The term “iodometry” describes the type of titration that uses a standardised sodium thiosulfate solution as the titrant, one of the few stable reducing agents where oxidisation of air is concerned. Iodometry is used to determine the concentration of oxidising agents through an indirect process involving iodine as the intermediary. In the presence of iodine, the thiosulphate ions oxidise quantitatively to the tetrathionate ions.
To determine the concentration of the oxidising agents, an unknown excess of potassium iodide solution is added to the weakly acid solution. The iodine, which is stoichiometrically released after reduction of the analyte, is then titrated with a standard sodium thiosulphate solution (Na2S2O3).
Titration involving iodine commonly uses a starch suspension as indicator. This suspension is a watery solution of starch with a few drops of bactericide added to prevent decomposition, as this would stop the starch behaving as an indicator.
Once the bond between the iodine (I2) and the helical chain of beta-amylose is formed it turns an intense blue.
Colour of the starch solution in the presence of I2. In the presence of I- ions the starch solution is colourless. Source: Istituto comprensivo di Tubirgo
Iodometric titration needs to be done in a weak acid environment which is why we need to remember that:
1. The iodine solution used needs to be at pH < 8.5 because at a base pH iodine disproportionates (a particular kind of oxidoreduction reaction where one substance partly oxidises and partly reduces);
2. Sodium thiosulphate needs a neutral or weak acid environment to oxidise with tetrathionate (in an alkaline solution we would get sulphate oxidation);
3. In a strong acid environment thiosulphate decomposes to S2;
4. In acid environments the iodide is oxidised to iodine as in the reaction below:
O2 + 4I- + 4H+ ↔2I2 + 2H2O
In the food industry, iodometry is widely used to determine the concentration of hydroperoxides in any given lipid matrix (oils and fats for human consumption).
Oxidation is a chemical process which is catalysed by various factors (presence of oxygen, levels of unsaturation in the oil, presence of metals, temperature) and leads to the formation of hydroperoxides. Determining the concentration of these chemicals is important because hydroperoxides have a negative effect on the acceptability of the fat matrix used, and on the food fried in it, and they also decompose easily, forming molecules which are dangerous for human health.
Initiation: in the presence of a catalyst, alkyl radicals (L·) form and these tend to accumulate.
Propagation: the alkyl radicals which have formed react with atmospheric oxygen to form a peroxyl radical (LOO· ). This radical can react with another available hydrogen atom (LH) to form another free radical (L·) and a hydroperoxide (LOOH).
Termination: hydroperoxides, which are highly unstable chemicals, decompose to produce additional free radicals and /or secondary oxidation products which accumulate and so increase the rancidity of oil.
Hydroperoxides in the presence of KI reduce as shown in the redox reaction below. The reaction is illustrated as the sum of the two half-reactions in fig. 1.
The iodine released is titrated using sodium thiosulphate at a known concentration with a starch indicator (blue colour). The number of equivalents of titrated iodine is the same as the number of hydroperoxides present in the sample as shown in the reaction in fig. 2. Thiosulphate is added until the blue colour disappears and the solution turns colourless. The turning point indicates that all the iodine released has been titrated.
The number of peroxides (NP) expressed in meq/Kg results from the following calculation:
V = ml of standard solution of Na2S2O3 used;
N = normality of Na2S2O3 solution;
p = weight of oil expressed in grams.
The term iodimetry, on the other hand, refers to titration using an iodine solution and is useful for determining substances that have reducing properties. The half-reaction is as follows:
I3- + 2e- ↔ 3I- E0 = 0.536 V
I3- : triodide ion (iodide ions have to be added to increase the solubility of iodine in water and these form the triodide complex).
Standard iodine solutions are of fairly limited use compared to oxidants because of their small electrode potential. This characteristic of the I3- /I- pair can sometimes be an advantage, however, because it makes it selective and therefore means that strong reducing agents can be determined in the presence of weak ones.
One interesting application of iodometry in the food industry is for determining sulphur dioxide (SO2) in wine.
Sulphur dioxide has several important functions:
Sulphur dioxide is added to the must and wine in the form of salts, like potassium bisulphate (KHSO3) which contains 53% in weight of sulphur dioxide, and the potassium metabisulphite (K2S2O5) which has a 69.5% concentration of sulphur dioxide.
The legal limit for white and rose wines is 210mg/L, and for reds 160mg/L.
Once sulphur dioxide is added to wine it does not remain free but oxidates in part and in part combines with other molecues:
Free SO2: found as such, or in the form of sulphurous acid (H2SO3) or potassium bisulphite, which is less efficient than gaseous sulphur dioxide and has no smell. The free form (either as a gas or an acid) is the most important because it inhibits the action of microoganisms and acts as an antioxidant.
Oxidated sulphur dioxide appears in the form of sulphur trioxide (SO3), sulphuric acid or potassium bisulphate.
Combined SO2: has no smell or taste and results from combination with substances which have aldehyde and/or ketone groups to form bisulphate compounds. The SO2, therefore, can combine with sugars, proteins and polyphenols. Combined sulphur dioxide is in equilibrium with the free form. This means that any reduction in the free form will result in a significant quantity of the combined form moving towards the free form. Thi is another of the advantages of using sulphur dioxide because it guarantees the stability of the product over time.
The total sulphur dioxide in the wine sample is determined through direct iodimetric titration using starch as the indicator. Before proceeding to titration, a 10% solution of sulphuric acid is added to the wine, thus reducing the sulphur trioxide (SO3) to sulphur dioxide (SO2). The titration reaction can be represented as:
SO2 + I2 + H2O → 2I- + SO3 + 2H+
The addition of an excess of I2 makes the solution turn dark blue indicating that all the sulphur dioxide in the sample has been titrated.
With red wines it is difficult to see the equivalence point because their intense red colour makes it difficult to perceive the colour change in the indicator. In this case it is better use potentiometric titration which indicates the equivalence point on the basis of changes to the solution’s potential.
A = ml of I2 used in the titration
N = normality of titrant solution
V = ml of wine used in the analysis
32 = equivalent weight of SO2The calculation is applied for the three forms of SO2, the sum equals total SO2.
13. Mohr method
14. Vohlard method