the two stages. If K 2 is not less than about 10 7, all the replaceable hydrogen may be titrated, e.g. sulphuric acid (primary stage — a strong acid), oxalic acid, malonic, succinic, and tartaric acids. 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

Weak acid and weak base. There is no sharp rise in the neutralisation curve and, generally, no simple indicator can be used. The titration should therefore be avoided, if possible. The approximate pH at the equivalence point can be computed from the equation ... 

Weak acids with weak bases. The titration of a weak acid and a weak base can be readily carried out, and frequently it is preferable to employ this procedure rather than use a strong base. Curve (c) in Fig. 13.2 is the titration curve of 0.003 M acetic acid with 0.0973 M aqueous ammonia solution. The neutralisation curve up to the equivalence point is similar to that obtained with sodium hydroxide solution, since both sodium and ammonium acetates are strong electrolytes after the equivalence point an excess of aqueous ammonia solution has little effect upon the conductance, as its dissociation is depressed by the ammonium salt present in the solution. The advantages over the use of strong alkali are that the end point is easier to detect, and in dilute solution the influence of carbon dioxide may be neglected. 

The neutralisation curves have been expressed by three constants corresponding to the first, second and third dissociations, viz. —... 

The value of the first constant was determined as 1-1 x IQ-2 by measurements of the conductivity of the free acid.3 The second constant was determined as 1-95 x 10 7 by conductivity measurements in solutions of NaH2P04.3 This value was confirmed by calculations from the neutralisation curve as determined by means of the hydrogen electrode.4 The value of the third constant, viz. 3-6 x 10-la, was first determined by measurements of the conductivity of ammonium phosphates and also by the distribution of the ammonia between water and chloroform.3 It was shown that this result was incompatible with the observed values of hydrion concentration during the later stages of neutralisation by a strong alkali. A calculation based on these values gave 3 = 3-0 x 10-12 in decimolar solutions.4... 

The results are closer to the theoretical if the solutions are saturated with sodium chloride. See also Neutralisation Curve, p. 166. 

The quantity, d[B]/dpH, is a quantitative measure of the buffering ability of a solution. It was introduced by Van Slyke (1922) as the buffer unit or buffer value , and is also known as the buffer capacity . It is the reciprocal of the slope of the pH-neutralisation curve. 

Flat neutralisation curve, quick and safe adjustment to mandatory pH range... 

N/P 1. Plot follows closely charge neutralisation curve in Fig. 6.11. 

Neutralisation Curves.— When a strong acid is titrated with a strong base, and the accompanying changes in pH are measured, a characteristic curve is obtained showing an enormous alteration in... 

Figure P.7 Neutralisation curves for acids and basts (all concentrations 0.1 mol dm ) I, strong acid II, strong base III, weak acid Ilia, very weak acid IV, weak base. A is equivalence point for I-IV B for I-II and C for II-III...

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