Ionisation percentage calculation

The percentage of HON molecules that are ionised is calculated as follows ... 

Calculate the percentage of ionisation of the drugs shown in Figure 2.1 at pH 7.0... 

The estimation of individual components and different synthetics was effectively achieved by gas chromatography after simple silylation (10). Figure 2 shows the separation obtained, clearly away from other volatiles in a capsicum extract. The individual capsaicinoids were quantitated in relation to vanillyl octanamide as the internal standard chosen, because it eluted just prior to the capsaicinoids and the response factors of the capsaicinoids with reference to this standard were close to unity. The standardised conditions were as follows Silylation with N,0-bis (trimethylsilyl)-trifluoroacetamide in tetrahydrofuran gave clear rapid reaction at room temperature. The silylated extracts were injected directly on to a stainless steel column of 2 m x 3 mm, filled with 3% SE-30 on Chromosorb-GHP, (100-120 mesh) the column temperature was programmed from 170° to 215°C at 4°/min. and held at 215°C for 10 minutes. The injection port temperature was important for rapid volatilisation of all the components without decomposition, and was fixed at 200°C. The flame ionisation detector temperature was kept at 250°C, and nitrogen flow at 20 ml/min. The percentages of the individual capsaicinoids were calculated from the areas of the peaks, the response factors, and the weight and area of the reference compound. 

Rearranging to facilitate calculation of the percentage ionisation leads to... 

Equation (1.8) applies to drugs that are weak acids and allows the fraction of the total dose that is ionised to be calculated for any pH if the pKa of the drug is known. The equation is sometimes written as the percentage ionised, which is simply given by... 

Weak acids and bases are often formulated as their salts to make them more water soluble. The ionised salts, however, do not cross biological membranes very well. Calculate the percentage of a dose of pentobarbital that will be ionised at plasma pH (7.4). The structure of pentobarbital is shown in Figure 1.8. 

To calculate the percentage ionised, use can be made of equations of the type... 

Weigh out accurately 0.7 g of dried cement into a 250 cm beaker. Disperse in water, add 10 cm cone. HCl and 150 cm hot water. Bring to the boil and keep hot for 5 minutes. Cool and transfer quantitatively to a 250 cm volumetric flask and make up to the marie. Dissolve in a 100 cm aliquot 0.191 g purest KCI and in another 100 cm aliquot 0.254 g purest NaCl. These chlorides are added as ionisation suppressors. Use the atomic spectrophotometer in the emission mode and measure the readings for the two solutions, using the manufacturer s recommended operating conditions. Use calibration standards to find [K] and [Na]. Calculate from the results of spectrophotometry the percentages of oxides of the elements determined. 

Weigh out accurately about 0.1 g of your preparation, dissolve in 30 cm of 0.1 M nitric acid. Heat until the solution changes colour to pink and no further colour change is observed. Add 1 cm of diphenylcarbazone/bromophenol blue indicator and titrate with 0.01 M mercuiy(Il) nitrate solution until the first tinge of blue purple does not disappear. Repeat to get concordant results. Calculate from the average results the % chloride and hence the percentage purity of your sample. (The chloride corresponds to the ionisable chloride + the chloride replaced by water on aquation (see below). 

Calculate the percentage ionisation and partition coefficient of procaine at pH 7.4. Partition coefficient of un-ionised procaine = 50, pKa = 9.0. 

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