ASTM D1091 test methods for phosphorus in lubricating oils and additives
OXIDATION OF THE SAMPLE
7. Scope
7.1 This test method covers a procedure for removal of organic material and subsequent conversion of phosphorus to phosphate ion in samples of unused lubricating oils, lubricating oil additives, and their concentrates.
8. Summary of Test Method
8.1 Organic material in the sample is destroyed and the phosphorus is converted to phosphate ion by oxidation with sulfuric acid, nitric acid, and hydrogen peroxide. The residual hydrogen peroxide is removed by diluting with water and evaporating several times to dense white fumes.
9. Apparatus
9.1 Digestion Flasks, Kjeldahl flasks, 300 mL, ground-glass stoppered.
9.2 Digestion Rack - A digestion rack constructed to hold one or more 300-mL Kjeldahl flasks at an angle of approximately 45° in such a fashion that direct heat is applied only to the bottom of the flask and such that the body and neck of the flask are insulated from the source of heat. Approximately three fourths of the neck of the flask should be cooled by air at atmospheric temperature, preferably by directing an air stream against the neck of the flask. A bunsen flame or high capacity electric heater are suitable heat sources.
10. Reagents
10.1 Hydrogen Peroxide (30 %), concentrated hydrogen peroxide (H2O2) (Warning - Concentrated solutions are highly toxic and strong oxidants.) containing no more than 0.0002 % phosphorus.
10.2 Nitric Acid (sp gr 1.42), concentrated nitric acid (HNO3).
10.3 Sulfuric Acid (sp gr 1.84), concentrated sulfuric acid (H2SO4).
10.4 White Oil, phosphorus-free.
10.5 Quality Control (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.
11. Procedure
11.1 Weigh out a portion of the material to be analyzed, in accordance with Table 1, into a 300-mL Kjeldahl flask. Any convenient method of transferring the test specimen may be used as long as care is taken to avoid getting the test specimen on the neck of the flask (see Note 1). Add H2SO4 (3 mL for the photometric procedure, or 10 mL for the gravimetric procedure) and a 6-mm glass bead (see Note 2), and swirl the flask to mix the contents.
11.2 To obtain satisfactory accuracy with the small amounts of phosphorus involved, it is necessary to take extensive precautions in handling. The usual precautions of cleanliness, careful manipulation, and avoidance of contamination should be scrupulously observed; also, all glassware should be cleaned before use, with cleaning acid or by some procedure that does not involve use of commercial detergents. These compounds often contain alkali phosphates, which are strongly absorbed by glass surfaces and are not removed by ordinary rinsing. It is desirable to segregate a special stock of glassware for use only in the determination of phosphorus.
NOTE 1 - The volume occupied by the glass bead (0.1 mL) can be ignored for ordinary work. Excessive bumping is encountered occasionally in the digestion of some organic phosphorus compounds. This bumping can be minimized by using a glass bead. Some difficulty can be experienced when using commercial boiling aids in obtaining a solution clear enough for photometric measurement of phosphorus (see Sections 12-18) even after centrifuging, due to the attrition of these boiling aids under the vigorous digestion procedure.
11.3 Make a blank determination following the same procedure and using the same amounts of all reagents and a similar size sample of phosphorus-free white oil. This blank is for use in the photometric method (see Sections 12-18).
11.4 Place the flask on the digestion rack under a hood and warm gently with a micro burner until the test specimen is charred, while cooling the neck of the flask, preferably by use of an air stream (see Note 2). Continue heating until dense white fumes appear (see Note 3). While boiling, continuously add 1 mL of HNO3 dropwise (see Note 4) to oxidize the organic material. When the HNO3 has boiled off and dense white fumes reappear, repeat the treatment with an additional 1 mL of HNO3(see Note 5). Continue the addition of HNO3 in 1-mL increments until the digestion mixture is no darker than a straw color, indicating that almost all the organic matter has been oxidized.
NOTE 2 - The amount of air used to cool the neck of the flask will at times have to be reduced or even shut off to allow vapors and fumes to leave the flask and to allow sample to come to dense white fumes. However, this should not be done until the test specimen is in a well-decomposed state; the air stream should be turned on again each time before the addition of the HNO3 or H2O2(see 11.4).
NOTE 3 - Excessive evaporation of H2SO4 should be avoided to minimize any loss of phosphorus that may occur. Care should be exercised to avoid heating above the liquid level. Since there is some indication that with test specimens containing inorganic compounds (that is, barium or lead salts) there can be losses of phosphorus due to sintering or fusion of the phosphate and sulfate to the glass, it is well to examine the dried vessel after use to detect any opaque film of fused material.
NOTE 4 - Unless the HNO3 is added dropwise, it can force excessive amounts of vapor from the flask and lead to loss of phosphorus containing fumes.
NOTE 5 - To minimize the loss of H2SO4 in the digestion process, it is advisable not to prolong the dense white fumes stage between addition of HNO3.
11.5 Cool the flask slightly and add 10 drops (0.5 mL) of H2O2. Heat until dense white fumes appear, and while boiling, cautiously add 1 mL of HNO3 dropwise. When the HNO3 has boiled off and dense white fumes reappear, repeat the treatment with H2O2 and HNO3 until the digestion mixture is colorless, at which time the organic material will be completely oxidized. Four treatments will usually suffice. The total amount of H2O2 used should be noted, and the same amount used for each test specimen and the blank.
11.6 When oxidation is complete, allow the flask to cool, wash down the mouth and neck with a minimum amount of water (5 mL), and mix the contents. Return the flask to the digestion rack and continue heating to the appearance of dense white fumes. Repeat the process of the addition of water and heating to dense fumes several times. This will remove all traces of H2O2. (Warning - Use extreme care in fuming, in accordance with 11.5, to remove all traces of H2O2 so that no color interference will be experienced when phosphorus is to be determined photometrically, as described in Sections 12-18.)
