ASTM D1091 test methods for phosphorus in lubricating oils and additives
GRAVIMETRIC METHOD
19. Scope
19.1 This test method covers the determination of total phosphorus in concentrations of 2 mass % or more, (see Note 10), calculated on the basis of the original sample, in samples treated by the acid-oxidation procedure described in Sections 7-11.

NOTE 10 - For phosphorus concentrations less than 2 mass %, see Sections 12-18.

20. Summary of Test Method
20.1 After oxidation of organic material in the test specimen and quantitative conversion of the phosphorus to phosphate ion, the phosphate ion is separated from interfering metals by precipitation as ammonium molybdiphosphate in nitric acid solution. After an ammoniacal solution of the phosphate ion is obtained, the phosphorus is precipitated as magnesium ammonium phosphate, ignited, and weighed as magnesium pyrophosphate.

21. Apparatus
21.1 Electric Muffle Furnace, capable of operating over a variable temperature range from 200 to 1100°C and of maintaining a temperature of 1050 more or less 50°C.

21.2 Filtering Crucible, 25-mL porcelain crucibles, having porous bottoms capable of retaining a fine precipitate.

22. Reagents
22.1 Ammonium Hydroxide (relative density 0.90), concentrated ammonium hydroxide (NH4OH).

22.2 Ammonium Hydroxide (3+5) - Mix 3 volumes of NH4OH (relative density 0.90) with 5 volumes of water.

22.3 Ammonium Hydroxide (1+24) - Mix 1 volume of NH4OH (relative density 0.90) with 24 volumes of water.

22.4 Ammonium Nitrate, NH4NO3 crystals.

22.5 Ammonium Nitrate Solution - Dissolve 50 g of NH4NO3 in water and dilute to 1 L.

22.6 Hydrochloric Acid (reative density 1.19), concentrated hydrochloric acid (HCl).

22.7 Magnesia Mixture - Dissolve 50 g of magnesium chloride (MgCl2•6H2O) and 100 g of ammonium chloride (NH4Cl) in 500 mL of water, add a slight excess of NH4OH, and allow to stand overnight. Filter, make the solution just acid with HCl, and dilute to 1 L.

22.8 Methyl Red Indicator Solution (1 g/L) - Dissolve 0.5 g of methyl red in 300 mL of alcohol (95 % ethyl alcohol or denatured alcohol conforming to Formula No. 3A of the Alcohol, Tobacco, and Firearms Bureau), and dilute with water to 500 mL.

22.9 Molybdate Reagent - Dissolve 100 g of ammonium molybdate (NH4)6•Mo7O24•4H2O) in 400 mL of water. Add 80 mL of NH4OH (rel dens 0.90) and filter if a precipitate appears. Mix 400 ml of HNO3(relative density 1.42) with 600 mL of water. Prepare the ammonium molybdate reagent from these solutions immediately before use by slowly mixing 1 volume of the ammonium molybdate solution with 2 volumes of the diluted HNO3, while stirring rapidly.

22.10 Nitric Acid (1+1) - Mix equal volumes of nitric acid (HNO3, relative density 1.42) and water.

22.11 QC Samples, preferably, portions of one or more liquid petroleum materials that are stable and representative of the samples of interest. These QC samples can be used to check the validity of the testing process, as described in Section 26.

23. Procedure for Samples Containing No Metals Other than Alkali Metals 23.1 Cool the Kjeldahl flask (see 11.5), transfer the solution to a 400-mL beaker, and wash the flask with small portions of water until the volume of solution is approximately 100 mL. Boil the solution for 5 to 10 min, cool to near room temperature, and add NH4OH (rel dens 0.90) until the solution is neutral to methyl red. Make the solution acid with HCl (relative density 1.19), and add 1 mL in excess.

23.2 Add 20 mL of magnesia mixture, slowly and while stirring, and cool the solution to below room temperature in an ice bath. Add NH4OH (relative density 0.90), slowly and while stirring constantly, until the solution is basic. Continue stirring until most of the precipitate has formed (see Note 11); then add 5 mL of NH4OH (relative density 0.90) in excess. Allow the precipitate to stand overnight.

23.3 Filter through a weighed porcelain filter crucible of fine porosity, wash with NH4OH (1+24), and dry in an oven. Place in a cool furnace, gradually raise the temperature to red heat, and ignite at 1050 more or less 50°C for 30 to 40 min. Repeat the ignition for similar periods until constant weight is reached.

NOTE 11 - For work of highest accuracy, it is generally necessary to test the precipitation technique on known inorganic samples. Reprecipitation sometimes aids in obtaining more accurate values.

24. Procedure for Samples Containing Metals Other than Alkali Metals 24.1 Cool the Kjeldahl flask (see 11.5), add 40 to 50 mL of water, cool to room temperature, and filter the solution through a medium-texture, ashless paper. Collect the filtrate in a 500-mL wide-mouth, glass-stoppered Erlenmeyer flask, and wash the Kjeldahl flask and filter paper thoroughly with water, adding the rinsings to the filtrate; discard the paper. Boil the solution for several minutes, and cool to near room temperature. Add NH4OH (relative density 0.90) until the solution is neutral to methyl red; then add HNO3(1+1) until the color just changes to red. Concentrate or dilute the solution to approximately 150 mL.

24.2 Add 15 g of NH4NO3 crystals and swirl until dissolved. Adjust the temperature to 35 to 40°C and add 240 mL of freshly prepared molybdate reagent. Stopper the flask, shake vigorously for 4 to 6 min, and allow to stand for at least 2 h, or preferably overnight. Filter the solution through a medium-texture, ashless paper. Wash the precipitate with NH4NO3 solution. Do not attempt to transfer all of the precipitate from the flask to the paper; however, reserve the flask for later treatment. Wash the precipitate several times with the wash solution but do not allow the stream of wash solution to strike the funnel above the edge of the paper as the precipitate has a tendency to creep.

24.3 Place a clean 400-mL beaker under the funnel, and dissolve the precipitate through the paper into the beaker with NH4OH (3+5). Use a little of the NH4OH to dissolve any of the precipitate that remained in the flask set aside in 24.2 and pour this solution through the paper. Wash the flask, funnel, and paper four times with hot water, once with NH4OH (3+5), and once again with water. Discard any residue remaining on the paper. Evaporate the solution to a volume of 90 to 100 mL, make the solution acid with HCl, and add 1 mL in excess. Disregard any molybdiphosphate precipitate that may appear at this point.

24.4 Add 20 mL of magnesia mixture, slowly while stirring, and cool the solution to below room temperature in an ice bath. Add NH4OH (relative density 0.90), slowly and while stirring constantly, until the solution is basic. Continue stirring until most of the precipitate has formed (see Note 10); then add 5 mL of NH4OH (relative density 0.90) in excess. Allow the precipitate to stand overnight.

24.5 Filter through a weighed porcelain filter crucible (fine porosity), wash with NH4OH (1+24), and dry in an oven. Place in a cool furnace, gradually raise the temperature to red heat, and ignite at 1050 more or less 50°C for 30 to 40 min. Repeat the ignition for similar periods until constant weight is reached.