________ is used to estimate the presence of unsaturation in oils and fats.

Question

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Correct — option (2), 'Iodine value'. The iodine value of an oil or fat is defined as the mass of iodine, in grams, taken up by a hundred grams of the sample, and it measures unsaturation directly. The chemistry behind the definition is the addition reaction of a halogen across a carbon-carbon double bond: each double bond in the fatty acid chains absorbs one molecule of halogen, so the quantity of iodine consumed is a straightforward count of the double bonds present. A highly unsaturated oil therefore has a high iodine value and a saturated fat a low one, which is why the number tracks the familiar distinction between liquid oils and solid fats — the more double bonds a chain carries, the more its kinks prevent close packing, and the lower the melting point. The measurement has practical uses beyond the laboratory. Drying oils used in paints and varnishes are identified by a high iodine value, because it is the unsaturation that lets them polymerise on exposure to air. The hydrogenation of vegetable oils to make vanaspati works by saturating those same double bonds with hydrogen over a nickel catalyst, so the iodine value of the product is markedly lower than that of the oil it was made from, and the fall in iodine value is the standard way of following the process. The other three quantities offered all say something about an oil, but none of them counts double bonds.

Why the other options are wrong — (a) The saponification value is the number of milligrams of potassium hydroxide needed to saponify one gram of the fat — that is, to hydrolyse its ester linkages into glycerol and the salts of the fatty acids, which are soap. What it measures is the average chain length, and therefore the average molecular mass, of the fatty acids present: a fat built from short chains has more ester groups per gram and so consumes more alkali, giving a high saponification value, while one built from long chains gives a low one. It is entirely indifferent to whether those chains contain double bonds, which is precisely the information the question asks for.  (c) The acid value is the number of milligrams of potassium hydroxide required to neutralise the free fatty acids present in one gram of the sample. It measures how much of the fat has already been broken down out of its ester form, and so it is used as an index of deterioration — a rising acid value in a stored oil signals hydrolytic rancidity and poor keeping quality. The quantity being counted is free acid groups, not double bonds; an oil can be thoroughly unsaturated and perfectly fresh, with a high iodine value and a very low acid value.  (d) Viscosity is a physical property, a measure of a liquid's resistance to flow, and it is not one of the standard analytical constants by which oils and fats are characterised in the way that the iodine, saponification and acid values are. It does vary with composition and strongly with temperature, so it cannot serve as a specific measure of unsaturation: two oils of quite different double-bond content can be brought to the same viscosity simply by warming one of them. The option is included as the one choice that does not belong to the family of chemically defined values at all.

Concept: Oils and fats are triglycerides — esters of glycerol with three fatty acid chains — and they are characterised in the laboratory by a small set of defined numbers, each of which isolates one property of those chains. The iodine value counts double bonds, since halogen adds across each one, and so measures unsaturation; it is high for liquid oils such as linseed, sunflower and safflower and low for solid fats. The saponification value measures the alkali needed to hydrolyse one gram of the fat and therefore reflects the average molecular mass of the fatty acids, being high for short-chain fats such as coconut oil and butter. The acid value measures free fatty acid and so indicates rancidity and storage quality. Behind all three lies the structural point that governs the whole subject: saturated chains are straight and pack closely, giving higher melting points and solids at room temperature, while unsaturated chains carry cis double bonds that put a permanent bend in the molecule, prevent close packing and give liquids. Hydrogenation removes those bends by adding hydrogen across the double bonds over a nickel catalyst, converting an oil into a semi-solid fat; the by-product of partial hydrogenation is the formation of trans fatty acids, which is why the process has become a public health as well as an industrial subject.

State PCS 2024 Q70 — Science & Technology PYQ with Answer

State PCS 2024 Q70 — Science & Technology PYQ with Answer