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MOISTURE DETERMINATION:

MOISTURE DETERMINATION PRESENTED BY G.PRASAD Ph. ANALYSIS UCPSC

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Why measure moisture content; Most natural products contain moisture. The water content per se is seldom of interest. Rather, it shows Whether a product intended for trade and production has standard Properties such as –Storability –Agglomeration in the case of powders –Microbiological stability –Flow properties, viscosity –Dry substance content –Concentration or purity –Commercial grade (compliance with quality agreements) –Nutritional value of the product –Legal conformity (statutory regulations governing food) Trade and industry are interested in the dry substance content o f Commercial goods . The water present in the product is taken into Account when the product is priced . Legal regulations and product Declarations define the difference between natural moisture and moisture added to the product.

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Selection of the suitable measurement method The selection of a suitable measurement method essentially depends on the following quantities: Requirements regarding accuracy, measurement range, repeatability, sensitivity –Type of bonding of the water –Desired information: water or moisture content –Measurement speed –Amount of sample –Physical properties of the sample (e.g. decomposition temperature) –Budget –Simplicity (operation or functionality) –Statutory regulations

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Methods of moisture content determination The moisture content influences the physical properties of a substance such as weight, density, viscosity, refractive index, electrical conductivity and many more. Over the years, a wide range of methods has been developed to measure these physical quantities and express them in the form of the moisture content. The measurement methods can logically be divided in the following procedures: –thermo gravimetric –chemical –spectroscopic

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Thermogravimetric methods Principle Thermo gravimetric methods are weighing-drying methods in which the samples are dried until their mass is constant. The mass change is interpreted as the evolved moisture. Drying ends with attainment of an equilibrium state, i.e. when the vapour pressure of the moist substance is equal to the vapour pressure of the surroundings . The lower the vapour pressure of the surroundings, the lower the residual moisture in the substance whenequilibrium is reached . Reducing the pressure lowers the vapour pressure of the surroundings and hence results in stricter drying conditions.

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Infrared drying Principle This method uses direct infrared radiation instead of hot circulating air to dry the sample. The infrared rays are absorbed by the sample and the absorbed energy causes the desired heating of the substance. Advantage; Important advantages of this method are the simple operating principle and the short drying time. Measurement results are available after a few minutes. Infrared dryers are supplied with an integrated balance and have a very compact and rugged construction

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Microwave drying; Principle; This method is based on the absorption of microwave radiation by the water molecules of the sample. This absorption generates heat and leads to vaporization of the volatile components . Measured variable is here also the total amount of volatile components. This is determined by weighing before and after drying

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Chemical methods 2.2.1 Karl Fischer titrations Principle The Karl Fischer method is used for many substances as a reference method. It is a chemical analysis procedure which is based on the oxidation of sulphur dioxide by iodine in a methanolic hydroxide solution. In principle, the following chemical reaction takes place: H2O + I2+ SO2+ CH3OH + 3RN →[ RNH]SO4CH3+ 2[RNH]I The titration can be performed volumetrically or coulometrically . In the volumetric method a Karl Fischer solution containing iodine is added until the first trace of excess iodine is present. The amount of iodine converted is determined from the burette volume of the iodine-containing Karl Fischer solution.

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In the coulometric procedure, the iodine participating in the reaction is generated directly in the titration cell by electrochemical oxidation of iodide until again a trace of unreacted iodine is detected. Faraday’s law can be used to calculate the amount of iodine generated from the quantity of electricity required. Application; The Karl Fischer titration is a moisture determination method specific for water and is suitable for samples with a high moisture content ( titrimetry ) and also for those with water contents in the ppm range ( coulometry ). It was originally developed for nonaqueous liquids , but is also suitable for solids if these are soluble or if the water they contain can be removed by heating in a stream of gas or by extraction

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Calcium carbide method; Principle In the calcium carbide method a sample of the moist substance is carefully mixed with excess calcium carbide resulting in the following reaction: CaC2+ 2H2O Æ Ca (OH)2 + C2H2 The amount of acetylene formed is determined either by measuring its volume or by the rise in pressure in a closed vessel

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Infrared spectroscopy for determination of the surface moisture Principle This moisture determination method is used solely to measure the surface moisture. A sample is irradiated with light ( electromagnetic radiation ). The intensity of the reflected spectrum forms the basis for determination of the moisture content. In the electromagnetic spectrum , the near infrared (NIR) region used comprises wavelengths between 800 nm and 2500 nm. In this region the three atom water molecule (H2O) has two prominent absorption bands at wavelengths 1.475 µm and 1.94 µm (harmonic vibrations and combination vibration). If a moist sample is irradiated with light of these wavelengths, part of the light is absorbed, a second part diffusely reflected and a third part passes through the sample (transmission).

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The diffuse reflected intensity is measured (spectrum), which is proportional (in a nonlinear relation) to the water concentration on the surface . With thin samples , the spectrum of the transmission is also evaluated on occasion.

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NMR (nuclear magnetic resonance) spectroscopy Principle 1H NMR spectroscopy determines the number of water nuclei in a substance. This can then be used to decide the amount of water in the sample. Two different types of 1H NMR spectroscopy are always used for the moisture content determination. In the first type the substance under investigation is positioned in a high frequency alternating magnetic field. The resonance behaviour of the water nuclei (spin of the protons) is a measure of the water content in the substance. In the second type water nuclei (protons) are deflected by a magnetic pulse. The spin flip of the proton induces a voltage in the receiver coil. Mathematical processing of the experimental signal results in an NMR spectrum, which provides information on the hydrogen atoms in the sample.

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Gas chromatography Principle The sample mixture must first be capable of vaporization without undergoing decomposition so that it can then be transported in gaseous form by an inert carrier gas through a separating column. The individual sample components are separated by the column on the basis of their different boiling points and the intermolecular interactions between the liquid, stationary phase in the separating column and the sample components in the mobile gas phase. The individual gas fractions exiting the column are usually detected by thermal conductivity(TC).

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T H A N K Y O U

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