Titration solvents

Griffin and Albaugh [102] describe a procedure whereby the neutral AOS product is converted to the parent acids by cation exchange and then titrated potentiometrically. A dilute aqueous solution of AOS is passed through cationic ion exchange resin in acid form. The acids are eluted with small portions of water and titrated potentiometrically using tetrabutylammonium hydroxide solution in a solvent mixture of 70 30 benzene/methanol. It is probable that the benzene can be replaced with toluene for safety reasons or that ASTM titration solvent (ASTM D664 toluene/propan-2-ol/water) can be used. 

This electrode filling is unsuitable with acetic acid or another related acid as the titration solvent, as in this medium possible leakage of sodium acetate would act as the inherent base. 

Composition limitation—Sample must be soluble in water or in one of the several special non-aqueous titration solvents available that can accommodate most petroleum fractions (except some of the heavy ends). 

Concentration—A wide range of concentrations can be accommodated by varying the amount of sample dissolved in the titration solvent. In many cases concentrations as low as a few ppm can be reported. 

Rhodes et al (31) determined a mixture containing ace-tylsalicylic acid, acetaminophen and salicylamide, potentiometrically by non-aqueous titration. The tit-rant was 0.1 N tetrabutylammonium hydroxide in benzene/ methanol mixture and the titration solvent was dimethyl-formamide. The difference in the pKa values for these weak acidic drugs (aspirin pKa 3-5, acetaminophen pKa 8.31) was sufficient to permit useful differentiation. 

Many sulfa drugs such as sulfanilamide, sulfathio-zole, and sulfathalidine can be titrated in ethylene-amine with tetrabutylammonium hydroxide. The ability of dimethylformamide to dissolve salts, polymers, and many organic compounds accounts for its wide use as a titration solvent. Inert solvents such as acetone can be used for titration of acids, acetonitrile for both acids and bases, and ethyl acetate for amines. A suitable non-aqueous medium such as methyl... 

Equation 4-4a represents a type of reaction called autoprotolysis which, as we will see later, is important in describing the utility of solvents as acid-base titration solvents. 

Titration solvent. Prepare 1 L of titration solvent by mixing the following 714 cm of glacial acetic acid, 134 cm of carbon tetrachloride, 116 cm of methanol, 18 cm of sulphuric acid (1+5) and 18 cm of a methanolic solution of mercury (II) chloride (lOOg ). 

M Room temp titrations solvent contains - 2% H2O (to dissolve gelatin) 12... 

Buffer Nonaqueous Basic—Add 10 mL of buffer stock Solution B to 100 mL of titration solvent. Use within 1 h. 

Titration Solvent—Add 500 mL of toluene (Warning—See Note 7) and 5 mL of water to 495 mL of anhydrous propan-2-ol. The titration solvent should be made up in large quantities, and its blank value determined daily by titration prior to use. 

Testing of Electrodes—Test the meter-electrode combination when first put into use, or when new electrodes are installed, and retest at intervals thereafter by dipping the electrodes into a well-stirred mixture of 100 mL of the titration solvent and 1.0 to 1.5 mL of 0.1-mol/L alcoholic KOH solution. For the meter-electrode combination to be suitable for use, the potential between the electrodes should change by more than 480 mV from the potential between the same electrodes when dipped in the nonaqueous acidic buffer solution (Note 14). 

Into a 250-mL beaker or a suitable titration vessel, introduce a weighed quantity of sample as prescribed in Table 1 and add 125 mL of titration solvent (Note 18). 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. If feasible, adjust the meter so that it reads in the upper part of the millivolt scale, for example 700 mV. 

Note 18—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. 

Remove the titration solution, rinse the electrodes and burette tip with the titration solvent, then with propan-2-ol and fin y with reagent grade water. Immerse the electrodes in water for at least 5 min before using for another titration to restore the aqueous gel layer of the glass electrode. Store the electrodes in reagent water when not in use. If the electrodes are found to be dirty and contaminated, proceed as in 8.1. 

For each set of samples, make a blank titration of 125 mL of the titration solvent. For manual titration, add 0.1-mol/L alcoholic KOH solution in 0.05-mL increments, waiting between each addition until a constant cell potential is reached. Record the meter and burette readings when the former becomes constant after each increment. For automatic titration proceed as in 11.4. 

Curve B—10.00 g of used crankcase oil plus 125 mL of titration solvent. Since no sharp Inflections are apparent, the end points are chosen at the meter readings obtained with the two nonaqueous buffer solutions. 

Curve C—10.00 g of oil containing a weak acid plus 125 mL of titration solvent. The end point Is chosen as the point at which the curve Is most neatly verticisl. 

C = alcoholic KOH solution used to titrate the sample to end point that occurs at a meter reading corresponding to acid nonaqueous buffer, mL, and D = alcoholic HQ solution used to titrate solvent blank to end point corresponding to C, mL. 

Electrodes—A platinum wire electrode pair with each wire approximately 12 mm long and 1 mm in diameter. The wires shall be located 5 mm apart and approximately SS mm below the level of the titration solvent. Clean the electrode pmr at regular intervals with 65 % nitric acid and rinse with distilled water before use. 

The test specimen taken shaU not exceed 10 mL and the volume of bromide-bromate titrant used shaU not exceed 10 mL and no separation of the reaction mixture into two phases shall occur during the titration. Difficulty may be experienced in dissolving specimen of the high boiling ranges in the titration solvent this can be prevented by the addition of a smaU quantity of toluene. 

Introduce 110 mL of titration solvent into the vessel and pipet in a 5-mL aliquot of the sample solution from the 50-mL volumetric flask. Switch on the stirrer and adjust to a rapid stirring rate, but avoid any tendency for air bubbles to be drawn down to the solution. 

Blanks—Make duplicate blank titrations of each batch of titration solvent and reagents by repeating the entire procedure, using S mL of 1,1,1-trichloroetihane in place of the test aliquot. Less than 0.1 mL of bromide-bromate solution should be required. If more than 0.1 mL is used, discard the analysis, prepare fresh titration solvent and fresh reagents and repeat e analysis. 

Blanks—Make duplicate blank titrations on each batch of titration solvent and reagents. Less than 0.10 mL of bromide-bromate solution should be required. 

The hydrogen sulfide-free sample is dissolved in an alcoholic sodium acetate titration solvent and titrated potentiometiically with sQver nitrate solution, using as an indicator the potential between a glass reference electrode and a sUver/sUver-sulfide indicating electrode. Under these conditions, the mercaptan sulfur is precipitated as silver mercaptide and the end point of the titration is shown by a huge change in ceD potential. 

Titration Solvent—Low molecular weight mercap-tans, as usually found in gasoline, are readily lost from the titration solution if an acidic titration solvent is used. For the determination of the higher molecular weight mercaptan as normally encountered in kerosines, aviation turbine fuels and distillate fuels, the acidic titration solvent is used to achieve more rapid equilibrium between successive additions of the titrant. 

Place electrode in operating position and immerse it in 100 mL of titration solvent containing 8 mL of Na2S solution. 

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