io6 Journal of Agricultural Research voi.x. N0.3 



mixture should be diluted to about 90 c. c. with distilled water, shaken 

 until solution is complete, and then made up to 100 c. c. with distilled 

 water. The manufacturer should furnish material which when treated 

 in this manner will provide a clear solution free from the odor of cresol. 



A satisfactory concentration for coloring milk is about 0.005 per cent, 

 and is obtained by adding 10 c. c. of a 0.5 per cent solution to i liter of 

 milk. Since the molecular weight of the sodium salt of dibromoortho- 

 cresolsulfonphthalein is 562, the above-described concentration is 

 approximately M/ 10,000. 



The color of milk containing about 0.005 P^r cent of bromcresol 

 purple may be approximately described as a deep glaucous gray.^ After 

 sterilization for 20 minutes at 15 pounds' pressure, the color is a tea- 

 green.^ When an alkali formation occurs, the color goes through a 

 series of blues, while in the case of an acid fermentation the color changes 

 to yellow. If the milk is digested, the color of the indicator stands 

 out clearly, but it is difficult to describe by reason of the dichromatic 

 nature of the transmitted light, which the writers have explained in a 

 previous paper .^ 



The cost of the new indicator is a factor which must be considered 

 by those who use extensively an indicator in milk cultures. In 191 6 

 a manufacturer made for the writers some bromcresol purple at $2 a 

 gram. At that price it costs 10 cents to color a liter of milk with the 

 concentration of dye which is recommended. In the same year they pur- 

 chased azolitmin at $5 an ounce. If that dye is used in the customary 

 concentration of o.i per cent, it costs, at the price given above, about 

 17K cents to color a liter of milk. Crude litmus is, of course, very much 

 cheaper, but the quality purchasable is not very satisfactory. As is well 

 known, litmus and azolitmin have lost prestige in the modem chemical 

 laboratory, and for that reason there is little incentive for the manu- 

 facturers to improve the quality of the samples placed on the market. 



It should be noted that the price paid for bromcresol purple was out 

 of all proportion to the current costs of the raw materials at the time 

 and to the cost of manufacture. It represents the trouble and risk in 

 manufacturing and marketing a new product for which at the time the 

 demand was very small. A fair estimate of the cost can not be made 

 at present. 



When litmus milk is sterilized, the litmus undergoes a temporary 

 reduction. In some laboratories whole milk is used for special pur- 

 poses. In this case the cream layer which is formed delays the diffusion 

 of oxygen, and the reoxidation of the dye becomes a slow process, involv- 

 ing considerable delay in the use of the litmus-milk tubes. Bromcresol 

 purple does not suffer such a reduction, and consequently milk tubes 

 containing it are ready for use directly after sterilization. 



' RiDGWAY, Robert, color standards and color nomenclature. Washington, D. C, iqm. 



2 Idem. PL 47. 



'Clark, W. M., and Lubs, H. A. 1917. Op. cit. 



