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Numerous aspects of the wet acidic digestion procedure for the assay of chromic acid in a small amount of feed and excreta were examined to study the digestibility of feed by marine fishes; these examined were the spectral absorption curves of solutions prepared by the wet acid digestion of chromic oxide (Fig. 1), the stability of chromic acid solution obtained (Tables 1, 2), the effects of the amount of perchloric acid added (Table 2) and the wavelength to pre-pare the concentration-optical density curve (Fig. 2). The results of the present work showed that the following procedures were adequate for the study of digestibility. Weigh 50-100mg sample containing 1-3mg chromic oxide, wrap in a piece of filter paper and transfer to a dry 100ml Kjeldahl flask. Add 5ml of concentrated nitric acid in such a manner that it will wash down the particles adhered on the inside of the flask and allow to stand for a short period. Heat flask over a micro-electric heater which has the holes in the asbestos board covered so as to allow more heat to come in contact with the flasks. Allow the sample to digest until white precipitate is obtained (for about 20 minutes). When black particles adhere to the neck or side of the flask, wash them down by turning the flask 180°. Turn off the heater, cool the flask and 3ml perchloric acid to the digestion mixture and then reheat until green colour changes to yellow, orange or red. The reversal change in colour frequently occurs if the flasks are cooled just after the change in colour from green to yellow, because of the insufficient oxidation of the content. Therefore, the extension of digestion for 10 minutes is necessary after the colour change. Cool slightly and add about 50ml distilled water. Cool to room temperature and make up to 100ml in a volumetric flask with distilled water. Allow to stand for a few minutes to precipitate inor-ganic material. Transfer solution gently from the volumetric flask to a colorimetric tube, and read optical density at 350mμ against distilled water. The standard curve obtained by the wet acid digestion technique is expressed by the following equation;, Y=0.2089X+0.0032, where Y is the optical density at 350mμ, and X is the chromic oxide content of the sample (mg/100ml).
Furukawa et al. (Sat,) studied this question.