Benzoic Acid Titration Method Using Indicators

The basic strength of a solid surface is defined as the ability of the surface to convert an adsorbed electrically neutral acid to its conjugate base, i.e. the ability of the surface to donate an electron pair to an adsorbed acid. The amount of base (basic sites) on a solid is usually expressed as the number (or mmol) of basic sites per unit weight or per unit surface area of the solid. It is also sometimes more loosely called basicity. There are two main methods for the measurement of strength and amount of basic sites benzoic acid titration method using indicators and geseous acid adsorption method. 

1 Benzoic Acid Titration Method Using Indicators 

For the reaction of an acid indicator BH with a solid base B, 

The first perceptible change in the color of an acid indicator occurs when about 10 percent of the adsorbed layer of indicator is in the basic form, i.e. when the ratio [B ]/[BH] reaches 0.1/0.9 ( = 0.11). Further increase in the intensity of the color is only perceptible to the naked eye when about 90 percent of the indicator is in the basic form, i.e. [B-]/[BH] = 0. 9/0.1 ( = 9). Thus the initial color change and the subsequent change in intensity are observed at values of H equal to p/ifsH 1 and pKm + 1 respectively. If we assume that the intermediate color appears when the basic form reaches 50 percent, i.e. when [B ]/[BH] = 1, we have = 

According to this assumption, the approximate value of the basic strength on the surface is given by the p/iBH value of the adsorbed indicator at which the intermediate color aj ears. Indicators which lend themselves to this method are listed in Table 2.3. Non-polar solvents such as benzene and isooctane are used for the indicators. 

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The basicity of each solid was measured by the benzoic acid titration method using Hammett indicators [4]. 

The British P 1993 [15] describes a procedure to determine benzoic acid in benzoic acid ointment that also contains salicylic acid. In this method, 2 g of the ointment is added to 150 mL water, and warmed until melted. The solution is titrated with 0.1 M NaOH, using phenolphthalein solution as the indicator. Salicylic acid is assayed by a spectrophotometric procedure, and its content subtracted from the total acid value to obtain the benzoic acid content. 

One such method is the combination of titration with spectrophotometric determination, which is well established (Poppi and Pasquini, 1993). These authors used multivariate calibration, optimizing the FIA titration system for the selective determination of a mixture of acetic acid, benzoic acid, and hydrochloric acid, using NaOH as the titrant. The mixture containing the indicator-sample-alkaline gradient was analyzed using a spectrophotometer at 340-680 nm. This methodology allows the analysis of mixtures of acids in synthetic solutions within 1-5 min, although it is important to have data on the differences between the pKa values of the acids concerned. 

The B,P, method of estimation is by direct titration in ethanol phenol-phthalein or phenol red may be used as indicator, the colour change being quite sharp in the presence of ethanol, phenolphthalein being less sensitive. 1 ml 0 5N NaOH = 0 06106 g. Benzoic acid may also be titrated in a non-aqueous medium such as dimethylformamide, an alkaline methoxide or tetrabutylammonium hydroxide being used as titrant (see p. 793). 

Hydrolysis would then give alkali arising both from Bu°Li and EtOU, so the titration with acid would overestimate the amoimt of Bu Li present. Several methods have been devised for dealing with this problem. In one of these a weighed amount of benzoic acid in a mixture of dimethyl-sulphoxide, 1,2-dimethoxyethane and ether or hydrocarbon is titrated with the organolithimn solution. The indicator is triphenylmethane, which develops a red colour when all the benzoic acid is used up ... 

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