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D664_12 (17a) 04-2018

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Into a 250-milliliter beaker or a suitable titration vessel, introduce a weight quantity of sample as recommended in table 1a of the written standard. See note 13 of the written standard and add 125 milliliters of titration solvent. See note 14 of the written standard. Alternatively, into a 125-milliliter beaker or a suitable titration vessel, introduce a weight quantity of sample as recommended in table 1b of the written standard and add 60 milliliters of titration solvent. Prepare the electrodes as directed in 8.1 of the written standard. Place the beaker of titration vessel on the titration stand and adjust its position so that the electrodes are about half immersed. Start the stir and stir throughout the determination at a rate sufficient to produce vigorous agitation without spattering and without stirring air into the solution. If it is suspected that the recommended sample size will foul the electrodes, a smaller sample size can be taken. Results using smaller sample size may not be equivalent to results obtained with the recommended sample size. The precision statement does not include results when using a smaller sample size. A titration solvent that contains chloroform-- warning, may be fatal if swallowed, harmful if inhaled, may produce toxic vapors if burned-- can be used in place of toluene to completely dissolve certain heavy residues of asphaltic materials. Results using chloroform may not be equivalent to results obtained using toluene. The precision statement does not include results when using chloroform. Select the right burette. Fill with a 0.1 mole per liter alcoholic potassium hydroxide solution and place the burette in position on the titration assembly ensuring that the tip is immersed about 25 millimeters in titration vessel liquid. Record the initial burette and meter cell potential readings. Add suitable small portions of 0.1 mole per liter alcoholic potassium hydroxide solution and wait until a constant potential has been established. Record the burette and meter readings. At the start of the titration and in any subsequent regions inflections where 0.1 milliliters of the 0.1 mole per liter potassium hydroxide solution consistently produces a total change of more than 30 millivolts in the cell potential, add 0.05 milliliter portions. In the intermediate regions plateau where 0.1 milliliters of 0.1 mole per liter alcoholic potassium hydroxide changes the cell potential less than 30 millivolts, add larger portions sufficient to produce a total potential change approximately equal to but not greater than 30 millivolts. Titrate in this manner until the potential change is less than 5 millivolts per 0.1 milliliters of potassium hydroxide and the cell potential indicates that the solution is more basic than the aqueous basic buffer. Remove the titration solution, rinse the electrodes and burette tip with the titration solvent then with isopropyl alcohol and finally with reagent grade water. Immerse the electrodes in water for at least five minutes before starting another titration to restore the aqueous gel layer of the glass electrode. After five minutes in the water, rinse the electrodes with isopropyl alcohol then the titration solvent before proceeding to the next titration. If the electrodes are found to be dirty and contaminated, proceed as in 8.1 of the written standard, store electrodes according to 8.3.3 of the written standard. Adjust the apparatus in accordance with the manufacturer's instructions to provide a dynamic mode of titrant addition. Verify that the instrument will determine the amount of strong acid when the initial millivolts of the test sample relative to the millivolt's reading of the aqueous acidic buffer indicates the presence of such acids. Record the volume of potassium hydroxide added to reach the millivolts of the pH for aqueous buffer. This value is used to calculate the strong acid number. Proceed with the automatic titration and record the potential metric curves or derivative curves as the case may be. Titrate with a 0.1 mole per liter alcoholic potassium hydroxide solution. The apparatus shall be adjusted or programmed such that when an inflection point suitable for use in the calculation is approached, the rate of addition of titrant and volume of titrant added are based on the change in slope of the titration curve. The titrant shall be added in increments of a suitable size to achieve a potential difference of 5 millivolts to 15 millivolts per increment. Increment value shall vary between 0.05 milliliter and 0.5 milliliter. The next increment shall be added if the signal does not change more than 10 millivolts in 10 seconds. The maximum waiting time in between increments shall not exceed 60 seconds. The titration can be terminated when the signal reaches the pH 11 buffer potential past 200 millivolts. An equivalence point is recognizable if the first derivative of the titration curve produces a maximum, which is significantly higher than the noise produced by electrostatic effects. See also 13.1.1 of the written standard. The goal of cleaning is to rinse the residue from the previous sample and to rehydrate the electrode. On completion of the titration, rinse the electrodes and burette tip with titration solvent. If clean, then rinse with 2-propanol and then with water. Immerse the electrodes in pH 4.5 to 5.5 water for at least three minutes to five minutes to rehydrate the aqueous gel layer of the glass electrode. Rinse with 2-propanol prior to beginning the next sample to remove the water. If sample residue remains after the rinse with titration solvent, another solvent such as toluene, xylene, heptane, or chloroform may be used for rinse. The rinse may be more effective if a beaker of solvent is used with strong stirring. Using automated equipment, cleaning may be done by rinsing with titration solvent, soaking with stirring in a solvent such as toluene, xylene, heptane, or chloroform for 45 seconds. Soaking briefly in 2-propanol to remove the solvent then soaking in pH 4.5 to 5.5 water three minutes to five minutes to rehydrate. Dip in 2-propanol all briefly to remove water before beginning the next sample. The same solvent cleaning beaker, 2-propanol beaker, and water beaker may be used for a short series of samples. They should be changed at reasonable intervals before contamination builds up. The user shall ensure that the electrode is adequately cleaned and hydrated. If electrodes are found dirty and contaminated, proceed as an 8.1 of the written standard. Store electrodes according to 8.3.3 of the written standard. When acid numbers about or below 0.1 are expected, better precision can be obtained by modifying the method in one or more ways such as by substituting 0.01 molar to 0.05 molar alcoholic potassium hydroxide solution, increasing the sample size above 20 grams or switching from a manual operated burette that is graduated in 0.05 milliliter divisions to an automated burette that can dispense smaller increments of the K08 solution if samples are being analyzed by manual titration. For each set of samples and for every new batch of titration solvent, perform a blank titration of 125 milliliter or 60 milliliter depending on the volume of the solvent that will be used for sample analysis. For manual titration, add 0.1 mole per liter alcoholic addition until a constant cell potential is reached. Record the meter and readings when the former becomes constant after each increment. For automatic titration, use the same mode of titration as for the determination of the acidic property of the sample but use smaller increments of titrant addition, 0.01 milliliter to 0.05 milliliter. Recheck the blank periodically based on the sample load. When strong acids are present and a strong acid number is to be determined, perform a blank titration of 125 milliliter or 60 milliliter, depending on the volume of the titration solvent that will be used for sample analysis. Add 0.1 mole per liter alcoholic hydrochloric acid solution in 0.01 milliliter to 0.05 milliliter increments in a manner comparable to that specified in 12.5.1 of the written standard.

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Posted by: mer.figueroa on Aug 23, 2018

D664_12 (17a) 04-2018

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