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4.12 DETERMINATION OF WATER

Karl Fischer Method​

      Principle The titrimetric determination of water is based upon the quantitative reaction of water with an anhydrous solution of sulfur dioxide and iodine dissolved in pyridine and methanol. The test sample may be titrated with Karl Fischer reagent directly or the analysis may be carried out by a residual titration procedure. In the residual titration, excess Karl Fischer reagent is added to the test sample, sufficient time is allowed for the reaction to reach completion, and the unconsumed Karl Fischer reagent is titrated with a standard solution of water in methanol. The residual titration procedure is applicable generally and avoids the difficulties that may be encountered in the direct titration of substances from which the bound water is released slowly.

      The stoichiometry of the reaction is not exact, and the reproducibility of a determination depends upon such factors as the relative concentrations of the Karl Fischer reagent ingredients, the nature of the inert solvent used to dissolve the test sample, and the technique used in the particular determination. Therefore, an empirically standardized technique is used in order to achieve the desired accuracy. Precision in the method is governed largely by the extent to which atmospheric moisture is excluded from the system. The titration of water is usually carried out with the use of anhydrous methanol as the solvent for the test sample; however, other suitable solvents may be used for special or unusual test sample.

      Apparatus Use a closed system consisting of a suitable titration vessel, previously dried at 105º and cooled in a desiccator, fitted with two platinum electrodes, a nitrogen inlet tube, a stopper which accommodates the burette tip, a vent tube protected by a suitable desiccant, and a magnetic stirrer, connected to a suitable electro-metric apparatus. This apparatus embodies a simple electrical circuit which serves to pass 5 to 10 μA of direct current between a pair of platinum electrodes immersed in the solution to be titrated. At the endpoint of the titration a slight excess of the reagent increases the flow of current to between 50 and 150 μA for 30 seconds or longer, depending upon the solution being titrated.

      Reagent

      KARL FISCHER REAGENT Dissolve 125 g of iodine in 170 ml of pyridine, add 670 ml of anhydrous methanol, and cool. To 100 ml of pyridine kept in an ice-bath, add liquid anhydrous sulfur dioxide until the volume reaches 200 ml. Slowly add this solution to the cooled iodine mixture, and shake well to dissolve the iodine. Allow the solution to stand for 24 hours before use. Preserve the reagent under refrigeration in a suitably sealed, glass-stoppered bottle, protected from light and from moisture in the air. This methanolic reagent is somewhat unstable and requires frequent standardization. A more stable reagent may be prepared by substituting 2- methoxyethanol for anhydrous methanol. A commercially available, stabilized solution of Karl Fischer reagent may be used.

      METHANOL SOLUTION Add sufficient water (usually 2 ml per litre) to anhydrous methanol so that each ml of the resulting methanol solution is equivalent to about 0.5 ml of Karl Fischer reagent.

      Standardization of the reagent

      WATER EQUIVALENCE OF KARL FISCHER REAGENT (F) Standardize the Karl Fischer reagent no longer than 1 hour before use by one of the following methods.

      Place enough anhydrous methanol in the titration vessel to cover the electrodes, and add sufficient Karl Fischer reagent to give the characteristic end-point colour, or 100±50 μA of direct current at about 200 mV of applied potential.

      For determination of trace amounts of water (less than 1 per cent), sodium tartrate may be used as a convenient water reference substance.

      Quickly add 120 to 300 mg of sodium tartrate (C₄H₄Na₂O₆.2H₂O), accurately weighed by difference, and titrate to the end-point. Calculate the water equivalence of the Karl Fischer reagent as follows:

          F = 2(18.02/230.08)(W/Vr),

where F = water equivalence of the Karl Fischer reagent in terms of mg of water per ml,

18.02 = molecular weight of water,

W = weight of sodium tartrate in mg,

230.08 = molecular weight of sodium tartrate, and

Vr = ml of Karl Fischer reagent consumed in the second titration.

      For the precise determination of significant amounts of water (more than 1 per cent), use water obtained by distillation as the reference substance.

      Quickly add between 25 and 250 mg of water, accurately weighed by difference, from a weighing pipette or from a pre-calibrated syringe or micropipette. Titrate to the end-point. Calculate the water equivalence of Karl Fischer reagent as follows:

      F = W/Vr

where F = water equivalence of the Karl Fischer reagent in terms of mg of water per ml,

W = weight of water in mg, and

Vr = ml of Karl Fischer reagent consumed in the second titration.

      KARL FISCHER REAGENT EQUIVALENCE OF METHANOL SOLUTION (f)

      Titrate a known volume of Karl Fischer reagent with methanol solution until the end-point is reached. Calculate the millilitres of Karl Fischer reagent equivalent to each ml of methanol solution as follows: 

      f = Vr/Vm

where f = ml of Karl Fischer reagent equivalent to each ml of methanol solution,

Vr = ml of Karl Fischer reagent used, and

Vm = ml of methanol solution used.

      Sample preparation For Pharmacopoeial articles, unless otherwise specified in the individual monograph, use an accurately weighed or measured amount of the specimen under test estimated to contain 10 to 250 mg of water.

      TABLETS AND CAPSULES Tablets, use accurately weighed powder from not less than 4 tablets ground to a fine powder in an atmosphere of about 10 per cent relative humidity.

      Capsules, use an accurately weighed portion of the mixed contents of not less than 4 capsules.

      OINTMENTS AND OILS Use an accurately weighed portion of about 1 to 2 g.

      AEROSOLS WITH PROPELLANT Store it in a suitable freezing unit having a temperature of not higher than 0º for at least 2 hours. After removing from the freezer, immediately open the container, and test 10.0 ml of the well-mixed sample. In titrating the sample, determine the end-point at a temperature of 10º or higher

      HYGROSCOPIC POWDERS Accurately weigh the immediate container. Using a suitable dry hypodermic needle and syringe, inject an appropriate volume of anhydrous methanol, accurately measured, into the container and shake to dissolve the contents. Using the same syringe, remove the solution from the container and transfer it to a titration vessel containing 20 ml of a mixture of carbon tetrachloride, chloroform and anhydrous methanol (2:2:1), previously titrated to the end-point as directed under Titration procedure. Repeat the procedure with a second portion of anhydrous methanol, accurately measured, add this washing to the titration vessel, and immediately titrate. Determine the volume, in ml, of Karl Fischer reagent equivalent to a portion of anhydrous methanol of the same total volume as that used to dissolve the sample and to wash the container and syringe, as directed under Titration procedure. Subtract this value from the volume, in ml, of Karl Fischer reagent obtained in the titration of the sample under test. Dry the container and its closure at 100º for 3 hours, allow to cool in a desiccator, and weigh. Determine the weight of the sample tested from the difference in weight from the initial weight of the container.

      Titration procedure Determine the water by Method I, unless otherwise specified in the individual monograph. In the case of a colourless solution that is titrated directly, the end-point may be observed visually as a change in colour from canary-yellow to amber. The reverse is observed in the case of a test sample that is titrated residually. More commonly, however, the end-point is determined electrometrically.

      METHOD I: DIRECT TITRATION Add 20 ml of anhydrous methanol to the titration vessel, and titrate with Karl Fischer reagent to the electrometric or visual end-point to consume any moisture that may be present. (Disregard the volume consumed, since it does not enter into the calculations.) Quickly add an accurately weighed or measured amount of the sample under test, mix, and again titrate with Karl Fischer reagent to the electrometric or visual end-point.

      METHOD II: RESIDUAL TITRATION Where the individual monograph specifies that the water content is to be determined by Method II, the residual titration procedure, transfer 20 ml of anhydrous methanol to the titration vessel, and titrate with Karl Fischer reagent to the electrometric or visual end-point. Quickly add an accurately weighed or measured amount of the sample under test, mix, and add an accurately measured excess of Karl Fischer reagent. Allow sufficient time for the reaction to reach completion, and titrate the unconsumed Karl Fischer reagent with standardized methanol solution to the electrometric or visual end-point.

      Calculation

Calculate the percentage of water in the sample as follows:

If titration Method I is used: Percentage of water in weighed samples = (Vr × F × 100)/Ws

Percentage of water in aerosols = Vr × F/(ml of sample used × 10)

Percentage of water in hygroscopic powders = [(Vr – Vb) × F × 100]/Ws

If titration Method II is used:

Percentage of water in weighed samples = [(Vr – Vm × f) × F × 100]/Ws

Percentage of water in aerosols = [(Vr – Vm × f) × F]/(ml of sample used × 10)

Percentage of water in hygroscopic powders = [(Vr – Vm × f – Vb) × F × 100]/Ws

Where f = ml of Karl Fischer reagent equivalent to each ml of methanol solution determined as directed in Karl Fischer reagent equivalence of methanol solution,

          F = water equivalence of the Karl Fischer reagent determined as directed in Water Equivalence of Karl Fischer reagent of this section,

        Vb = ml of Karl Fischer reagent equivalent to the anhydrous methanol used as a sample solvent,

       Vm = ml of methanol solution used

        Vr = ml of Karl Fischer reagent used, and

       Ws = weight of the sample in mg.

Coulometric Titration

      Principle The Karl Fischer reaction is used in the coulometric determination of water. Iodine, however, is not added in the form of a volumetric solution but is produced in an iodide-containing solution by anodic oxidation. The reaction cell usually consists of a large anode compartment and a small cathode compartment that are separated by a diaphragm. Other suitable types of reaction cells (e.g., without diaphragms) may also be used. Each compartment has a platinum electrode that conducts current through the cell. Iodine, which is produced at the anode electrode, immediately reacts with water present in the compartment. When all the water has been consumed, an excess of iodine occurs, which usually is detected electrometrically, thus indicating the end-point. Moisture is eliminated from the system by pre-electrolysis. Changing the Karl Fischer reagent after each determination is not necessary since individual determinations can be carried out in succession in the same reagent solution. A requirement for this method is that each component of the test sample is compatible with the other components, and no side reactions take place. Samples are usually transferred into the vessel as solutions by means of injection through a septum. Gases can be introduced into the cell by means of a suitable gas inlet tube. Precision in the method is predominantly governed by the extent to which atmospheric moisture is excluded from the system; thus, the introduction of solids into the cell is not recommended, unless elaborate precautions are taken, such as working in a glove-box in an atmosphere of dry inert gas. Control of the system may be monitored by measuring the amount of baseline drift. This method is particularly suited to chemically inert substances like hydrocarbons, alcohols, and ethers. In comparison with the volumetric Karl Fischer titration, coulometry is a micromethod.

      Apparatus Any commercially available apparatus consisting of an absolutely tight system fitted with the necessary electrodes and a magnetic stirrer is appropriate. The instrument’s microprocessor controls the analytical procedure and displays the results. Calibration of the instrument is not necessary, as the current consumed can be measured absolutely.

      Reagent See Reagent under Karl Fischer Method. 

      Sample preparation Where the sample is a soluble solid, dissolve an appropriate quantity, accurately weighed, in anhydrous methanol or other suitable solvents. Liquids may be used as such or as accurately prepared solutions in appropriate anhydrous solvents.

      Where the sample is an insoluble solid, the water may be extracted using a suitable anhydrous solvent from which an appropriate quantity, accurately weighed, may be injected into the anolyte solution. Alternatively an evaporation technique may be used in which water is released and evaporated by heating the specimen in a tube in a stream of dry inert gas, this gas being then passed into the cell.

      Procedure Using a dry syringe, quickly inject sample preparation, accurately measured and estimated to contain 0.5 to 5 mg of water, or as recommended by the instrument manufacturer into the anolyte, mix, and perform the coulometric titration to the electrometric end-point. Read the water content of sample preparation directly from the instrument’s display, and calculate the percentage that is present in the substance. Perform a blank determination, and make any necessary corrections.

      Calculation One mole of iodine corresponds to 1 mole of water, a quantity of electricity of 10.71 C corresponds to 1 mg of water.

Azeotropic Distillation Method

      Apparatus The apparatus (see figure) consists of a glass flask (A) connected by a tube (D) to a cylindrical tube (B) fitted with a graduated receiving tube (E) and a reflux condenser (C). The receiving tube (E) is graduated in 0.1-ml subdivisions so that the error of reading is not more than 0.05 ml. The source of heat is preferably an electric heater with rheostat control or an oil-bath. The upper portion of the flask and the connection tube may be insulated with asbestos.

Method Clean the receiving tube and the condenser of the apparatus by a suitable method, thoroughly rinse with water, and dry.

      Introduce 200 ml of toluene and about 2 ml of water into the dry flask. Distil for about 2 hours, allow to cool to room temperature and read the water volume to an accuracy of 0.05 ml. Place in the flask a quantity of the substance, weighed to the nearest centigram, expected to give about 2 to 3 ml of water. If the substance is of a pasty character, weigh it in a boat of metal foil. Add a few pieces of porous material and heat the flask gently for 15 minutes. When the toluene begins to boil, distil at the rate of 2 drops per second until most of the water has distilled over, and then increase the rate of distillation to about 4 drops per second. When the water has all distilled over, rinse the inside of the condenser tube with toluene. Continue the distillation for 5 minutes, remove the heat, allow the receiving tube to cool to room temperature, and dislodge any droplets of water which adhere to the walls of the receiving tube. When the water and toluene have completely separated, read the volume of water and calculate the percentage present in the substance using the expression:

      

where p = the weight in g of the substance to be examined,

          n = the volume in ml of water obtained in the first distillation, and

          n’ = the total volume in ml of water obtained in the two distillations.