1920] AGRICULTURAL CHEMISTRY AGROTECHNY. 11 



The colorimetric method recommeiided by Barnett and Chapman (E. S. R., 

 39, p. 9) is considered to be less accurate than lias been stated on account 

 of the difficulty in matching shades and the fact that the scale colors do not 

 completely match those of the broth and are not permanent. 



Directions are given for the preparation of ordinary and double-strength 

 broth and the control of the reaction of the finished media. For the latter, 

 instead of titrating the broth again, two small samples of the medium are 

 diluted in test tubes with an equal amount of distilled water. To one tube 

 is added 2 drops of 0.5 per cent alcohol solution of phenolphthalein and to the 

 other 2 drops of 0.02 per cent solution of cresol red. If the reaction is correct 

 (pH 7.6) no color change should occur in the first tube, while a rose or pink 

 color develops in the second tube. 



Iron sulphid as an indicator in acidinietry and a new quantitative 

 method for the determination of zinc, .1. Houben {Ber. Deut. Chem. GeselL, 

 52 {1919), No. 8, pp. 161 3-1 621). —The author suggests the use of iron sulphid 

 as an indicator in acid-base titrations and also in the determination of zinc. 

 For the latter the slightly acid solution of the zinc salt is saturated with hydro- 

 gen sulphid, a crystal of iron sulphate is added, and the mixture titrated with 

 n/4 or n/5 sodium hydroxid or borax solution with constant shaking to the 

 end point of a milk-coffee or milk-chocolate color. 



Colorimetric determination of titration -curves without buffer mixtures, 

 L. J. Gillespie {Jour. Amer. Chem. Soc, 42 {1920), No. 4, pp. 742-748, fig. 1).— 

 The author describes a method devised at the Bureau of Plant Industry, U. S. 

 Department of Agriculture, for securing titration curves colorimetrically with- 

 out the use of buffer mixtures. In place of these mixtures a color standard 

 is used consisting of a series of pairs of test tubes, one tube of each pair 

 containing dilute alkali and the other dilute acid ; and both containing to- 

 gether 10 drops of indicator solution of suitable strength, the drop ratio vary- 

 ing from 1:9 to 9:1. The indicators used are those recommended by Clark 

 and Lubs (E. S. R., 36, p. 111). .A .table has been constructed giving the H-ion 

 exponent for each pair of tubes with the different indicators and varying pro- 

 portions of indicators as determined by means of the buffer mixtures of Clark 

 and Lubs (E. S. R., 3.5, p. 801). The standard tubes are u.sed in determining the 

 H-ion exponent by a technique similar to that proposed by Barnett and Chap- 

 man (E. S. R., 39, p. 9) with the use of a single comparator. 



The technique of the procedure is described in detail, and its value and 

 limitations are dLscussed. 



The relation between the total acidity, the concentration of the hydro- 

 gen-ion and the taste of acid solutions, R. B. Harvey {Jour. Avier. Chem. 

 Soc, 42 {1920), No. 4, pp. 712-714, fig. l).—To determine whether the taste of 

 an acid solution is due to the H-ion concentration or the total acidity of the 

 solution, a comparison was made at the Bureau of Plant Industry, U. S. De- 

 partment of Agriculture, between the tastes of sodium acetate and acetic 

 acid buffer mixtures at different H-ion concentrations and at different acidi- 

 ties for each H-ion concentration. The acid taste of the buffer solutions was 

 expressed in terms of the normality of hydrochloric acid of the same degree 

 of sourness, since at the dilutions used the H-ion concentration and total 

 acidity of this acid were practically the same. 



The sourness of the acetate-buffer solution as thus measured was found 

 to be a function of both the H-ion concentration of the buffer solution and 

 its total acidity. " It is desirable, therefore, in determining the acidity of 

 acid foods to give both of these values in order that their desirability as edible 

 products can be determined." 



