122 REPORT OF COMMITTEE 



To test for acid, it is recommended that in place of the illogical 

 titrimetric method, determinations of hydrogen-ion concentra- 

 tion be made by the colorimetric method described by Clark 

 and Lubs (1917a). In accurate research work the exact shade 

 of the indicator should be compared with that obtained in 

 standard "buffer" solutions, and results recorded in terms of 

 Ph. In laboratories where these standard solutions cannot be 

 obtained, it is better to record results simply as + or — , accord- 

 ing to the reaction of the culture to litmus, than to use the titra- 

 tion method. Under such conditions it is possible, however, to 

 obtain a rough idea of the hydrogen-ion concentration by the 

 use of Clark and Lubs' series of indicators without making 

 accurate determinations of Ph- Four different degrees of acidity 

 can be easily distinguished by this simple method in sugar broth 

 with initial reaction of neutrality. The indicator reactions for 

 these different degrees of acidity are listed in table 1, together 

 with the approximate range of Ph to which each corresponds. 

 In the absence of accurate determinations, these degrees of 

 acidity may be recorded by the indefinite terms, "weak," "mod- 

 erate," "strong" and "very strong," or by the symbols +, 

 + +, -f- + -h, and -h + + +. 



Gas production is ordinarily measured in percentage of gas in 

 the closed arm, and the ratio of H : CO2 by means of absorption 

 with NaOH, using the methods described in laboratory manuals 

 (filling open arm with 4 per cent NaOH, allowing gas to enter 

 open arm, shaking and returning gas to closed arm). As this 

 method is far from accurate, it is recommended for provisional 

 use only. 



The fermentation test is ordinarily of no significance for 

 organisms of series III and IV because of their poor growth in 

 broth. Sometimes these organisms can be studied in some 

 other liquid medium in which they do give good growth; but 

 often it is necessary to use agar slants. In such a case, use a 

 sugar agar containing brom cresol purple (see p. 117). Litmus 

 can be used, but is unsatisfactory. Often gas-production can 

 be detected in agar cultures by the presence of cracks and air 

 bubbles but, as a test for gas, agar slants are not as reliable as 

 fermentation tubes. 



