Hübl method The basic principle of iodine value was originally introduced in 1884 by A. V. Hübl as "
Jodzahl". He used iodine alcoholic solution in presence of
mercuric chloride () and
carbon tetrachloride () as fat solubilizer. The residual iodine is titrated against sodium thiosulfate solution with starch used as endpoint indicator. This method is now considered as obsolete.
Wijs/Hanuš method J. J. A. Wijs modified the Hübl method by using
iodine monochloride (ICl) in glacial acetic acid, which became known as ''Wijs's solution'', dropping the reagent. :I2{} + \underset{(blue)}{starch}{} + 2 Na2S2O3 -> 2 NaI{} + \underset{(colorless)}{starch}{} + Na2S4O6 is calculated from the formula : :\textrm{IV} = \frac{(\textrm{B} - \textrm{S}) \times \textrm{N} \times 12.69}{\textrm{W}} :Where: :* is the difference between the volumes, in
mL, of sodium thiosulfate required for the blank and for the sample, respectively; :* is the
normality of sodium thiosulfate solution in Eq/ L; :*12.69 is the conversion factor from
mEq sodium thiosulfate to grams of iodine (the
molecular weight of iodine is ); :* is the weight of the sample in grams. The determination of IV according to Wijs is the official method currently accepted by international standards such as
DIN 53241-1:1995-05,
AOCS Method Cd 1-25,
EN 14111 and
ISO 3961:2018. One of the major limitations of is that halogens does not react stoichiometrically with
conjugated double bonds (particularly abundant in some
drying oils). Therefore, Rosenmund-Kuhnhenn method makes more accurate measurement in this situation.
Kaufmann method Proposed by H. P. Kaufmann in 1935, it consists in the
bromination of the double bonds using an excess of
bromine and anhydrous
sodium bromide dissolved in
methanol. The reaction involves the formation of a
bromonium intermediate as follows: Then the unused bromine is reduced to
bromide with iodide (). :Br2 + 2 I- -> 2 Br- + I2 Now, the amount of iodine formed is determined by back-titration with sodium thiosulfate solution. The reactions must be carried out in the dark, since the formation of bromine radicals is stimulated by light. This would lead to undesirable side reactions, and thus falsifying a result consumption of bromine. For educational purposes, Simurdiak et al. (2016) The original method uses
pyridine dibromide sulfate solution as halogenating agent and an incubation time of 5 min.
Other methods Measurement of iodine value with the official method is time-consuming (incubation time of 30 min with Wijs solution) and uses hazardous reagents and solvents. :\text{IV} =\frac{2 \times 126.92 \times \text{no. of double bonds} \times 100}{\text{molecular weight}} Accordingly, the IVs of
oleic,
linoleic, and
linolenic acids are respectively 90, 181, and 273. Therefore, the IV of the mixture can be approximated by the following equation : IV can be also predicted from
near-infrared,
FTIR and
Raman spectroscopy data using the ratio between the intensities of and bands. High resolution
proton-NMR provides also fast and reasonably accurate estimation of this parameter. == Significance and limitations ==