Methylene Blue absorption Method:

Carboxyl groups present in the cellulose may be determined by methylene blue absorption method. This method is based on the principle that when cellulose containing carboxyl group is titrated with a standard solution of methylene blue, under specified conditions, the dye (mehtylene blue) cation is quantitatively absorbed and retained by the carboxylate anion in the cellulose.

The drop in concentration of the dye in the treating solution can then be found out colorimetrically and the amount of carboxyl groups present in the cellulose can be calculated in terms of milliequivalent/100 g of dry cellulose. For this purpose pure methylene blue is required.

In this study first of all, a calibration curve is obtained with the known concentration of methylene blue solutions ( 2-16 mg/litre with increment of 2 mg/litre). The curve will be between absorbance and known concentrations of methylene blue solutions. 

After plotting the curve, in the second step, the concentration of  methylene blue solution left after treatment of cellulose is determined using colorimeter. The concentration of the dye in a solution to be estimated by finding out the absorbance of a solution at 620 nm  wavelength. Following reagents are used in this study:

Reagent-1: Mehtylene blue solution: 0.2 moles/litre

Reagent-2: Potassium dihydrogen phosphate solution: 0.625 milli mole/litre

Reagent-3: Sodium hydroxide solution: 0.4 mole/litre

To start study a stoke solution of methylene blue dye should be prepared by taking methylene blue, potassium di hydrogen phosphate and sodium hydroxide in the above proportion.