ASTM D94 standard test method for saponification number of petroleum products
METHOD B - POTENTIOMETRIC TITRATION
14. Apparatus
14.1 Erlenmeyer Flask and Condenser - An Erlenmeyer flask, 250 or 300-mL capacity, alkali-resistant (see Note 2), to which is attached a straight or mushroom-type reflux condenser. The straight-type condenser is fitted to the flask with a ground-glass joint; the mushroom-type condenser must fit loosely to permit venting of the flask. Water reflux condensers can also be used instead of air condensers.

14.2 Hot Plate - A suitable hot plate heated by either electricity or steam (see 6.2).

14.3 Potentiometric Titrator Automatic, Recording or Manual - High-precision titrator capable of distinguishing the carbonate from pure caustic in the titration of ACS reagent grade KOH by hydrochloric acid (HCl).

14.4 Electrodes - High-quality electrodes must be used. Cleaning and maintenance of the electrodes are vital to their satisfactory operation.

14.4.1 Combination Glass Electrode, or a suitable glass electrode and a suitable reference electrode; either silver chloride (AgCl)/saturated alcoholic lithium chloride (LiCl) or saturated potassium chloride (KCl) inverted glass sleeve calomel electrode can be used.

14.5 Magnetic Stirring Bars - Because, on titration, two immiscible liquid phases appear, and potassium chloride (KCl) is precipitated, stirring conditions are critical, and very vigorous stirring is essential (see Note 16).

NOTE 16 - If a large magnetic stir bar is used, only slow speeds are possible, and if too small a stir bar is used, the highest rotational speeds cause insufficient agitation of the bulk of the liquid. The optimum magnetic stir bar has been found to be a 2.5 by 0.5-cm plain polytetrafluoroethylene (PTFE)-coated cylinder. This magnetic stirrer is not needed if a propeller or paddle stirrer is used.

14.6 Tall Form Beakers - 250 or 300-mL size Berzelius type tall-form glass beaker with or without a spout (see Note 17).

NOTE 17 - The potentiometric titration cannot be performed in the Erlenmeyer flask used in digestion. The small opening of this flask will not accommodate the electrodes, unless a combination electrode is used.

14.7 Stirrer, Buret Stand, Titration Vessel - A typical cell assembly is shown in Fig. 2. The propeller or paddle stirrer is not needed if a magnetic stirrer is used.

14.7.1 Stirrer, either mechanical or electrical, with variable speeds and with propeller or paddle of chemically inert material. If an electrical stirrer is used, it must be grounded so that disconnecting or connecting the power to the motor will not produce a permanent change in meter reading during the course of titration. A magnetic stirrer with stirring bar can be used provided it meets the above conditions.

14.7.2 Buret, 10 or 20-mL, graduated in 0.05-mL divisions and calibrated with an accuracy of 60.02 mL, or an automatic buret of similar accuracy.

14.7.3 Titration Stand, suitable to support the beaker, electrodes, stirrer, and buret. An arrangement that allows for the removal of the beaker without disturbing the electrodes, buret, and stirrer is desirable (see Note 18).

NOTE 18 - Some apparatus can be sensitive to interference by static electricity, shown by erratic movements of recorder pen or meter indicator, when the titration assembly (beaker and electrodes) is approached by the operator. In that case, surround the beaker closely with a cylinder of copper gauze that is electrically grounded.

15. Reagents
15.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.

NOTE 19 - Commercially available reagents may be used in place of laboratory preparations, provided they meet the specifications outlined.

15.2 Purity of Water - Unless otherwise indicated, references to water shall be understood to mean reagent water as defined by Type III in Specification D1193.

15.3 Alcohol - 95 % ethanol (Warning - See 7.3) (see Note 4) or 95 % ethanol to which has been added 10 volume % of methanol (see 7.3, Note 4, and Note 5), or absolute alcohol.

15.4 Aqueous Hydrochloric Acid Standard Solution (0.5 M) - Standardize frequently enough to detect molarity changes of 0.0005, by titrating with standard alcoholic KOH solution (see 15.7 and Note 6).

15.5 Butanone (Methyl Ethyl Ketone), technical grade. Store in dark or brown bottles. (Warning - See 4.1.)

15.6 Naphtha, (Warning - Extremely flammable. Harmful if inhaled. Vapors can cause flash fire.) ASTM Precipitation Grade (or Petroleum Spirit-60/80 or hexanes) (Warning - Combustible. Vapor harmful.) Petroleum spirit shall conform to the current IP 136.

15.7 Alcoholic Potassium Hydroxide Standard Solution (0.5 M) - Prepare approximately 0.5 M solution by dissolving KOH in the alcohol specified in 7.3. Allow the solution to settle in a dark place. Filter the solution, and allow to stand for 24 h before using.

15.7.1 Alternatively prepare 0.5 or 0.1 M alcoholic KOH by mixing commercially available KOH ampule (which is carbonate free) with 95 % alcohol. Using this type solution gives consistent blanks and does not give multiple breaks (see Note 7).

15.7.2 The KOH solutions shall be standardized by titrating with standard potassium hydrogen phthalate solution (see 15.8 and Note 7).

15.8 Potassium Hydrogen Phthalate - (C8H5KO4) 0.1 M Standard Solution - Weigh 2.0422 more or less 0.0002 g of potassium hydrogen phthalate that has been dried at 110 more or less 5°C for 1 h into a 100-mL volumetric flask. Dissolve in distilled or deionized water. Some heating can be necessary to dissolve the solid. Dilute to 100 mL with distilled or deionized water after the solution has cooled.

15.9 Stoddard Solvent, technical grade. (Warning - See 7.10.)

15.10 Potassium Chloride, Aqueous (3.0 M) - Prepare by dissolving 225.2 g reagent grade KCl in 1.0 L of distilled or deionized water.

15.11 Xylene, reagent grade. (Warning - See 7.11.) See 15.1.

15.12 Chlorobenzene, reagent grade. See 15.1.

16. Preparation, Testing, and Maintenance of Electrode System
16.1 Preparation of Electrodes:
16.1.1 If the calomel electrode is to be changed from nonaqueous to aqueous bridge, drain out the nonaqueous solution, wash with water and methanol, then rinse the outer jacket (salt bridge) several times with 3.0 M aqueous KCl electrolyte solution. Finally, fill the outer jacket with 3.0 M aqueous KCl electrolyte solution up to the filling hole.

16.1.2 When using the sleeve-type electrode, carefully remove the ground-glass sleeve and thoroughly wipe both ground surfaces. Replace the sleeve loosely and allow a few drops of electrolyte to drain through to flush the ground-glass joint and to wet the ground surfaces thoroughly with electrolyte. Set the sleeve firmly in place, refill the outer jacket with the 3.0 M aqueous KCl electrolyte solution, and rinse the electrode with chlorobenzene.

16.1.3 When in use, the electrolyte level in the calomel electrode should be kept above that of the liquid in the titration beaker to prevent entry of contaminants into the salt bridge. When not in use, fill the calomel electrode with 3.0 M aqueous KCl electrolyte solution, leave the bung in the filling orifice, and immerse both electrodes in distilled water, keeping the level of the electrolyte above that of the distilled water.

16.2 Testing of Electrodes - Test when new electrodes are installed, and retest once a month thereafter by standardizing 10 mL of 0.5 M alcoholic KOH using 0.5 M aqueous HCl.

16.3 Maintenance of Electrodes:
16.3.1 Clean the glass electrode (see Note 17) at least once every week during continual use by immersing in cold chromic acid cleaning solution (Warning - See 16.3.3) or Nochromix solution.

16.3.2 Drain the calomel electrode at least once each week and refill with fresh 3.0 M aqueous KCl electrolyte as far as the filling hole. Ascertain that crystallized KCl is present. Maintain the electrolyte level in the calomel electrode above that of the liquid in the titration beaker of vessel at all times.

16.3.3 Immerse the lower halves of the electrodes in water when not in use. Do not allow them to remain immersed in titration solvent for any appreciable period of time between titrations. Although the electrodes are not extremely fragile, handle them carefully at all times. (Warning - Cleaning the electrodes thoroughly, keeping the ground-glass joint free of foreign materials, and regularly testing the electrodes are very important in obtaining repeatable potentials because contamination can introduce uncertain erratic and unnoticeable liquid contact potentials, resulting in nonrepeatable results.)

16.3.4 At the end of the blank titration and between successive titrations a thin film of KCl crystals coats the electrode and the titrant delivery tip. Use a jet of water from a plastic squeeze bottle to remove it. Then rinse the electrode by immersion in a beaker full of distilled water for a few seconds. Dry the electrode by blotting with a paper towel; do not rub the electrode.

16.3.5 At the end of a set of sample titrations a mixture of KCl crystals and of sample fractions coats the electrode and titrant delivery tip. Cleaning is performed by immersion in a titration beaker containing the following:
50 mL of Stoddard Solvent
38 mL of 2-Propanol
38 mL of Distilled Water

Stir the solution for a time long enough to dissolve the coating; typically less than 1 min. Remove the traces of washing solution from the electrode with a jet of water from a plastic squeeze bottle. Then rinse the electrode further by immersion in a beaker full of water stirred for a few seconds. Dry the electrode by blotting with a paper towel; do not rub the electrode.

16.3.6 The electrode must be held firmly in a steady holder so that it does not wobble when the liquid is vigorously stirred. Electrode wobbling creates electrical noise, which interferes with the determination of the end points, resulting in nonrepeatable results.

17. Blank Determination
17.1 Make a blank determination concurrently with each set (see Note 8) (one or more) of samples as follows: Measure accurately from a buret or volumetric pipet (see Note 9) into the Erlenmeyer flask 25 more or less 0.03 mL of alcoholic KOH solution and 25 more or less 1 mL of butanone (methyl ethyl ketone) or one of the alternative solvents. Connect the condenser to the flask, and heat for 30 min (see 8.1 and Note 10) after refluxing begins. Immediately add 50 mL of ASTM precipitation naphtha (Warning - See 7.6) (see also Note 11 and Note 12) by cautiously pouring the naphtha down the condenser (disconnect condenser if mushroom type is used), and potentiometrically titrate the blank while hot, without reheating, with 0.5 M HCl.

17.2 Transfer the solution into a 200-mL borosilicate beaker, taking care to wash the flask with two 10-mL portions of naphtha (see 7.6).

17.3 Place the titration solution with a magnetic stir bar on a magnetic stir plate. Immerse the electrodes as far as possible without touching the magnetic stir bar. Adjust the speed of stirring to maximum possible without causing excessive aeration or splattering of solution.

17.4 Titrate the blank solution with 0.5 M aqueous HCl added at the rate of 2 mL/min, using potentiometric titrator. Two inflections with corresponding equivalence points are expected. The first one corresponds to the free KOH (record as VB in 21.1) and the second one to the small amount of potassium carbonate (K2CO3) generally present in commercial (KOH) (see Fig. 3).

17.5 A pre-addition of titrant used in the blank to expedite the titration time.

18. Sample
18.1 Make sure that the sample appears homogenous. Choose the size of the sample so that the back-titration volume is from 40 to 80 % of the blank, but do not exceed a 20-g sample weight (see Note 14).

19. Procedure
19.1 Weigh the specimen to the nearest 0.01 g (record as W in 21.1) such as by difference, from a small beaker into the Erlenmeyer flask. Add 25 more or less 1 mL of butanone or one of the alternative solvents (Warning - See 4.1), followed by 25 more or less 0.03 mL of alcoholic KOH solution (Warning - see 7.3) measured accurately from a buret or volumetric pipet) (see Note 6).

19.2 Dissolve the difficult to dissolve samples such as lubricants and additive first in 15 to 25 mL of Stoddard Solvent (Warning - See 7.10) or xylene (Warning - See 7.9) before adding butanone (Warning - See 4.1).

19.3 Connect the condenser to the flask and heat for 30 min after refluxing begins (see 8.1 and Note 10). Immediately add 50 mL of ASTM precipitation naphtha (Warning - Do not pour naphtha while the flask is on the hot plate) and (see 7.6) by cautiously pouring the naphtha down the condenser (see Note 11) (disconnect condenser if mushroom-type before adding the naphtha).

19.4 Titrate the solution while hot (without reheating) with 0.5 M aqueous HCl. Follow the titration procedure used for the blank (see Section 17) using the potentiometric titrator. Fig. 3 shows a complete titration curve for a sample. The titration of the sample differs from the blank in that no pre-addition of titrant is made and the titration conditions are much less critical.

19.4.1 The potential readings are fairly constant. The reading before any addition of titrant is -520 mV. The first inflection is moderately sharp and takes place around -300 mV. The second inflection is extremely sharp and takes place around 50 mV.

20. Quality Control Checks
20.1 Confirm the performance of the test procedure by analyzing a quality control (QC) sample that is, if possible, representative of the samples typically analyzed.

20.2 Prior to monitoring the measurement process, the user of the method needs to determine the average value and the control limits of the QC sample (see ASTM MNL 7).

20.3 Record the QC results and analyze by control charts or other statistical equivalent technique to ascertain the statistical control status of the total testing process (see ASTM MNL 7). Any out-of-control data should trigger investigation for root cause(s). The results of this investigation may, but not necessarily, result in process recalibration.

20.4 The frequency of QC testing is dependent on the criticality of the quality being measured, the demonstrated stability of the testing process, and customer requirements. Generally, a QC sample should be analyzed each testing day. The QC frequency should be increased if a large number of samples are routinely analyzed. However, when it is demonstrated that the testing is under statistical control, the QC testing frequency may be reduced. The QC precision should be periodically checked against the precision listed in the Precision and Bias section of this method to ensure data quality.

20.5 It is recommended that, if possible, the type of QC sample that is regularly tested be representative of the samples routinely analyzed. An ample supply of QC sample material should be available for the intended period of use, and must be homogenous and stable under the anticipated storage conditions.

20.6 See ASTM MNL 7, and Practice D6299 for further guidance on QC and Control Charting techniques.