Quinine and its salts may be assayed by innumerable methods ranging from gravimetry, titrimetry, spectrophotometry, spectrofluorimetry, potentiometry to newer methods such as fluoroimmunoassay etc. Estimation of quinine in cinchona bark can also be done by several methods, principal of which are gravimetry, titrimetry and spectrophotometry.

A. Method I (Spectrophotometric assay)

1. Mix 2g finely powdered bark and 0.5 g of finely powdered calcium oxide (CaO) with about 10 ml water to make a smooth homogenous paste. It is set aside for 10 min.
2. To the above paste taken in a 200 ml standard flask, add 150 ml 95% ethanol and shake vigorously. Set aside for 1 h with occasional shaking.
3. Make up to volume with ethanol, shake and filter through a Whatman filter paper. Collect the filtrate carefully to minimize ethanol evaporation.
4. Pipette 25 ml of the ethanol extract into a 50 m conical flask and add 20 mg bentonite. Shake well and filter.
5. To 8 ml of the filtrate taken in a standard flask, add 5 ml of 0.1 N HCl and make up to volume with water. This is the test solution.
6. Prepare five graded standard stock solutions of pure quinine sulphate in 0.1 N HCl so as to contain 1.6, 3.2, 4.8, 6.4 and 8 mg of anhydrous quinine sulphate. Transfer 5 ml each of the prepared solutions into a series of five 100 ml standard flasks, add 8 ml ethanol into each and make up to volume with water.
7. Determine the absorbance of the test and standard solutions at 380 nm using a solution of acid and ethanol in water as blank. Calculate the percentage of quinine as sulphate from standard curve prepared.

B. Method II (Non-aqueous titration method, IP)

1. Weigh accurately about 0.2 g of quinine sulphate, dissolve in a mixture of 10 ml chloroform and 20 ml acetic anhydride.
2. Determine the end point potentiometrically using glass electrode and calomel reference electrode containing a saturated solution of potassium chloride in water.
3. Perform a blank determination and make any necessary correction.
4. Each ml of 0.1 M perchloric acid is equivalent to 0.02490 g of quinine sulphate