J01N£ OF AGKICDLTIIAL RESEARCH 



Vol. X Washington, D. C, July i6, 1917 No. 3 



A SUBSTITUTE FOR LITMUS FOR USE IN MILK 



CULTURES 



By Wm. Mansfield Clark and Herbert A. Lubs, of the 'Research Laboratories , Dairy 

 Division, Bureau of Animal Industry, United States Department of Agriculture 



The color changes which occur in litmus-milk cultures may be due not 

 only to changes in the hydrogen-ion concentration of the medium but 

 to reduction or even destruction of the dye. Thus, in any given case 

 there may be obtained a composite picture which may happen to be 

 more or less characteristic of a particular organism but which at the same 

 time is difficult to analyze. It is not to be denied that such a complex 

 picture may be of some value to a trained observer, but its complexity 

 obscures that clear and simple view which should distinguish a good 

 cultural test. 



Dibromoorthocresolsulfonphthalein, which the writers have described 

 as a reliable and brilliant indicator for the colorimetric determination of 

 hydrogen-ion concentration,^ is reduced with difficulty. In most cases 

 it may be used even in the presence of active bacterial growths without 

 being appreciably reduced, and it will therefore continue to show changes 

 in the reaction of the medium without the confusing efifect of reduction. 



For laboratory parlance the writers have suggested that dibromoortho- 

 cresolsulfonphthalein be called "bromcresol purple." Its preparation 

 has been described in previous papers,^ and it may now be purchased 

 in this country; but in purchasing this compound the full chemical name 

 should always be used. For ordinary indicator purposes a 0.04 per cent 

 aqueous solution of the monosodium salt is recommended, but as a stock 

 solution for the present purpose a solution of the salt containing 0.5 

 per cent of the acid is suggested. 



This solution may be prepared as follows: 0.5 gm. dibromoorthocresol- 

 sulfonphthalein should be ground to a fine powder in a glass mortar and 

 14 c. c. of Njio sodium hydroxid added, and the mixture stirred well. 

 This is approximately 1.5 equivalent parts of sodium hydroxid. The 



' Clark, W. M., and Lubs, H. A. the colorimetric determination op hydrogen ion concen- 

 tration AND ITS applications IN BACTERIOLOGY. PT. i-ui. In JonT. Bact., V. 2, no. I, p. 1-34, 4 fig.; 

 no. 2, p. 109-136, fig. 5-7; no. 3, p. 191-236, fig. 8. 1917. References, v. 2, no. 3, p. 233-236. 



2 Lubs, H. A., and Clark, W. M. a note on the sulphonephthaleins as indicators for the 

 colorimetric determination OP hydrogen-ion CONCENTRATION. /« JouT. Wash. Acad. Sci., v 6, no. 

 14, p. 481-483. 1916. 



Journal of Agricultural Research, Vol. X, No. 3 



Washington, D. C. July 16, 1917 



iy Key No. A— 30 



(105) 



