VPB 112: General Veterinary Biochemistry (1+1)

DETERMINATION OF pH

• pH can be determined by using
  • Indicator Solutions (approx.)
  • pH strips or pH papers (approx.)
  • pH meter (accurately)

Indicator Solutions

• They are weak acids or weak bases whose unionized molecules exhibit one color whereas their anion or cation will have a different color.

Examples

S.No.	Indicator	pH range	Color Change
1	Methyl Orange	3.0 - 4.4	Red - Yellow
2	Methyl Red	4.4 - 6.2	Red - Yellow
3	Phenolphthalein	8.3 -10.0	Colorless - Red
4	Bromophenol Blue	3.0 – 4.0	Yellow - Blue
5	Thymol Blue	1.2 - 2.8	Red - Yellow
6	Cresol Red	7.2 - 8.8	Yellow - Red

pH strips / pH Papers

• These are paper strips impregnated with different indicators, so that when a piece of these papers are dipped in the unknown solution, the paper color changes; the color formed is compared with the reference color and the corresponding pH values available along with the pH strips. Thus the pH of the unknown solution is determined approximately.

pH Meter

• pH meter with glass electrode accurately determines the pH value of biochemical system.
• The potential of the glass electrode [Eg] related to the external reference electrode [Eref] is related to pH as follows:

       pH = Eg-Eref / 0.0591 

• The pH meter measures the EMF of a concentration of cell formed from a reference electrode, the test solution and a glass electrode sensitive to hydrogen ion.