SOP: Determination of Equivalent Weight of Pectin

GIFSQ — Global Institute for Food Safety & Quality

Principle

The equivalent weight is used for calculating the anhydrouronic acid content and the degree of esterification. It is found by titrating the free carboxyl groups of the pectin with standard sodium hydroxide solution to pH 7.5, using either phenol red or Hinton’s indicator.

Apparatus required

• 250-ml conical flask
• Mortar

Reagents

a. Ethanol (5 ml to moisten the sample).

b. 0.1 N standard sodium hydroxide.

c. Phenol red indicator: grind 0.1 g of the dry powder in a mortar with 28.2 ml of 0.01 M NaOH; dilute to 250 ml with distilled water. (Use either this or Hinton’s indicator.)

d. Hinton’s indicator: mix together 20 ml of 0.4% bromothymol blue, 60 ml of 0.4% phenol red, 20 ml of 0.4% cresol red and 20 ml of distilled water; use the sodium salts of the indicators for preparing the solutions. (Use either this or phenol red indicator.)

e. Sodium chloride (1 g, to sharpen the end point).

f. Carbon dioxide-free distilled water: boil distilled water for 15 min; cool to room temperature; protect from atmospheric carbon dioxide.

Sample preparation

Use pectic substance that is ammonia- and ash-free.

Procedure

Weigh 0.5 g of the pectic substance (ammonia- and ash-free) into a 250-ml conical flask

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Moisten with 5 ml of ethanol

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Add 1 g of sodium chloride to sharpen the end point; add 100 ml of carbon dioxide-free distilled water and 6 drops of phenol red or Hinton’s indicator; make sure that all the pectic substance has dissolved and that no lumps are retained on the sides of the flask

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Titrate slowly (to avoid possible deesterification) with 0.1 N NaOH until the colour of the indicator changes (pH 7.5); the colour change should persist for at least 30 sec; Hinton’s indicator gives a magenta end point

Calculation

Equivalent weight = (Wt of sample × 1000) / (ml of alkali × Normality of alkali)

Where,

• Wt of sample = weight of the pectic substance taken (g)
• ml of alkali = volume of 0.1 N NaOH used in the titration (ml)
• Normality of alkali = normality of the sodium hydroxide solution used

Precautions

• Use ammonia- and ash-free pectic substance.
• Titrate slowly to avoid possible deesterification.
• Make sure no lumps of pectic substance are retained on the sides of the flask.
• The colour change at the end point should persist for at least 30 sec.
• The neutralized solution can be used for methoxyl determination.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Owens, H.S., R.M. McCready, A.D. Shepherd, S.H. Schultz, E.L. Pippen, H.A. Swenson, J.C. Miers, R.F. Erlandsen & W.D. Maclay, Methods used at Western Regional Research Laboratory for Extraction and Analysis of Pectic Materials, AYC-340, Western Regional Research Laboratory, Albany, California (1952) — cited in the book.