Peroxidegetal

Bepaling van het peroxidegetal

2.5.5. PEROXIDE VALUE

The peroxide value I_P is the number that expresses in milliequivalents of active oxygen the quantity of peroxide contained in 1000 g of the substance as determined by the methods described below.

Place 5.00 g (m g) of the substance to be examined in a 250 ml conical flask fitted with a ground-glass stopper. Add 30 ml of a mixture of 2 volumes of chloroform R and 3 volumes of glacial acetic acid R. Shake to dissolve the substance and add 0.5 ml of saturated potassium iodide solution R. Shake for exactly 1 min then add 30 ml of water R. Titrate with 0.01 M sodium thiosulphate adding the titrant slowly with continuous shaking until the yellow colour is almost discharged. Add 5 ml of starch solution R and continue the titration, shaking vigorously, until the colour is discharged (n1 ml of 0.01 M sodium thiosulphate). Carry out a blank test under the same conditions (n2 ml of 0.01 M sodium thiosulphate). The volume of 0.01 M sodium thiosulphate used in the blank titration must not exceed 0.1 ml.

I_p = 10 ( n₂ - n₁) / m

Place 50 ml of a mixture of 2 volumes of trimethylpentane R and 3 volumes of glacial acetic acid R in a conical flask and replace the stopper. Swirl the flask until the substance to be examined (m g; see Table 2.5.5.-1) has dissolved. Using a suitable volumetric pipette, add 0.5 ml of saturated potassium iodide solution R and replace the stopper. Allow the solution to stand for 1 min � 1 s, thoroughly shaking the solution at least three times, then add 30 ml of water R.

Table 2.5.5.-1

Titrate the solution with 0.1 M sodium thiosulphate (V1 ml), adding it gradually and with constant, vigorous shaking, until the yellow iodine colour has almost disappeared. Add about 0.5 ml of starch solution R1 and continue the titration, with constant shaking especially near the end-point, to liberate all of the iodine from the solvent layer. Add the sodium thiosulphate solution dropwise until the blue colour just disappears.

If the titration uses less than 0.5 ml of 0.1 M sodium thiosulphate repeat the determination using 0.01 M sodium thiosulphate (V1 ml) with vigorous, constant shaking.

NOTE: There is a 15 s to 30 s delay in neutralising the starch indicator for peroxide values of 70 and greater, due to the tendency of trimethylpentane to float on the surface of the aqueous medium and the time necessary to adequately mix the solvent and the aqueous titrant, thus liberating the last traces of iodine. It is recommended to use 0.01 M sodium thiosulphate for peroxide values below 150. A small amount (0.5 per cent to 1.0 per cent (m/m)) of high HLB emulsifier may be added to the reaction mixture to retard the phase separation and decrease the time lag in the liberation of iodine.

Carry out a blank determination (V0 ml). If the result of the blank determination exceeds 0.1 ml of titration reagent, replace the impure reagents and repeat the determination.

IP = 1000 ( V₁ - V₀ ) c / m

The saponification value IS is the number that expresses in milligrams the quantity of potassium hydroxide required to neutralise the free acids and to saponify the esters present in 1 g of the substance.

Introduce the prescribed quantity of the substance to be examined (m g) into a 250 ml borosilicate glass flask fitted with a reflux condenser. Add 25.0 ml of 0.5 M alcoholic potassium hydroxide and a few glass beads. Attach the condenser and heat under reflux for 30 min, unless otherwise prescribed. Add 1 ml of phenolphthalein solution R1 and titrate immediately with 0.5 M hydrochloric acid (n1 ml of 0.5 M hydrochloric acid). Carry out a blank test under the same conditions (n2 ml of 0.5 M hydrochloric acid).

I_S = 28.05 (n₂ - n₁) / m

10 september 1999