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Water, simply hydrogen atoms chemically bonded to an oxygen atom. It is one of the nutrients vital to the body, and is present in all foods but importantly in varying quantities. In foods, it can be found in three forms:
Bound Water- chemically bonded water found in crystals or as hydrates
Adsorbed Water-water with no chemical interaction to the food but has physical presence usually in the form of a monolayer to the food's surface.
Free Water -this form of water is present in the matrix of the food and is an entirely separate entity.
Albeit water is not technically a nutrient, it is responsible for 50% of our body weight and plays many important roles in the body, namely acting as a solvent, providing a medium for reactions and being a mobiliser of reactants just to name a few. The understanding of these types of water and the amount of water present in foods today are quite essential in the advancement of technological process based on food preservation and safety. Due to this, methods needed to be developed which would be useful in determining the water content in foods and to a lesser extent the forms of water present in food.
Considerably, there are many techniques which can be used to ascertain the moisture content of foods but they are basically of two types: volumetric and gravimetric methods. These two type of methods envelopes all the available methods as they fall under either one of these categories. Gravimetric methods usually involve actually drying the sample and the difference in weight is assumed the amount of water that was present in the sample. Examples of gravimetric methods would be the Air and Vacuum Oven Methods for determining moisture content. Volumetric methods on the other hand involve the reflux of a food sample with a liquid immiscible with water and thus the condensed water can be separated and its volume measured. Example of this method is the Dean and Stark Method. Importantly, it must be noted all moisture determination techniques would not be suitable for all samples, factors such as the physical properties of the sample and the time it takes to obtain a result would be considered.
A popular method of moisture determination in food is the Karl Fischer Method. Karl Fischer was a German chemist who worked at a petroleum company in the 1930s. He is responsible for developing this technique with improvements to this technique done by Eugen Scholz and Helga Hoffman. Karl Fischer Titration (titration is a technique which determines the concentration of a substance by reacting it with a solution of known concentration and is often indicated by a colour change or electrical measurement) is a technique used in the analysis of moisture content in solids, liquids or gases (Aurand,2010). It revolves around the quantitative reaction between water, iodine and sulphur dioxide occurring in the company of a molecularly small alcohol (like methanol) and an organic base like pyridine and can be done either by volumetric or the coulometric titration method. The Karl Fischer Titration is popular and widely accepted due to certain key advantages that it possesses. These include
High Accuracy and Precision
Small sample size
Short analysis duration
Fitness for use with solids, liquids and gases
Selectivity or determination of micro or trace amounts of water
It is however restricted by the fact that the method must be able to adapt to specific samples.
Principle of Karl Fischer Titration
Fundamentally, the reaction between iodine and sulphur dioxide in a aqueous medium is what this technique is based on. The reaction which occurs is as follows:
I2+ SO2+2H2O → 2HI+ H2SO4
Fischer realized that modifications to this reaction make it a simple way of determining the amount of water present in a non-aqueous system comprising excess sulphur dioxide. In modifying this reaction, he used methanol (a primary alcohol), which acted as a solvent and pyridine (a base) which acted as a buffering agent. The modified reaction now read:
py ·I2+ py ·SO2+H2O+ py → 2py ·HI+ py ·SO3
with py representing pyridine.
Classically, when pyridine is used in Karl Fischer Titration, it is a reagent which is basic in nature and is a ligand which complexes the I2 and SO2, ultimately lowering the vapour pressure of both I2 and SO2, thus displacing the equilibrium further to the right of the reaction equation.
While the titration is going on pySO3 may react with water which changes the stocichiometry of H2O and I2 from 1:1 to 2:1.
py+·SO3- +H2O → C5H5NH +SO4H-
In order to negate this, excess anhydrous methanol is added to the reaction since it has the ability to react with py+.SO3- which ultimately decreases the concentration of py+.SO3-
Pyridine is a carcinogen and noxious reagent. In today's experiments however, mostly pyridine free reagents which also contain imidazole or primary amines are employed. Methanol can also be substituted with 2-(2-Ethoxyethoxy) ethanol or another compatible alcohol.
Basically, what occurs in this reaction is the alcohol reacting with sulphur dioxide and the base and forming an alkyl sulphite salt as the intermediate. This intermediate then undergoes oxidation by iodine to form an alkyl sulphate salt. It is this oxidation reaction which consumes water. The reaction is as follows:
Simplifying this, this technique is based on the redox reaction occurring between iodine and sulphur dioxide in the company of a small chain alcohol and an organic base. It occurs in two steps:
Step 1:the alcohol, suphur dioxide and base(RN) react and forms an alkylsulphite intermediate
CH3OH + SO2 + RN « [RNH]SO3CH3
Step 2: the alkylsulphite reacts with iodine and the water present in the sample:
[RNH]SO3CH3 + I2 + H2O + 2RN « [RNH]SO4CH3 + 2[RNH]I
And since water and iodine are equimolar in consumption, once the amount of iodine used up is known then the amount of water that was existent in the sample would be known.
The Karl Fischer Titration technique is pH sensitive so therefore care must be taken in controlling the pH of the solution since the reaction rate is directly dependent on the pH of the solvent. The reaction occurs relatively normal between pHs' of 5 to 8 with any pH lower than 5 causing a reduction of the speed of the titration while any pH higher than 8 results in an increase titration rate.
The Karl Fischer reagent is very unstable and decomposes on standing ( especially rapid just after preparation) and it is therefore recommended that the reagent is prepared a day or two before use and standardizing it against a standard solution of water in methanol.
Types of Karl Fischer Titration
There are two types or approaches to the Karl Fischer titration technique. These methods are the:
Volumetric titration method
Coulometric titration method
The Volumetric Titration Method
With this method, the iodine being used in the reaction is dissolved beforehand in the water determination test solution (TS) and the water content is ascertained by measuring the amount of iodine used up since the amount of water present is equivalent to the iodine consumed.
Apparatus: For this method an automatic burette, a back titration flask, a stirrer and equipment necessary for amperometric titration at constant voltage or potentiometric titration at constant current is employed. The apparatus used in this method should be protected against atmospheric moisture since the water determination test solution is hygroscopic. Calcium chloride or silica gel is used for moisture protection usually.
Procedure: The temperature used for the standardization of the test solution should be the same temperature used for the titration of the sample so as to protect it from moisture.
A variable resistor in circuit should be used with the above mentioned apparatus and the resistor is manipulated so as to apply a definitive voltage (mV) between two platinum electrodes which are submerged in the solution awaiting titration. The difference in current (μA)is measurable during the dropping of the water determination test solution during the Amperometric titration at constant voltage. During the titration sudden changes in current in the circuit is noted with normalcy in returning very quickly but as the titration culminates the difference in current occurs for a given time which is usually longer than half a minute and this is where the end point of the titration is determined.
Alternatively, in potentiometric titration at constant current, when adjusting the resistor a definite current passes within the two platinum electrodes and any change in potential (mV) is recorded during the dropping fthe water determining TS. As the titration progresses, the indicated value shown on the potentiometer in the circuit falls abruptly from a polarization state of several hundred (mV) to a non-polarization state but returns to normalcy within seconds. At the culmination of the titration, the non-polarization state exists continuously (longer than 30 seconds) and allows for the determination of the end point of the titration when this state arises.
When using back titration with the amperometric titration method at constant voltage, the micro ammeter needle is out of scale while excessive amounts of water determination TS is left behind. But it undergoes a rapid return to its initial position when the end point of the titration is reached. The same thing occurs when the potentiometric titration method is used. A definite voltage is applied when the end point is reached.
Titration of water with water determination TS is done using one of two methods, namely:
Direct Titration: 25ml of methanol is placed in a dry titration flask and titrated with water determining TS until the end point is reached. The sample is then weighed and transferred immediately into the titration flask and stirred until it dissolves. The solution is then titrated water determination TS up to the end point with vigorous stirring.
When the sample interacts with the Karl Fischer reagent, the water in the sample can be removed through heating or by passing it under a stream of nitrogen gas with introduction into the titration vessel being done with a water evaporation device.
The following equation is used to calculate the amount of water:
Water = volume of TS for water determination consumed x f (mg) X 100%
Weight of sample (mg)
Where f= the number of mg of water (H2O) corresponding to 1 ml of water determination TS.
Back Titration: methanol is placed in a dry titration vessel and titrated against water determination TS. Then a quantity of the sample is accurately weighed and transferred immediately into the reaction vessel. There is then the addition of an excessive amount of water determination TS followed by 30 minutes of stirring whilst protecting the solution from atmospheric moisture. It is then titrated with Water-Methanol Standard Solution under vigourous stirring.
The following equation below can be used to calculate the amount of water:
Water (H2O) =
Weight of Sample (mg)
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
The Coulometric Titration Method
Here, the iodine is firstly produced by electrolysis of the iodide containing reagent and the water content in the sample is determined by the measurement of the quantity of electricity that is required for the electrolysis and is based on the quantitative reaction of water an d the produced iodine.
Apparatus: This is made up of an electrolytic cell used mainly for the production of iodine, a stirrer, a titration flask and a potentiometric titration set up at constant current. The electrlyitc cell comprises an anode and cathode parted by a diaphragm. The anode is submerged in the anolyte solution and the cathode in the catholyte solution, both for water determination with both electrodes usually being made up of platinum mesh.
Again the test solution should be guarded against atmospheric moisture.
Procedure: An anolyte for water determination is placed in a titration vessel and a pair of platinum electrodes are immersed in this solution, at constant current. The iodide production system which is filled with catholyte for water determination is immersed in the anolyte solution.
The electrolytic system is then switched on then the contents of the titration vessel are made anhydrous. Then a weighed sample is added quickly to the vessel and dissolved by stirring. The titration is done under vigorous stirring up to the end point. The amount of electricity need for this iodine production is measured and used to calculate the water content of the sample.
The direct relationship between coulombs and iodine enables us to determine the amount of iodine produced whilst titrating. The relationship which exists is:
Current (A) x time (sec) = Coulombs (C) I + W KF REACTION
According to Faradays Law:
2 x 96,485 Coulombs are needed to generate 1 mole of iodine and this iodine subsequently reacts 1 to 1 with the water in the KF reaction