SummaryThe overall kinetic solvent isotope effects on the acid catalyzed hydrolysis of a series of 2-aryl-2-diazocarboxylic esters ArCN2COOCH3, and one 2-aryl-2-diazocarboxamide C6H5CH2CON (CH,), vary inversely with the reactivity of the substrate, between limits of 3.14 and 1.46. A linear Hammett plot for the hydrolysis rates of the a-diazocarboxylic esters indicates that there is no mechanistic change for the hydronium-ion-catalyzed reaction. The relation between hydrolysis rate and buffer acid concentration deviates from linearity for high values of the latter. It is shown on the basis of the solvent isotope effects for the non-linear region that this deviation does not stem from a mechanistic change caused by the buffer base component. The specific salt effects on the general acid-catalyzed reaction are discussed.
Summary.A high pressure liquid chromatography (HPLC) procedure has been developed for the determination of vitamin A in milk and cheese. Recovery studies and reproducibility data showed an average recovery of 93_+ 6% and a SD of 4%. We also report results obtained from HPLC in comparison with the colorimetric procedure.Colorimetry is the conventional method for the determination of vitamin A in food products 2. After saponification of the sample, vitamin A is extracted into petroleum ether, the extract is purified on an aluminium oxide column, then vitamin A is reacted with antimony trichloride generating the characteristic blue color. A maximum of 6 analyses daily can normally be carried out and the quantity of laboratory material used is quite substantial. TLC and densimetric methods have also been used for assaying vitamins 3, the results obtained with these methods, however, are not very reproducible for the determination of vitamin A 4. The method we describe here permits the determination of 30 samples per day, and the glassware used could be reduced by one third. The results are reproducible with a relative SD of + 4%. High pressure liquid chromatography (HPLC) has been successfully utilized for the measurement of vitamin A in pharmaceuticals 5,6. It is still seldom applied to food products which are not enriched with vitamin A. Descriptions of this technique 7,8 usually do not include much information on the reproducibility, yield and comparison with the convential method of Cart-Price 2. Our description of the HPLC method for measuring vitamin A is accompanied by the results of the extraction yield and also compares them with the results obtained by the Cart-Price 2 procedure.
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