ASTM D974 method for acid and base number by color-indicator titration
9. Procedure for Acid Number
9.1 Into an appropriate size Erlenmeyer flask or a beaker, introduce a weighed quantity of the sample as given in Table 1. Add 100 mL of the titration solvent and 0.5 mL of the indicator solution, and without stoppering, swirl until the sample is entirely dissolved by the solvent. If the mixture assumes a yellow-orange color, proceed as directed in 9.2; if it becomes green or green-black, proceed as directed in Section 10.
NOTE 11 - In routine analysis, the indicator may be pre-mixed with the titration solvent before adding to the sample.
9.2 Without delay, titrate at a temperature below 30°C (Note 14). Add 0.1 M KOH solution in increments and mix to disperse the KOH as necessary (see Note 12). Shake vigorously near the end point, but avoid dissolving carbon dioxide (CO2) in the solvent. (In the case of acidic oils, the orange color changes to a green or green-brown as the end point is approached.) When the solution first turns green or green-brown, reduce the increment size to dropwise (manual buret) or between 0.01 and 0.05 mL (automated buret). Continue until a persistent green or green-brown end point is reached (see Note 13) and held for a minimum of 15 s after the addition of the last increment or if it reverses with two drops of 0.1 M HCl.
NOTE 12 - When acid numbers about or below one are expected, better precision can be obtained by substituting 0.01 or 0.05 M solutions in 9.2 and 9.3. This substitution was not included in the development of a precision statement.
NOTE 13 - To observe the end point of dark-colored oil, shake the flask vigorously to produce momentarily a slight foam when the color change occurs as the last few drops of titrant are added and observe the titration under a white fluorescent lamp at bench top level.
NOTE 14 - The temperature can be measured by any suitable temperature measuring device.
NOTE 15 - An automated photometric device may also be used to detect the titration end point. However, the precision estimates given in Section 15 may not apply to this mode of titration.
9.3 Blank - Perform a blank titration on 100 mL of the titration solvent and 0.5 mL of the indicator solution, adding 0.1-mL or less increments of the 0.1 M KOH solution.
9.3.1 The titration solvent usually contains weak acid impurities which react with the strongly basic components of the sample. To correct the base number for the sample, determine an acid number blank upon the solvent.
10. Procedure for Base Number
10.1 If the titration solvent containing the dissolved sample assumes a green or greenish-brown color after the indicator is added (9.1), carry out the titration as described in 9.2, but use 0.1 M HCl and titrate until the green-brown color changes to orange.
10.2 Blank - Perform a blank titration as directed in 9.3.
11. Procedure for Strong Acid Number
11.1 Introduce approximately 25 g of a representative sample, weighed to the nearest 0.1 g, into a 250-mL separatory funnel and add 100 mL of boiling water. Shake vigorously and drain the water phase, after separation, into a 500-mL titration flask. Extract the sample twice more with 50-mL portions of boiling water, adding both extracts to the titration flask. To the combined extracts add 0.1 mL of methyl orange indicator solution and, if the solution becomes pink or red, titrate with 0.1 M KOH solution until the solution becomes golden brown in color. If the initial color is not pink or red, report the strong acid number as zero. (See Note 15.)
11.2 Blank - Into a 250-mL Erlenmeyer flask, introduce 200 mL of the same boiling water as used for the sample titration. Add 0.1 mL of methyl orange indicator solution. If the indicator color is yellow-orange, titrate with 0.1 M HCl to the same depth and shape of color obtained in the titration of the sample. If the indicator color is pink or red, titrate with 0.1 M KOH solution to the same end point as that used in the sample titration.
