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It is important that the water content of both raw and finished goods be determined at all stages of the manufacturing process since the quality of the product depends on it. The presence of unwanted moisture in products such as kerosene, transformer insulation oil and brakes oil, can have disastrous consequences.
In the pharmaceutical industry, it is important to know the amount of water present in the active ingredients of a drug in order to correctly predict its lifetime, stability and effectiveness. In the food industry, the water content of both raw materials and the finished foodstuff needs to be carefully monitored. The technique most commonly used for these analyses is the Karl Fischer titration. This titration is based upon the oxidation of sulphur dioxide by iodine in the presence of water.
Theory of determining moisture content
This is a technique used to determine the concentration of a substance in solution by adding to it a standard reagent of known concentration in carefully measured amounts until a reaction of definite and known proportion is completed, as shown by a color change or by electrical measurement, and then calculating the unknown concentration.
What is Karl-Fischer titration?
Karl Fischer is an analytical technique designed to measure the water/moisture content in solids, liquids or gaseous substances. It involves the reaction of water, iodine and sulphur dioxide in the presence of an alcohol such as methanol and an organic base such as anhydrous pyridine.
Reaction 1: CH30H + SO2 +RN ↔ [RNH]SO3CH3
Reaction 2: [RNH]SO3CH3 + I2 + H2O + 2RN ↔ [RNH]SO4CH3 + 2[RNH]I
The first reaction involves the formation of the akylsulfite intermediate. In the second reaction, the akylsulfite reacts with the water and iodine.
In the second reaction, water and iodine are used up in equimolar amounts. As such, if the amount of iodine consumed is known, then the amount of water present in the sample will also be known.
Types of Karl-Fisher titration
Volumetric titration: In the volumetric titration method the iodine is present within the reagent. Iodine, required for reaction with water, is previously dissolved in water, and the water content is determined by measuring the amount of iodine consumed as a result of reaction with water in a sample. Volumetric titration is used in samples containing 0.1% - 100% of water. The volumetric system is used for the titration of solids, liquids and gases and it allows for the modification of temperature.
Coulometric titration: In the coulometric titration method, the iodine is produced at an electrode. Iodine is generated by the electrolysis of the reagent containing iodide ion, and then, the water content in a sample is determined by measuring the quantity of electricity which is required for the electrolysis (the production of iodine), based on the quantitative reaction of the generated iodine with water. Coulometric titration is used in samples containing 0.001% - 0.1% of water. The coulometric system is used for the titration of liquids and gases and has no option for the modification of temperature.
Both methods use bi-potentiometric titration to measure the amount of iodine consumed by the water.
Bi-potentiometric titration: is a technique designed to track the extent of a reaction by measuring the changes in electrical conductivity of the reaction solution.
Bi-potentiometric titration: determining the amount of iodine
Consider the second reaction that occurs in Karl Fischer titration:
[RNH]SO3CH3 + I2 + H2O + 2RN ↔ [RNH]SO4CH3 + 2[RNH]I
This is a redox reaction since both oxidation and reduction is occurring. The sulphur in the akylsulfite has been oxidized and the iodine has been reduced.
Oxidation reaction: SO3CH32- ® SO4CH32- + 2e- reducing agent
Reduction reaction: I2 + 2e- ® 2I- (E°= 0.54 V) oxidizing agent
From the two oxidation and reduction equations, it can be seen that sulphur has changed oxidation state from +4 to +6 whereas iodine has moved from o to -1. As such, alkylsulfite is the reducing agent and iodine is the oxidizing agent.
The reduction of iodine causes iodine to take up electrons produced by the oxidation of sulfur, and this causes a change in the electrical potential of the system. The change in potential is detected by an electrode called the double platinum electrode).
Practice of determining moisture content
Apparatus: Automatic burette, titration flask, back-titration flask, stirrer, equipment for bi-potentiometric titration
Reagents: sulphuric acid, water, iodine, menthanol and pyridine.
Procedure: Direct titration
Measure 25ml of methanol for water determination and place in a dried titration flask. Titrate with water determination TS (a reagent) to the end point.
Accurately weigh a quantity of the sample containing 10 to 50mg of water then transfer it quickly into the titration flask, and dissolve by stirring.
Titrate the solution with water determination TS to the end point under vigorous stirring. When the sample is insoluble in the solvent, powder the sample quickly, accurately weigh a suitable amount of the sample, and transfer it quickly into the titration vessel, stir the mixture for 30 minutes while protecting it from moisture.
Perform a titration under vigorous stirring. When the sample interferes with the Karl Fisher reaction, water in the sample can be removed by heating under a stream of nitrogen gas and introduced into the titration vessel by using a water-evaporation device.
Take 20ml of methanol for water determination in the dried titration vessel, and titrate with water determination TS.
Accurately weigh a suitable quantity of the sample containing 10
50 mg of water, transfer the sample quickly into the titration vessel, add an excessive and definite volume of water determination TS.
Stir for 30mins, protecting from atmospheric moisture, and then titrate the solution with Water
Methanol Standard Solution under vigorous stirring.
Where: f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS,
f* = the number of mg of water (H2O) in 1 ml of Water
Methanol Standard Solution
Apparatus: An electrolytic cell for the production of iodine, a stirrer, titration flask and a potentiometric titration system.
Reagents: sulphuric acid, water, iodine, menthanol and pyridine.
Procedure: 1. Take a suitable volume of an anolyte for water determination in a titration vessel.
2. Immerse in this solution a pair of double platinum electrodes for potentiometric titration at constant current.
3. Then, immerse the iodide production system filled with a catholyte for water determination in the anolyte solution.
4. Switch on the electrolytic system and make the content of the titration vessel anhydrous.
5. Next, take an accurately weighed amount of the sample containing 1
5mg of water, add it quickly to the vessel, and dissolve by stirring.
6. Perform the titration to the end point under vigorous stirring.
7. When the sample is insoluble in the anolyte, powder it quickly, and add an accurately weighed amount of the sample to the vessel.
8. After stirring the mixture for 5
30 minutes, while protecting from atmospheric moisture, perform the titration with vigorous stirring.
9. Determine the quantity of electricity (C) [electric current (A) - time (s)] required for the production of iodine during the titration, and calculate the percentage water content in the sample. When the sample interferes with the Karl Fisher reaction, water in the sample can be removed by heating under a stream of nitrogen gas, and introduced into the titration vessel by using a water-evaporation device.
Because water determination TS (a reagent) is extremely hygroscopic, the titration apparatus should be protected from atmospheric moisture. For this reason, silica gel or calcium chloride for water determination is usually used for moisture protection.