ASTM D128 standard test methods for analysis of lubricating grease
GLYCERIN (QUANTITATIVE)
25. Scope
25.1 This test method is intended for the determination of free glycerin in grease and for glycerin present as fat in grease. The test method is applicable to greases containing 0.03 to 1.6 % of glycerin and is accurate even in the presence of ethylene and propylene glycols.
26. Summary of Test Method
26.1 To determine free glycerin in grease, the sample is refluxed with dilute sulfuric acid and n-hexane. The warm mixture is extracted with warm water, the washings neutralized and oxidized with potassium periodate, (Warning - May be irritating to skin and eyes. May react vigorously with reducing agents) and the resulting solution titrated with standard sodium hydroxide solution (Warning - Poison. Causes eye and skin damage). Glycerin oxidizes to 2 mol of formaldehyde and 1 mol of formic acid, as follows:
C3H8O3 + 2KIO4 →2HCHO + HCOOH + 2KIO3 + H2O
26.2 To determine free glycerin and glycerin combined as fat, aqueous sodium hydroxide is added before refluxing, instead of sulfuric acid. After saponifying, the soaps are decomposed by refluxing with mineral acid, the n-hexane is evaporated, and the determination completed as for free glycerin.
Determination of Free Glycerin
27. Procedure
27.1 Weigh 10 g of the sample into a 250-mL Erlenmeyer flask and add dilute H2SO4(3 mL of concentrated H2SO4 plus 20 mL of distilled water) and 20 mL of n-hexane.
27.2 Reflux the mixture on a steam bath until the grease is disintegrated. Allow the contents of the flask to cool to room temperature. If contents remain liquid, follow the procedure prescribed in 27.3. If contents congeal or if solid particles form, follow the procedure prescribed in 27.4.
27.3 Add 50 mL of n-hexane and transfer the contents of the flask quantitatively to a 500-mL separator, washing the flask with 50 mL of n-hexane and 25 mL of distilled water, and introducing these washings into the separator. Shake the contents of the separator and allow the two layers to separate. Draw off the water layer into a second separator containing 100 mL of n-hexane. Wash the first separator with two 30-mL portions of distilled water and add these washings to the second separator.
27.4 Heat contents of the flask to approximately 63°C and transfer quantitatively to a 500-mL separator, washing the flask with 50 mL of warm n-hexane and 25 mL of warm distilled water, and introducing these washings into the separator. Keep the contents of the separator warm, shake, and allow the two layers to separate. Draw off the water layer into a second separator containing 100 mL of n-hexane. Wash the first separator with two 30-mL portions of warm distilled water and add these washings to the second separator.
27.5 Shake the second separator, allow the contents to separate, and filter the water layer into a 300-mL ground-glass stoppered Erlenmeyer flask. Wash the second separator once with 20 mL of distilled water and filter the washings into the same Erlenmeyer flask. The n-hexane solutions may then be discarded.
27.6 Add three drops of methyl red indicator solution (see Note 10) to the aqueous solution and make almost neutral with NaOH solution (240 g/L). Add two more drops of the indicator solution and carefully titrate the solution to a copper-colored end point with 0.05 N NaOH solution.
NOTE 10 - The indicator is red in a neutral or acid solution. An alkaline (aqueous unadulterated) solution is yellow. In the glycerin determination the solution is copper-colored, corresponding to faint alkalinity.
27.7 Blank - Prepare a blank by adding 23 mL of H2SO4 (3 + 20) to 80 mL of distilled water in a 300-mL glass-stoppered Erlenmeyer flask, and neutralize the solution to the methyl red end point as described in 27.6.
27.8 Add 3 g of cp KIO4 to each flask. Tie the stoppers carefully in place and shake the flasks for 2 h. (A mechanical shaker is recommended for this purpose.) Remove excess KIO4 by filtering the solutions through close-textured paper and collect the filtrates in clean, 300-mL, glass-stoppered Erlenmeyer flasks. Add six drops of methyl red indicator solution to each flask and titrate the solutions to a copper-colored end point with 0.05 N NaOH solution. Record the amount of NaOH solution used.
Determination of Total Glycerin (Free and Combined)
