570 J. HOWARD BROWN 



5. Calculate the reserve acidity and reserve alkalinity as indicated 

 on the record sheet. The buffer index is the sum of the reserve acidity 

 plus the reserve alkalinity. 



Record these values on the record sheet. 



Notes 



Greater accuracy is attained by using N/20 NaOH and N/20 HCl 

 containing phenol red and methyl red respectively in the same concen- 

 tration as is present in the color standards, as suggested by Hurowitz, 

 Mej^er and Ostenberg (1915). 



If sufficient volumes of N/20 NaOH or HCl are added to tubes 

 II or III to change the volume of the contents appreciably, the volume 

 of the contents of the color screen, tube I, should be increased equally 

 by the addition of distilled water or of N/20 alkali or acid without 

 indicator. 



Determinations should be made in dupHcate, at least until the techni- 

 cian has perfect confidence in his results. 



Record of titration of medium 



Medium Fermented Veal Infusion Bouillon No. A9 



Sample (x) = 1 cc. of medium in each tube. 



Initial hydrogen ion concentration (n) = pH 7.3. 



Titration of reserve acidity with N/20 NaOH from initial pH 7.3 to pH 8.0 



Sample I Sample II 



Burette reading (a) 0.00 0.15 



Burette reading (b) 0.15 0.3 



Difference (c) 0. 15 0. 15 



Average (d) 0-15 



Reserve acidity = d X 5 -4- x = 0.75 



Titration of reserve alkalinity with N /20 HCl from initial pH 7.3 to pH 5.0 



Sample I Sample II 



Burette reading (e) 1.32 2.28 



Burette reading (f) 2.28 3.26 



Difference (g) 0.96 0.98 



Average (h) 0. 97 



Reserve alkalinity = h X 5 -4- x = 4.85 



Calculation of buffer index; 



Reserve acidity, Rh (pH n — 8) =0.75 

 Reserve alkalinity, Ron (pH n — 5) = 4.85 



Buffer index, BI (pH 8 — 5) =5.60 



