D2896_12

Calculate the quantity of sample required from its expected base number as follows-- approximate weight of sample, g, grams, equals 10 divided by the expected base number. For the back titration procedure, see 17.2 of the Written Standard. It may be necessary to use a smaller sample weight. Weigh the sample into the titration beaker, applying the limits shown as follows. A maximum of 10 grams should be taken for analysis. It is especially important for procedure B that great care be exercised in obtaining accurate weights, particularly for the high base number samples which require small sample weights. Add 60 milliliters of titration solvent to the sample. Place the sample on the titration stand and stir the solution until the sample is dissolved. If the solvent of the sample proves difficult, dissolve it in 40 milliliters of chlorobenzene in the titration beaker. Then add 20 milliliters of glacial acetic acid. Prepare the electrodes as directed in 10.1, 10.2, and 10.3 of the Written Standard. Position the electrodes in the solution so that they are immersed as far as possible. Continue stirring throughout the determination at a rate sufficient to produce vigorous agitation without spattering and without stirring air into the solution. Adjust the meter so that it reads in the upper part of the millivolt scale-- for example, 700 millivolts. For simple meters without this adjustment, it may be necessary to incorporate a source of potential in series with the electrode. A 1.5-volt dry cell and potential divider is suitable. Fill the burette with 0.1 normal perchloric acid solution and place the burette in position in the titration assembly, taking care that the tip is immersed below the level of the surface of the liquid in the beaker. Record the initial burette and meter cell potential readings. Add suitable small portions of titrant. And after waiting until a constant potential has been established-- note 12 of the Written Standard-- record the burette and meter readings. At the start of the titration and in any subsequent regions, inflections, where 0.1 milliliters of titrant consistently produces a total change of more than 0.03 volts, corresponding to 0.5 pH scale unit in the cell potential, add 0.05 milliliters portions in the intermediate regions, plateaus, where 0.1 millimeter increments change the potential by less than 0.03 volts. Add large portions sufficient to produce a total potential change approximately equal to but not greater than 0.03 volts. Titrate in this manner until the potential changes less than 0.005 volts, corresponding to 0.1 pH scale unit per 0.1 millimeters. Consider the cell potential constant when a change is less than 0.005 volts per minute. Adjust the instrument in accordance with the manufacturer's instructions and set the titration speed at 1.0 milliliters per minute maximum. On completion of the titration, remove the beaker and proceed with cleaning of the electrodes. See 10.3 of the Written Standard. Store the electrodes in water when not in use. See 10.1 of the Written Standard. For each set of samples, make a blank on 60 milliliters of titration solvent. For a manual titration, add 0.1 normal perchloric acid solution in 0.05 milliliter increments, waiting between each addition until a constant cell potential is established. Record meter and burette readings after each increment. Follow the procedure in 12.6.2 of the Written Standard for an automatic titration.