Titration, benzoic acid

Figure 7. Benzoic acid titration curves in water and in the presence of an equal volume of octanol.

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

Shimadzu 240 Spectrophotometer. SEM photographs of the samples were taken using an Hitachi S-520 and for XRD analysis, a Phillips (Holland) instrument was used with CuK radiation. Basicity was measured using benzoic acid titrations. 

Table 1 shows the characteristics of the hydrotalcites prepared by varying the Mg/Al ratio between 2 to 7. The surface area of the calcined catalysts varied between 90 to 142mV The maximum surface area is obtained on hydrotalcite with Mg/Al ratio=4. Acidity measured by n-butylamine titration decreased with increasing Mg/Al ratio. Basicity measured by benzoic acid titration increased with increasing Mg/Al ratio from 2 to 4.5 and there is no further increase with increase in Mg/Al ratio. 

Benzoic Acid Titration Method In this method (4) powered alumina samples were first activated in stream of N2 gas at 200°C. About 0.5g of the sample is weighed and taken into sample vial closed with a septum. Acid indicator (1 wt.% dissolved in benzene dried over molecular sieve 4A) is added to this sample and kept for 24 hrs. by constantly shaking the vial... 

Tert-butyl catechol is a weakly acidic molecule with a pKj value of 11.4(6) and hence basic alumina should favour its adsorption by assuming simple acid-base interactions. Hence, we measured the basicity of alumina. The change in surface chemistry due modification of alumina surface with alkali and acid can be measured by two methods, namely (1) Adsorption /Desorption of Gaseous acid molecule (CO2) (2) Benzoic acid titration method... 

Table 5 Titre values for benzoic acid titrations...

Basicities determined by the benzoic acid titration (Table V) did not appear to be quantitative as the green color reappeared periodically... 

Table V. Determination of Basicity (mmole/g) by Benzoic Acid Titration...

A facile explanation of the failure of acidity to enhance activity of any of the catalysts while base enhances the activity of a poor catalyst without affecting the active ones is that the Claus active catalysts owe their activity to basic sites. In general, alumina shows strong surface acidity when activated at 450 °C in vacuum, and intensive investigation has shown this to happen in Lewis acid sites (8). There is also evidence that basic sites occur on alumina. Schwab and Krai (9) obtained evidence for basic sites by boron trifluoride adsorption on alumina, and Yamadaya et al. (10) reported up to 0.4 mmole/g of basic sites on alumina from benzoic acid titrations. Pines and Manassen (11) obtained indirect evidence of such sites from studies of the dehydration of primary alcohols. 

Sulfur dioxide adsorption at 250°C (2 torr sulfur dioxide) on Chromosorb-A containing 3.9% sodium hydroxide amounted to 0.02 mmole/g compared with 0.06 mmole/g basic sites by benzoic acid titration. This suggests that sulfur dioxide is adsorbed on the basic sites. The sulfur dioxide adsorption corresponded to 6% coverage of the total surface of the catalyst. 

Seventy-seven percent of the benzoyl peroxide formed benzoic acid by hydrogen abstraction. This was done by using styrene as the only monomer and the resultant benzoic acid titrated is due to abstraction of hydrogen by benzoyloxy radical. 

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. 

Benzoic Acid Titration Method Using Indicators... 

The basicities of some of the oxides were measured by benzoic acid titration using indicators as shown in Fig. 3.1. Basic strengths of Rb20 outgassed at 643 K tmd CS2O outgassed at 573 K exceed H = 26, which is the critical strength for superbases. 

Graphite reacts with tdkali metals to give lamellar compounds in which alkali metals are present in the form of monolayers separated by one or more carbon layers. The basicities measured by benzoic acid titration are shown in Fig. 3.3. The strongest basic sites are H = 18 both for potassium and cesium intercalated compounds. 

In the form of single component oxides, one important feature is their basic property. Basic property was measured by benzoic acid titration. Basicity variation of La203 at H =12.2 as a function of calcination temperature was measured. The maximum basicity upon unit weight basis was observed with pretreatment at 773 K. The measured basicity correlates well with the catalytic activity for diacetone alcohol decomposition. ... 

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