11. Procedure for Base Number and Strong Base Number
11.1 Calculate the quantity of sample required for its expected base number as follows:
A = 7/E
where:
A = approximate mass of sample, g and
E = expected base number.
11.1.1 Take a maximum of 5 g and a minimum of 0.1 g for analysis. The precision of weighing is as follows:
11.2 Into a 250-mL titration beaker or a suitable titration vessel, introduce a weighed quantity of sample as prescribed in 11.1.1 and add 125 mL of titration solvent (Note 10). Prepare the electrodes as directed in 8.2. Place the beaker or titration vessel on the titration stand and adjust its position so that the electrodes are about half immersed. Start the stirrer, and stir throughout the determination at a rate sufficient to produce vigorous agitation without spattering and without stirring air into the solution.
NOTE 10 - Some automatic titrators do not accept a beaker size that contains 125 mL of titration solvent. In such cases, a lesser amount of solvent in the range from 75 to 100 mL is acceptable.
11.3 Select and fill a suitable buret with the 0.1 M alcoholic HCl solution and place the buret in position on the titration assembly, taking care that the tip is immersed about 25 mm in the liquid in titration vessel. Record the initial buret and meter (cell potential) readings.
11.4 Titration - The reaction of the hydrochloric acid with the basic components is very slow with most titrations for base number. As a result, these titrations are not at equilibrium. Because of this, the titration conditions are tightly specified and must be strictly adhered to in order to achieve the precision as stated.
NOTE 11 - See Appendix X1 for techniques for reducing the titration time of a sample. Pre-dosing techniques have been found to provide satisfactory results although the precision using these techniques has not been determined.
11.4.1 Whether the titration is carried out manually or automatically, the following procedure of fixed increment, fixed time addition of titrant must be followed. Add 0.1 M HCl in increments of 0.100 mL throughout the titration with a 90-s pause between each incremental addition. Take millivolt readings at the end of each 90 s interval. Continue as above until a potential is reached which is 100 mV past the meter reading corresponding to that found for the standard acidic buffer solution (acidic buffer potential). If the volume of titrant required to reach this potential (100 mV past the acidic buffer potential) is greater than 4.0 mL, reduce the sample size by one-half and repeat the titration.
11.4.1.1 The meter readings of potential difference are plotted manually or automatically against the respective volumes of titrant, and the end point taken as described in 12.1.
11.4.2 On completion of the titration, remove the titration vessel and rinse the electrodes and buret tip with the titration solvent, then with water, then again with titration solvent. (Soak electrodes in distilled water for at least 5 min before using for another titration.) Store the glass electrode in deionized or distilled water and the reference electrode in a saturated solution of LiCl in isopropyl alcohol when not in use (see 8.1).
11.4.3 Blanks - For each set of samples, make a blank titration of the same volume of titration solvent used for the sample. For the base number blank, add 0.1 M alcoholic HCl solution in 0.05-mL increments, waiting 90 s between each addition, until a potential which is 100 mV past the buffer potential (see 11.4) is reached. For the strong base number blank, add titrant under the same conditions until the potential corresponding to the basic buffer solution is reached.
12. Calculation
12.1 If an inflection (see Note 12) occurs in the potential region between the acidic buffer potential (see 11.4) and a point 100 mV past this potential, mark this inflection as the end point. If no inflection occurs in the above mentioned potential region, mark as the end point the point on the curve that corresponds to the acidic buffer potential. See Fig. 2 for examples of end points.
NOTE 12 - An inflection point is generally recognizable by inspection whenever at least five successive cell potential changes, D, caused by the addition of the corresponding five increments of titrant, exhibit a maximum as illustrated by an example in the following table.
12.2 Calculate the base number and strong base number as follows:
Base number, mg KOH/g = [(A - B) x M x 56.1]/W
Strong base number, mg KOH/g = [(CM + Dm) x 56.1]/W
where:
A = alcoholic HCl solution, mL, used to titrate the sample to the end point (nonaqueous acidic buffer or inflection - see 12.1),
B = alcoholic HCl, mL, used to titrate the solvent blank to the same potential at which the sample end point occurs,
M = molarity of the alcoholic HCl solution,
W = sample, g,
C = alcoholic HCl solution, mL, used to titrate the sample to an end point that occurs at a meter reading corresponding to the nonaqueous basic buffer (see 7.2)
D = alcoholic KOH solution, mL, used to titrate the solvent blank to the potential corresponding to C, and
m = molarity of the alcoholic KOH solution.