SOP: Thiocyanogen Value of Oils and Fats
GIFSQ — Global Institute for Food Safety & Quality
Principle
The thiocyanogen (SCN) value (T.V.) is a measure of the unsaturation of fats and oils, expressed as the equivalent number of centigrams of iodine absorbed per gram of sample (% iodine absorbed). Thiocyanogen does not add to fatty acids such as linoleic and linolenic in the same proportion as iodine; hence, by combining the thiocyanogen value with the actual iodine value, the oleic, linoleic and linolenic acid contents can be calculated. The method is applicable to all fats which do not contain conjugated systems. With regard to the addition of thiocyanogen, 1 mole of fatty acid with 1 double bond absorbs 1 mole of SCN, the fatty acid with 2 double bonds absorbs approximately 1 mole, and with 3 double bonds absorbs 2 moles of SCN.
Apparatus required
- Florence flask, 3000 ml, with a cold-finger condenser
- Buchner funnel
- Amber coloured glass-stoppered bottle (for storing the thiocyanogen solution)
Reagents
a. Potassium iodide solution: dissolve 150 g of potassium iodide in water and make up to 1000 ml.
b. 0.1 N potassium dichromate solution.
c. 0.1 N Na₂S₂O₃ (sodium thiosulphate) solution.
d. Dehydrated glacial acetic acid: to 2000 ml of acetic acid in a 3000 ml Florence flask, add 100 ml of acetic anhydride; insert a cold-finger condenser, circulate cold water through the condenser and boil on a hot plate or oil bath for 3 hr at about 135°C; cool and store in a glass-stoppered bottle.
e. Lead thiocyanate, Pb(SCN)₂: dissolve 250 g of neutral lead acetate in 500 ml of distilled water and 250 g of potassium thiocyanate in another 500 ml of distilled water; mix the two solutions with continuous stirring; filter through a Buchner funnel and wash the precipitate with distilled water and then with ether; dry the precipitate in a desiccator over P₂O₅ for 8–10 days. The precipitate should be white or only slightly coloured; if it is considerably coloured, it should not be used.
f. 0.2 N thiocyanogen solution: suspend 50 g of lead thiocyanate in 500 ml of anhydrous acetic acid; dissolve 5.1 ml of bromine in another 500 ml portion of anhydrous acetic acid; add the bromine solution to the lead thiocyanate suspension in small portions with mixing until the lead thiocyanate solution is colourless; allow the precipitated lead bromide and excess lead thiocyanate to settle; filter using a Buchner funnel (refilter if needed to get a clear solution); store in an amber coloured glass-stoppered bottle at 18°C to 21°C.
g. Starch solution (indicator).
Procedure
Clean the glass apparatus thoroughly using cleaning mixture and dry; dry the oil or melted fat using anhydrous sodium sulphate and filter
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Weigh accurately about 0.2 to 1.2 g of the sample depending upon its thiocyanogen value (the higher the value, the smaller the weight taken)
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Add 25 ml of the thiocyanogen solution; mix and store in a dark place at 18 to 21°C for 24 hr; make at least 3 blank determinations alongside the sample
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Titrate one of the blanks immediately after adding potassium iodide and water (see below), and treat the other two exactly as the sample
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After 24 hr, remove the sample; add 1.66 g of dry powdered potassium iodide and mix by swirling for 2 min (mix the blanks for 3 min); add 30 ml of distilled water
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Titrate with 0.1 N Na₂S₂O₃ using 1 to 2 ml of starch solution as indicator
Precautions
- If the titre value of the thiocyanogen reagent exceeds by 0.2 ml of 0.1 N Na₂S₂O₃ between the titrations of the blanks carried out immediately and after 24 hr, the reagent is not satisfactory.
- The thiocyanogen method is intended for fats containing oleic acid, linoleic acid and linolenic acid; apart from these acids, only small percentages of higher unsaturated acids should be present, and conjugated double bonds should be absent.
Calculation
The thiocyanogen value is expressed in terms of the iodine value:
(Blank titre − Sample titre) × N of Na₂S₂O₃ × 12.69
T.V. = ───────────────────────────────────────────────────────────────
Wt of sample (g)
Calculation of oleic, linoleic and linolenic acid contents: the constants for the fatty acids are — linolenic acid (X): I.V. 273.7, T.V. 167.1; linoleic acid (Y): I.V. 181.1, T.V. 96.7; oleic acid (Z): I.V. 89.9, T.V. 89.3; saturated and unsaponifiable matter (S): I.V. 0, T.V. 0 (the I.V. and T.V. of unsaponifiable matter are not actually zero, but this is assumed for purposes of calculation).
When no linolenic acid is present:
181.1 Y + 89.9 Z = 100 × I.V. 96.7 Y + 89.3 Z = 100 × T.V. S = 100 − (Y + Z) Y = 1.194 × I.V. − 1.202 × T.V. Z = 2.421 × T.V. − 1.293 × I.V. S = 100 − (Y + Z)When linolenic acid is present: the saturated fatty acid content must be determined independently, so that three equations with three unknowns can be drawn up:
273.7 X + 181.1 Y + 89.9 Z = 100 × I.V. 167.1 X + 96.7 Y + 89.3 Z = 100 × T.V. X + Y + Z = 100 − S X = 1.5902 × T.V. − 0.12901 × I.V. + 1.3040 × S − 130.40 Y = 1.3565 × I.V. − 3.2048 × T.V. − 1.6423 × S + 164.23 Z = 1.6146 × T.V. − (I.V. term) − 0.6617 × S − 66.17
Note: in the available text, the Z equation of case 2 is garbled at the iodine-value term; it is carried here with that term shown as printed. The unsaponifiable matter’s I.V. and T.V. are assumed zero only for purposes of calculation. This method of calculation is used only when gas chromatographic facilities are not available for determining the fatty acid make-up.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Official and Tentative Methods, 3rd edn., American Oil Chemists’ Society, Vol. 1, Cd 2-38 (1975) — calculation reproduced with the permission of AOCS.