Glassware

The manufacturers’ tolerances for the volumes of a number of items of glassware are given in Chapter 1. The larger the volume measure the smaller the tolerance percentage is of the nominal volume. Thus, for a Grade A 1 ml pipette the volume is within ± 0.7% of the nominal volume, whereas for the 5 ml pipette the volume is within ± 0.3% of the nominal volume. If greater accuracy than those guaranteed by the tolerances is required, then the glassware has to be calibrated by repeated weighing of the volume of water contained or delivered by the item of glassware. This exercise is also useful for judging how good one’s ability to use a pipette is, since weighing of the volumes of water dispensed correctly several times from the same pipette should give weights that agree closely.

Primary standards and standard solutions

Primary standards are stable chemical compounds that are available in high purity and which can be used to standardise the standard solutions used in titrations. Titrants such as sodium hydroxide or hydrochloric acid cannot be considered as primary standards since their purity is quite variable. So, for instance, sodium hydroxide standard solution may be standardised against potassium hydrogen phthalate, which is available in high purity. The standardised sodium hydroxide solution (secondary standard) may then be used to standardise a standard solution of hydrochloric acid. Table 3.1 lists some commonly used primary standards and their uses.

Table 3.1 Primary standards and their uses

Strong acid/strong base titrations

Figure 3.1 shows the titration curve obtained from the titration of a strong acid with a strong base. The pH remains low until just before the equivalence point, when it rises rapidly to a high value. In many titrations a coloured indicator is used, although electrochemical methods of end-point detection are also used. An indicator is a weak acid or base that changes colour between its ionised and un-ionised forms; the useful range for an indicator is 1 pH either side of its pKa value. For example, phenolphthalein (PP) pKa 9.4 (colour changes between pH 8.4 and pH 10.4) undergoes a structural rearrangement as a proton is removed from one of its phenol groups when the pH rises, and this causes the colour change (Fig. 3.2). Methyl orange (MO) pKa 3.7 (colour changes between pH 2.7 and pH 4.7) undergoes a similar pH-dependent structural change. Both these indicators fall within the range of the inflection of the strong acid/strong base titration curve.

Fig. 3.1 The change in pH as 25 ml of 1 M HCl are titrated with 1 M NaOH.

Fig. 3.2 The structural rearrangement responsible for the colour change in phenolphthalein.

There are only a few direct strong acid/strong base titrations carried out in pharmacopoeial assays.

Strong acid/strong base titrations are used in pharmacopoeial assays of: perchloric acid, hydrochloric acid, sulphuric acid and thiamine hydrochloride.

Weak acid/strong base and weak base/strong acid titrations

On addition of a small volume of the strong acid or strong base to a solution of the weak base or weak acid, the pH rises or falls rapidly to about 1 pH unit below or above the pKa value of the acid or base. Often a water-miscible organic solvent such as ethanol is used to dissolve the analyte prior to the addition of the aqueous titrant.

Figure 3.3 shows a plot of pH when 1 M NaOH is added to 25 ml of a 1 M solution of the weak acid aspirin.

Fig. 3.3 Titration curve for 25 ml of a 1.0 M solution of aspirin (pKa 3.5) titrated with 1.0 M NaOH.

In the case of aspirin, the choice of indicator is restricted by where the inflection in its titration curve lies; PP is suitable as an indicator whereas MO is not.

In the example of the titration of quinine with hydrochloric acid (Fig. 3.4), MO is a suitable indicator because it falls within the inflection of the titration curve whereas PP is not suitable.

Fig. 3.4 25 ml of a 1.0 M solution of quinine (pKa 8.05) titrated with 1.0 M hydrochloric acid.

Some acids or bases can donate or accept more than one proton, i.e. 1 mole of analyte is equivalent to more than 1 mole of titrant. If the pKa values of any acidic or basic groups differ by more than ca 4, then the compound will have more than one inflection in its titration curve. Sodium carbonate is a salt of carbonic acid and it can accept two protons. The pKa values of carbonate and bicarbonate are sufficiently different (pKa 10.32 and 6.38) for there to be two inflections in the titration curve. The two stages in the titration are:

In a titration of sodium carbonate, the first inflection is indicated by PP and the whole titration by MO (Fig. 3.5).

Fig. 3.5 Titration of 1 M sodium carbonate with 1 M HCl.

Self-test 3.3

How many inflections do the following substances have in their titration curves when titrated with a strong base? Draw the predominant forms of these substances which would exist at pH 14.

Answers: oxalic acid 1, fumaric acid 1, citric acid 1, phenylalanine 2

Weak acid/strong base titration is used in the pharmacopoeial assays of: benzoic acid, citric acid, chlorambucil injection, mustine injection, nicotinic acid tablets and undecanoic acid.

Estimation of esters by back titration

Excess of sodium hydroxide is added to the ester. The following reaction occurs:

The XSNaOH is back titrated with HCl using PP as an indicator.

This procedure is used in pharmacopoeial assays of: benzyl benzoate, dimethyl phthalate, ethyl oleate, methyl salicylate, cetostearyl alcohol, emulsifying wax, castor oil, arachis oil, cod liver oil and coconut oil.

Saponification value

The assay of fixed oils provides a special case of ester hydrolysis since they are triesters of glycerol. The saponification value for a fixed oil is the number of mg of potassium hydroxide (KOH) equivalent to 1 g of oil. A high value means rancidity, a low value possible adulteration with mineral oil. Almost all edible oils have a saponification value between 188 and 196. Hydrolysis of the fixed oil is carried out with ethanolic KOH.

This procedure is used in the pharmacopoeial assays of: castor oil, cod liver oil, cotton seed oil, almond oil and sesame seed oil.

Acid values are also determined for fixed oils. The acid value for a substance is the number of mg of KOH required to neutralise 1 g of the test substance when it is titrated with 0.1 M ethanolic KOH to a PP end-point. This value is quoted for many fixed oils in order to eliminate rancid oils, which contain large amounts of free fatty acid. Typically acid values for fixed oils are in the range of 1–2.

Estimation of alcohols and hydroxyl values by reaction with acetic anhydride (AA)

Alcohols can be determined by reaction with excess acetic anhydride (AA) (Fig. 3.7). This is a useful titrimetric method because the alcohol group is difficult to estimate by any other means.

Fig. 3.7 Estimation of benzyl alcohol by reaction with acetic anhydride.

The excess AA and acetic acid may be back titrated with NaOH using PP as an indicator.

In a related assay, a hydroxyl value is determined for a fixed oil. A 1:3 mixture of AA in pyridine is used in the determination; the pyridine is present as a catalyst. The hydroxyl value may be defined as:

Reaction with acetic anhydride is used in pharmacopoeial assays of benzyl alcohol and dienestrol, and determination of hydroxyl values of castor oil, cetosteryl alcohol and cetomacrogol.

Theory

Non-aqueous titration is the most common titrimetric procedure used in pharmacopoeial assays and serves a double purpose, as it is suitable for the titration of very weak acids and bases and provides a solvent in which organic compounds are soluble. The most commonly used procedure is the titration of organic bases with perchloric acid in acetic acid. These assays sometimes take some perfecting in terms of being able to judge the end-point precisely.

The theory is, very briefly, as follows: water behaves as both a weak acid and a weak base; thus, in an aqueous environment, it can compete effectively with very weak acids and bases with regard to proton donation and acceptance, as shown in Figure 3.8.

Fig. 3.8 Competition of water with weak acids and bases for proton acceptance and donation.

The effect of this is that the inflection in the titration curves for very weak acids and very weak bases is small, because they approach the pH limits in water of 14 and 0 respectively, thus making end-point detection more difficult. A general rule is that bases with pKa < 7 or acids with pKa > 7 cannot be determined accurately in aqueous solution. Various organic solvents may be used to replace water since they compete less effectively with the analyte for proton donation or acceptance.

Non-aqueous titration of weak bases

Acetic acid is a very weak proton acceptor and thus does not compete effectively with weak bases for protons. Only very strong acids will protonate acetic acid appreciably according to the equation shown below:

Perchloric acid is the strongest of the common acids in acetic acid solution, and the titration medium usually used for non-aqueous titration of bases is perchloric acid in acetic acid. Addition of acetic anhydride, which hydrolyses to acetic acid, is used to remove water from aqueous perchloric acid. Weak bases compete very effectively with acetic acid for protons. Oracet blue, quinalidine red and crystal violet (very weak bases) are used as indicators in this type of titration. A typical analysis is shown in Figure 3.9 for L-3,4-dihydroxyphenylalanine (LDOPA).

Fig. 3.9 Analysis of L-3,4-dihydroxyphenylalanine (LDOPA) by non-aqueous titration.

When the base is in the form of a salt of a weak acid, removal of an anionic counter ion prior to titration is not necessary, e.g. for salts of bases with weak acids such as tartrate, acetate or succinate. However, when a base is in the form of a chloride or bromide salt, the counter ion has to be removed prior to titration. This is achieved by the addition of mercuric acetate; the liberated acetate is then titrated with acetous perchloric acid. This is illustrated in Figure 3.10 for the example of phenylephrine HCl.

Fig. 3.10 The analysis of phenylephrine hydrochloride by non-aqueous titration.