Moisture content determination (Q.C)

8/11/2019 Moisture content determination (Q.C)

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Weight per millilitre

The weight per millilitre of a liquid is the weight

in g of 1 mL of a liquid when weighed in air at

20C, unless otherwise specified in the

monograph.

The weight per millilitre is determined by dividing

the weight in air, expressed in g, of the quantity

of liquid that fills a pycnometer at the specifiedtemperature by the capacity, expressed in mL, of

the pycnometer at the same temperature.


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The capacity of the pycnometer is ascertained from theweight in air, expressed in g, of the quantity of waterrequired to fill the pycnometer at that temperature.

The weight of a litre of water at specified temperatureswhen weighed against brass weights in air of density0.0012 g per mL is given in the following table.Ordinary deviations in the density of air from theabove value, here taken as the mean, do not affect the

result of a determination in the significant figuresprescribed for Pharmacopoeial substances.


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DETERMINATION OF WATER / MOISTURE


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IT IS DONE BY;

Karl Fischer titration

Karl Fischer titrationis a classic titrationmethod in analytical chemistrythat uses

coulometricor volumetrictitration to

determine trace amounts of waterin a

sample.
http://en.wikipedia.org/wiki/Titrationhttp://en.wikipedia.org/wiki/Analytical_chemistryhttp://en.wikipedia.org/wiki/Coulometryhttp://en.wikipedia.org/wiki/Volumehttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Volumehttp://en.wikipedia.org/wiki/Coulometryhttp://en.wikipedia.org/wiki/Analytical_chemistryhttp://en.wikipedia.org/wiki/Titration


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Coulometric titration

The main compartment of the titration cellcontains the anodesolution plus the analyte. Theanode solution consists of an alcohol (ROH), abase (B), SO2andI2. A typical alcohol that may beused is methanol.

The titration cell also consists of a smallercompartment with a cathode immersed in the

anode solution of the main compartment. Thetwo compartments are separated by an ion-permeable membrane.
http://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Sulfur_dioxidehttp://en.wikipedia.org/wiki/Sulfur_dioxidehttp://en.wikipedia.org/wiki/Anode


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Method I

The semi-micro determination of water is

based upon the quantitative reaction of water

with sulfur dioxide and iodine in a suitable

anhydrous medium in the presence of a base

with sufficient buffering capacity.


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Apparatus:

The apparatus consists of a titration vessel with:

2 identical platinum electrodes;

tight inlets for introduction of solvent and titrant;

an inlet for introduction of air via a desiccant;

a sample inlet fitted with a stopper or, for liquids,

a septum.


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Inlet systems for introduction of dry nitrogen or foraspiration of solvents may also be fitted.

The titration is carried out according to the instrumentsupplier's instructions. Care is taken throughout thedetermination to avoid exposure of reagents and solventsto atmospheric moisture. The end-point is determinedusing 2 identical indicator electrodes connected to anelectrical source that maintains between the electrodeseither a constant current or a constant voltage. Wheredirect titration is used , addition of titrant causes either a

decrease in voltage where constant current is maintainedor an increase in current where constant voltage ismaintained, until the end-point is reached. Instrumentswith automatic end-point detection are commonly used.


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Method IA:

Introduce into the titration vessel methanol R, orthe solvent indicated in the monograph or

recommended by the supplier of the titrant.Where applicable for the apparatus used,eliminate residual water from the measurementcell or carry out a pre-titration. Introduce the

substance to be examined rapidly and carry outthe titration, stirring for the necessary extractiontime.


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Method IB:

Introduce into the titration vessel methanol R, or thesolvent indicated in the monograph or recommended bythe supplier of the titrant. Where applicable for theapparatus used, eliminate residual water from themeasurement cell or carry out a pre-titration. Introduce thesubstance to be examined rapidly and in a suitable state ofdivision. Add an accurately measured volume of the titrant,sufficient to give an excess of about 1 mL or the prescribedvolume. Allow to stand protected from light for 1 min or

the prescribed time, with stirring. Titrate the excess ofreagent using methanol R or the prescribed solvent,containing an accurately known quantity of water.


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The accuracy of the determination with the

chosen titrant must be verified for each

substance to be examined. The following

procedure, given as an example, is suitable for

samples containing 2.5-25 mg of water.


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The water content of the substance to be

examined is determined using the

reagent/solvent system chosen. Thereafter,

sequential known amounts of water R are

added in an appropriate form (at least 5

additions) and the cumulative water content

determined after each addition. Calculate thepercentage recovery (r) at each point using the

following expression:


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Method II: Determination of water bydistillation

The apparatus consists of a glass flask (A)

connected by a tube (D) to a cylindrical tube (B)fitted with a graduated receiving tube (E) andreflux condenser (C). The receiving tube (E) isgraduated in 0.1 mL. The source of heat is

preferably an electric heater with rheostat controlor an oil bath. The upper portion of the flask andthe connecting tube may be insulated.


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Method: Clean the receiving tube and the condenser of theapparatus, thoroughly rinse with water, and dry. Introduce 200 mL

of toluene R and about 2 mL of water R into the dry flask. Distil for 2h, then allow to cool for about 30 min and read the water volumeto the nearest 0.05 mL. Place in the flask a quantity of thesubstance, weighed with an accuracy of 1 per cent, expected to giveabout 2 mL to 3 mL of water. If the substance has a pastyconsistency, weigh it in a boat of metal foil. Add a few pieces of

porous material and heat the flask gently for 15 min. When thetoluene begins to boil, distil at the rate of about two drops persecond until most of the water has distilled over, then increase therate of distillation to about four drops per second. When the waterhas all distilled over, rinse the inside of the condenser tube withtoluene R. Continue the distillation for 5 min, remove the heat,

allow the receiving tube to cool to room temperature and dislodgeany droplets of water which adhere to the walls of the receivingtube. When the water and toluene have completely separated, readthe volume of water and calculate the content present in thesubstance as millilitres per kilogram, using the formula:


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Method III (Coulometric titration)

Principle:

The coulometric titration of water is based upon the quantitative reaction

of water with sulfur dioxide and iodine in an anhydrous medium in the

presence of a base with sufficient buffering capacity. In contrast to the

volumetric method, iodine is produced electrochemically in the reactioncell by oxidation of iodide.

The iodine produced at the anode reacts immediately with the water and

the sulfur dioxide contained in the reaction cell. The amount of water in

the substance is directly proportional to the quantity of electricity up until

the titration end-point. When all of the water in the cell has been

consumed, the end-point is reached and thus an excess of iodine appears.


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Apparatus:

The apparatus consists of a reaction cell,electrodes and magnetic stirrer. The reaction cell

consists of a large anode compartment and asmaller cathode compartment.

Depending on the design of the electrode, bothcompartments can be separated by a diaphragm.

Each compartment contains a platinumelectrode. Liquid or solubilised samples areintroduced through a septum, using a syringe.


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Method:

Fill the compartments of the reaction cell with electrolytereagent for the micro determination of water R accordingto the manufacturer's instructions and perform thecoulometric titration to a stable end-point. Introduce theprescribed amount of the substance to be examined intothe reaction cell, stir for 30 s, if not otherwise indicated inthe monograph, and titrate again to a stable end-point. Incase an oven is used, the prescribed sample amount isintroduced into the tube and heated. After evaporation of

the water from the sample into the titration cell, thetitration is started. Read the value from the instrument'soutput and calculate if necessary the percentage or amountof water that is present in the substance.


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Verification of the accuracy:

Between two successive sample titrations,introduce an accurately weighed amount of

water in the same order of magnitude as theamount of water in the sample, and perform thecoulometric titration. The recovery rate is withinthe range from 97.5 per cent to 102.5 per cent

for an addition of 1000 g of H2O and in therange from 90.0 per cent to 110.0 per cent forthe addition of 100 gof H2O.


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LOSS ON DRYING

Test determines the amount of volatile matter of anykind that is driven of under specified conditions.

Take 1-2g of substance, reduce particle size if necessaryand place in already dried (for 30 min) glass stopperedshallow bottle .

Weigh the bottle containing the sample. Shake the bottle to evenly distribute the test

substance. Remove the stopper of bottle and Place the loaded

bottle and stopper in drying chamber. Dry the sample at temperature specified in

monograph, close the bottle. Cool in desiccator &weigh


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If the substance melts at a lower temperature

than that specified for the determination of

Loss on drying, maintain the bottle with its

contents for 1 to 2 hours at a temperature 5 to10 below the melting temperature, then dry

at the specified temperature.

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Moisture content determination (Q.C)