ASTM D500 Test Methods of Chemical Analysis of Sulfonated and Sulfated Oils
ACIDITY AS FREE FATTY ACIDS OR ACID NUMBER
Test Method A. In the Absence of Ammonium or Triethanolamine Soaps
53. Scope
53.1 This method of analysis covers the determination of the acidity as free fatty acids existing in a sample of sulfonated or sulfated oil or both by titrating the sample dissolved in a solvent. It is not applicable in the presence of ammonium or triethanolamine soaps or salts, or other compounds that do not react neutral to phenolphthalein when dissolved in alcohol.

54. Reagents
54.1 Alcohol-Ether Mixture - Mix 50 mL of neutral alcohol with 25 mL of ether.

54.2 Phenolphthalein Indicator Solution (10 g/L) - Dissolve 1 g of phenolphthalein in 100 mL of ethanol (95 %).

54.3 Sodium Hydroxide, Standard Solution (0.5 N) - Prepare and standardize a 0.5 N solution of sodium hydroxide (NaOH). Potassium hydroxide (KOH) may be substituted for NaOH.

55. Procedure
55.1 The procedure consists of titrating a solution of the sample in a mixture of alcohol and ether in the presence of phenolphthalein indicator. Dissolve 10 g of the sample in an alcohol-ether mixture, add 5 drops of phenolphthalein indicator solution, and titrate the solution with 0.5 N NaOH or KOH solution until the pink color persists after vigorous shaking.

56. Calculation
56.1 Calculate the acidity as free fatty acids, as follows:
K = (C x E)/W
where:
K = acidity as free fatty acids or acid number, mg of KOH/g,
C = millilitres of NaOH or KOH solution required for titration of the sample,
E = strength of NaOH or KOH solution, mg of KOH/mL, and
W = weight of sample, g.

Test Method B. In the Presence of Dark Colored Oils but in the Absence of Ammonium or Triethanolamine Soaps (Brine Method)
57. Scope
57.1 This test method covers the determination of the acidity as free fatty acids existing in a sample of sulfonated or sulfated oil or both by titrating a water solution of the sample with phenolphthalein as the indicator. It is not applicable in the presence of ammonia or triethanolamine soaps or acid salts.

58. Reagents
58.1 Ethyl Ether.

58.2 Phenolphthalein Indicator Solution (10 g/L) - Dissolve 1 g of phenolphthalein in 100 mL of ethanol (95 %).

58.3 Sodium Chloride (NaCl).

58.4 Sodium Hydroxide, Standard Solution (0.5 N) - Prepare and standardize a 0.5 N solution of sodium hydroxide (NaOH).

59. Procedure
59.1 The procedure consists of titrating a water solution of the sample in the presence of salt, ether, and alcohol with phenolphthalein as the indicator. Dissolve 10 g of the sample in 100 mL of water contained in a 250-mL, glass-stoppered, Erlenmeyer flask, warming to obtain solution, if necessary. After cooling, add 30 g of NaCl, 25 mL of ether, and 50 mL of neutral alcohol (Note 13). Add 5 drops of phenolphthalein indicator solution and titrate the contents of the flask with 0.5 N NaOH solution until the first pink tinge appears in the water layer, shaking vigorously after each addition of the reagent.

NOTE 13 - At this stage, methyl orange indicator solution may be added and the total alkalinity determined as described in the determination of organically combined sulfuric anhydride, Test Method A (18.1.1).

60. Calculation
60.1 Calculate the acidity as free fatty acids as described in the determination of acidity as free fatty acids, Test Method A (Section 56).

NOTE 14 - In case the total alkalinity was determined on the same
sample the acidity of the free fatty acids is calculated as follows:
K = [(C x E)/W] - A
where:
K = acidity as free fatty acids or acid number, mg of KOH/g,
C = millilitres of NaOH solution required for titration of the sample,
E = strength of NaOH solution, mg of KOH/mL,
A = total alkalinity, and
W = weight of sample, g.

Method C. In the Presence of Ammonium or Triethanolamine Soaps
61. Scope
61.1 This test method covers the determination of the acidity as free fatty acids in the presence of ammonium or triethanolamine soaps, or both, existing in a sample of sulfonated or sulfated oils, or both, by calculation from the total alkalinity and the free-and-alkali-combined fatty acids.

62. Procedure
62.1 The procedure consists of two determinations: namely; (a) total alkalinity, and (b) free-and-alkali-combined fatty acids.
62.1.1 Total Alkalinity - Determine the total alkalinity as described in the determination of organically combined sulfuric anhydride, Test Method A (18.1.1).

62.1.2 Free-and-Alkali-Combined Fatty Acids - Extract the total sulfated and sulfonated fatty matter as described in the determination of total active ingredients (35.1 and 35.2), but omit the dehydration of the extract (Note 15). Add exactly 0.5 mL of 0.5 N NaOH solution to the extract, evaporate the ether to a volume of about 25 mL, add approximately 50 mL of neutral alcohol, and titrate the mixture with 0.5 N NaOH solution to a phenolphthalein end point.

NOTE 15 - In the presence of acetates, wash the ether layer carefully with concentrated NaCl solution until 1 or 2 drops of 0.5 N NaOH solution added to the final wash water are neutral to phenolphthalein indicator.

63. Calculation
63.1 Calculate the free-and-alkali-combined fatty acids as described in the determination of acidity as free fatty acids, Test Method A (Section 56). Calculate the acidity as free fatty acids in the presence of ammonium or triethanolamine soaps, or both, as follows:
K = J - A
where:
K = acidity as free fatty acids or acid number, mg of KOH/g,
J = free and alkali-combined fatty acids, and
A = total alkalinity.