ASTM D1091 test methods for phosphorus in lubricating oils and additives
PHOTOMETRIC (MOLYBDIVANADO) METHOD
12. Scope
12.1 This test method covers determination of total phosphorus in concentrations of less than two mass % (see Note 6), calculated on the basis of the original test specimen, in samples treated by the acid-oxidation procedure described in Sections 7-11.

NOTE 6 - For phosphorus concentrations greater than or equal to two mass %, see Sections 19-25.

13. Summary of Test Method
13.1 After oxidation of organic material in the test specimen and quantitative conversion of the phosphorus to phosphate ion, the acidity of the digestion mixture is adjusted and the mixture diluted to suitable volume. Solutions of ammonium vanadate and ammonium molybdate are added in the order named. The addition of the molybdate solution to the acid vanadate-phosphate mixture results in the formation of a heteropoly acid, molybdivanadophosphoric acid, which is yellow in color. Although the exact composition of molybdivanadophosphoric acid is uncertain, solutions of this compound, when formed in accordance with carefully prescribed conditions, have been found to conform to the Beer-Lambert law for optical transmittance measurements made at 420 to 470 nm as a function of phosphorus content.

14. Apparatus
14.1 Photoelectric Photometer - A spectrophotometer capable of isolating a 5-nm spectral band at 430 and 460 nm is a suitable instrument for use in this determination. The instrument should be equipped with auxiliary facilities for handling 1, 2, and 5-cm cells, and a supply of these should be available. Other instruments such as photoelectric filter photometers may also be used.

NOTE 7 - While not as desirable as photometers, visual color comparators can also be used, if necessary.

15. Reagents
15.1 Ammonium Molybdate Solution - Dissolve 50 g of ammonium molybdate (NH4)6Mo7O24•4H2O) in warm water and dilute to 1 L. Filter before using.

15.2 Ammonium Vanadate Solution - Dissolve 2.5 g of ammonium vanadate (NH4VO3) in 500 mL of hot water, add 20 mL of concentrated nitric acid (HNO3 relative density 1.42), and dilute to 1 L.

15.3 Phosphate, Standard Solution (1 mL = 0.1 mg P) - Dissolve 0.4393 g of potassium dihydrogen phosphate (KH2-PO4) in water and dilute to 1 L. For best work, the salt should be twice recrystallized and vacuum-dried before use.

15.4 Sulfuric Acid (relative density 1.84), concentrated sulfuric acid (H2SO4).

15.5 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.

16. Calibration and Standardization
16.1 Introduce 0, 0.4, 0.8, 1.6, 2.4, 4.0, 4.8, 8.0, 16, 24, and 32 mL of standard phosphate solution into 100-mL ground-glass-stoppered volumetric flasks. Add sufficient H2SO4 of any convenient concentration such that the final acid concentration after dilution to 100 mL will be 0.5 N. Dilute to 55 to 60 mL, and add 10 mL of ammonium vanadate solution and ammonium molybdate solution, in the order named, with adequate mixing between additions. Dilute to 100 mL, close with a ground-glass stopper, and mix thoroughly. Allow to stand at least 45 min but no longer than 60 min to develop the color.

16.2 Using the 1-cm cell and with the wave length set at 460 nm, adjust the photometer to read 100.0 % transmittance with the zero phosphate (reagent blank) standard. Although absorption cells are usually very closely matched, for best work it is recommended that two cells be used and that one be reserved for the blank and the other for the standard or sample solutions. Obtain transmittance measurements on solutions containing 0.4, 0.8, 1.6, 2.4, and 3.2 mg of phosphorus. These standards should give measurements falling between 90 and 20 % respectively. After making a measurement, return to the reagent blank cell. This should check the 100.0 % setting within 0.2 %. Repeat the reading of the standard and return to the blank. Obtain three readings in all of each standard solution. Using semilog graph paper, plot the average transmittance as a function of phosphorus content. The resultant curve should be a straight line.

16.3 In a similar manner, prepare calibration curves at 460 nm for the 2 and 5-cm cells, selecting concentrations from the series of standards that give readings between 20 and 90 %.

16.4 Finally, prepare a calibration curve for the 5-cm cell, using a wavelength setting of 430 nm. At this wavelength the molybdivanadophosphoric acid has a higher optical density, and the curve obtained will have a steeper slope. The advantages of having this calibration at the second wavelength are two-fold: (1) it provides increased sensitivity in the region of low concentrations, and (2) it provides an independent confirmation of measurements made at 460 nm. Agreement between measurements at both wavelengths is a criterion of the absence of interference.

17. Procedure
17.1 To the cooled, decomposed sample in the Kjeldahl flask (see 11.5), add by visual observation sufficient H2SO4 to bring the acidity to approximately one half the concentration present at the beginning of the acid-oxidation procedure (see 11.1). This step may not always be necessary (see Note 8). Cool the flask and contents and transfer to a 100-mL volumetric flask, using approximately 50 mL of water. (Warning - Extreme care should be exercised when adding water to H2SO4. It is advisable to add the water slowly, a small amount at a time, allowing it to run down the side of the flask, which is adequately cooled.)

NOTE 8 - The acidity of the solution after acid oxidation is critical, since interference occurs from the appearance of an orange-yellow color, which forms in a neutral or too acid solution. The acidity for proper development of the desired color should be in the range from 0.4 N to 0.6 N in H2SO4. Adjustment of acidity can not be required when the losses of H2SO4 have been kept to a minimum in the fuming steps of the acid-oxidation procedure; however, it may be necessary to further evaporate H2SO4 in order to bring the acidity of the solution to approximately optimum normality.

17.2 Add 10 mL each of ammonium vanadate solution (see Note 9) and ammonium molybdate solution. It is important that these solutions be added in the order named, with adequate mixing between additions, to ensure the reproducible composition of the complex. Dilute to 100 mL, stopper with a ground-glass stopper, and mix thoroughly. Allow to stand at least 45 min but no longer than 60 min to develop the color. Maintain the temperature of this solution within 5°C of the temperature at which the calibration was performed.

NOTE 9 - Remove the last trace of hydrogen peroxide since very little hydrogen peroxide is required to develop the maximum color of the vanadium-hydrogen peroxide complex. Any trace of H2O2 will be evident by the reddish brown color obtained upon the addition of the vanadium reagent. When such is the case, the sample must be discarded and the acid-oxidation step will have to be repeated on a new test specimen.

17.3 When any insoluble matter is present, transfer a portion of the solution to a centrifuge tube, centrifuge at 1200 rpm for 5 min, and decant the clear supernatant liquid into the absorption cell. If desired, a portion of the solution may be drawn off by means of a filter stick. Avoid the use of filter paper as the colored complex may be adsorbed on it.

17.4 When the approximate phosphorus content is known, the path length of the absorption cell should be chosen to give a transmittance between 25 and 50 %. It is desirable to employ conditions such that readings fall within this range to reduce the error in the photometric measurement. If the phosphorus content is unknown, the analyst, with experience, will be able to select the best cell to use by visual observation. In the case of test specimens that prove to be too highly colored to be read directly, transfer an appropriate aliquot to another volumetric flask and dilute with the reagent blank solution in order to maintain all the reagent concentrations at the proper level. Make all measurements at 460 nm, except for the extremely low concentrations (below 0.25 mg P/100 mL), which shall be made at 430 nm. In making the readings, adjust the galvanometer to 100.0 % with the reagent blank solution in the light path. Insert the sample in the light path, read the percentage transmittance to 0.1 %, and return to the reagent blank, which should check the original setting within 0.2 %. Readjust to 100.0 if necessary and repeat, obtaining at least three readings on the samples. These should agree within 0.2 %. Use the average of these readings to obtain the phosphorus content from the calibration curves.

17.5 Overall Blank - Although a reagent blank solution is used in preparing the calibration curves, an overall blank determination should be carried through on a sample of phosphorus-free white oil. No phosphorus should be detectable in such a blank.

18. Calculation
18.1 Calculate the percentage of phosphorus as follows:
Phosphorus, mass % = (Ps - Pb) x D x 100)/(1000 x S)
where:
Ps = milligrams of phosphorus in test specimen read from standard curve,
Pb = milligrams of phosphorus in overall blank read from standard curve,
D = dilution factor, if an aliquot is used (see 17.4), and
S= mass of test specimen.