Assay of Hydroxypropoxy Group in Hydroxypropyl Cellulose by United States Pharmacopeia-Titration Method

Hydroxypropyl cellulose (HPC) is a derivative of cellulose with both water soluble and organic solubility, used as food additive and sieving matrix for DNA separation. Hydroxypropyl Cellulose is partially substituted poly (hydroxypropyl) ether of cellulose. It contains NLT 53.4% and NMT 80.5% of hydroxypropoxy groups, calculated on the dried basis. It may contain ether suitable anticaking agents, and 0.6 percent of silica (SiO 2 ). The average sample result has been determined 67.3% which was conforms to USP/NF specifications for Hydroxypropyl cellulose.


Introduction
Hydroxypropyl cellulose (Cellulose, 2-hydroxypropyl ether) (HPC) is an ether of cellulose where some of the hydroxyl groups of the cellulose have been hydroxypropylated forming -OCH 2 CH (OH) CH 3 groups using propylene oxide. Chemical formula of Hydroxypropyl ether of cellulose is [C 6 H 7 O 2 (OH) x (OCH 2 CHOHCH 3 ) y (OCH 2 CH[R w ]CH 3 ) 2 ] n . R is a substituent comprising "w" hydroxypropoxyl group. Structural formula is shown at Figure 1. HPC is a nonionic water soluble cellulose ether with a remarkable combination of properties. It combines organic solvent solubility, and surface activity. HPC is a white or yellowish-white powder or granules, hygroscopic after drying soluble in cold water in glacial acetic acid, in anhydrous ethanol, in methanol and in propylene glycol and in a mixture of 10 parts of methanol and 90 parts of methylene chloride giving colloidal solutions, sparingly soluble or slightly soluble in acetone, practically insoluble in hot water, in ethylene glycol and in toluene. [1] HPC is used as a lubricant for severe dry eye syndromes. [2,3,4] HPC is also used as a thickener, a low level binder and as an emulsion stabilizer with E number E 463 . In pharmaceuticals it is used as a binder [5] in tablets. HPC is also used as a food additive, sieving matrix for DNA separations by capillary and microchip electrophoresis [6].
Hydroxypropyl cellulose is a physiologically inert substance, in a study of rats fed hydroxypropyl cellulose or unmodified cellulose at levels up to 5% of their diet; it was found that the two were biologically equivalent in that neither was metabolized. HPC is not absorbed from the gastrointestinal tract and is quantitatively excreted in the faces.

Equipment
• Analytical balance • Thermometer • Timer • Hydroxypropoxy determination apparatus (reaction flask). The boiling or reaction flask, consisting of a 125-mL conical-bottom boiling flask modified to provide a thermocouple (or thermometer) well and an inlet with a 1.0-mm capillary tip for nitrogen and water is fitted with a distillation head that leads to a condenser. The reaction flask is immersed in an oil bath equipped with an electric heater capable of heating the bath at the desired rate and maintaining the temperature at 155°. The distillate is collected in a flask. [NOTE-the tube from the condenser to the flask must be below the surface of the liquid in the flask to ensure the capture of all of the acetic acid formed [ Figure 2] [1].

Procedure Analysis
Transfer about 65 mg of Hydroxypropyl Cellulose, previously dried at 105° for 1 hour and accurately weighed, into the reaction flask. Add 5 mL of water, and swirl gently for 5 minutes. Add 10 mL of chromium trioxide solution (30 g in 70 mL). Assemble the apparatus as shown in Figure 2 and immerse the reaction flask in the oil bath slightly above the level of the chromium trioxide solution. Start the condenser cooling water, and pass nitrogen gas through the flask at a rate of about 70 to 75 mL per minute. Raise the temperature of the oil bath to 155° during a 30-minute period, and maintain it at this temperature throughout the determination. [NOTE-too rapid an initial rise in temperature results in high blank determinations. Monitor the temperature of the reaction mixture in the reaction flask using a thermocouple or thermometer in a well, as shown in Figure 2. When a reaction mixture temperature of 102 ± 1° is reached, add water through the water inlet until the reaction mixture temperature drops to 97 ± 1°. Continue this 97° to 102° temperature cycle until 100 mL of distillate has been collected. Detach the condenser from the distillation head, and wash with water, collecting the washings in the flask containing the distillate. Titrate the solution with 0.02 N sodium hydroxide VS to a pH of 7.0 ± 0.1, using an expanded-scale pH meter equipped with glass and calomel electrodes. Record the volume, V, of the 0.02 N sodium hydroxide used, then add 500 mg of sodium bicarbonate and 10 mL of 2 N sulfuric acid. After evolution of carbon dioxide has ceased, add 1 g of potassium iodide, insert the stopper in the flask, shake the mixture, and allow the solution to stand in the dark for 5 minutes. Titrate the liberated iodine with 0.02 N sodium thiosulfate VS to the sharp disappearance of the yellow iodine color, adding a few drops of starch TS to confirm the endpoint. Record the volume, Y, required. This titration, Y mL, multiplied by the empirical factor, K, appropriate to the particular apparatus and reagents in use (see calculation below), gives the acid equivalent not caused by acetic acid. The acetic acid equivalent is (V = KY) mL of 0.02N sodium hydroxide.
Obtain the empirical factor, K, for the apparatus by performing a blank determination in which the Hydroxypropyl Cellulose is omitted. The acidity of the blank for a given apparatus and given reagents is in a fixed ratio to the oxidizing equivalent of the distillate in terms of sodium thiosulfate: in which V B is the volume, in mL, of 0.02 N sodium hydroxide required in blank run; N 1 is the normality of the 0.02 N sodium hydroxide; Y B is the volume, in mL, of 0.02 N sodium thiosulfate required in blank run; and N 2 is equal to the normality of the 0.02 N sodium thiosulfate. Calculate the percentage of hydroxypropoxy groups (-OCH 2 CHOHCH 3 ) by the formula: in which V A is the volume, in mL, of 0.02 N sodium hydroxide required for titration of the sample; N 1 is the normality of the 0.02 N sodium hydroxide; K is the empirical factor; Y A is the volume, in mL, of 0.02 N sodium thiosulfate required for titration of the sample; N 2 is the normality of the 0.02 N sodium thiosulfate; and W is the quantity, in g, of sample used. Each mL of 0.02 N sodium hydroxide is equivalent to 1.502 mg of hydroxypropoxy groups (-OCH 2 CHOHCH 3 ) [7].
The results obtained as a percentage of hydroxypropoxy content may be converted to terms of average molecular substitution of glucose units by means of the accompanying graph (Figure 3)

Calculations for Hydroxypropylcellulose
65.0 mg of hydroxypropylcellulose was swirled in 5 mL water for 5 min. To this was added 10 mL of 30g/70 mL Cr 2 O 3 solution to optimize dissolution of sample. Take ½ hr to reach operating temperature, with argon flowing, on oil bath, as per official monograph.
Titrated with 0.01947 N NaOH (as standardized with HCl) to pH 7.00. Sample was required 28.90 mL of NaOH. A blank was required 0.20 mL of NaOH. Then add 0.5 g NaHCO 3 and 10 mL of 2N H 2 SO 4 ; after production of CO 2 (g) ceases, add 1 g KI, stopper and shake, kept in dark for 5 min, and titrated with 0.020M thiosulfate. Sample was required 0.80 mL thiosulfate; blank was required 0.02 mL thiosulfate (Table 1)