NUTRIENT MEDIA 



keep the hydrogen ion concentration fixed. Dilution does not 

 change the concentration. Mixtures of acid and alkaline salts 

 which will give hydrogen ion concentration ranging from Ph 

 values of 5. (acid) to 9. (alkali) have been prepared for use in 

 bacteriology. To these solutions may be added dye indicators. 

 Different dyes have different ranges of color changes. The most 

 useful are the following: 



Range] of r Ph) 

 5.2 6.8 

 6.0 7.6 

 6.8 8.4 



Brom cresol purple yellow-purple 



Brom thymol blue yellow-blue 



Phenol red yellow-red 



Litmus which has so long served a useful purpose in the laboratory 



has a Ph of about 6.8. A convenient method for practical use in 



the laboratory is as follows : 



Materials : 



1. Chemically clean test tubes. 



2. Freshly distilled water. 



3. N/i5 KH2PO4 (primary phosphate) solution. (1.078 gm. to litre of 

 water.) 



4. N/i5 Na 2 HPO 4 (secondary phosphate) solution (11.996 gm. per litre 

 of water.) 



5. Phenolsulphonphthalein .01 percent water solution (or phenol red). 



6. N/20 NaOH, N NaOH. 



7. N/20 HC1, N HC1. 



Phosphate mixture for Ph values 7.0 to 8 o. 



Ph value 



Amt. of primary 



phos. in c.c. 



19.4 



14.2 



9-7 

 6-5 

 4-3 

 2-5 

 Procedure. 



Add 20 c.c. freshly distilled water to chemically clean test tube. Add 

 10 drops phenol red and 5 c.c. of medium to be titrated. Compare with 

 standard. 



7.0 - 



7-8 

 8.0 



Amt. of secondary phos. 



in c.c. 



Quantities to be 

 mixed to get in- 

 dicated Ph value. 

 5 drops indicator 

 to each. 



30.6 

 35-8 

 40.3 

 43-5 

 45-7 

 47-5 



