TM-34, Determination of Phosphorus in Lubricants

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This method determines the percent of phosphorus in lubricants. Reagents and Apparatus

(a) Nickel crucible with a capacity of about 25 ml.

(b) Ammonium Molybdate solution. To an 800 ml beaker add 600 ml H2O, 65 g ((NH4)6)Mo7O24•4H2O, 225 g NH4NO3 and 15 ml NH4OH, all reagent grade. Heat gently until the crystals have dissolved, filter through Whatman No. 41 paper, and dilute to 1 liter.

(c) Phenolphthalein indicator solution. Dissolve 1 g phenolphthalein in 100 ml of 95% C2H5OH.

(d) 0.15 N Sodium Hydroxide solution. Dissolve 6 g of Reagent Grade NaOH in H2O and dilute to 1 liter.

(e) Potassium Permanganate solution. Dissolve 25 g of Reagent Grade KMnO4 in H2O and dilute to 1 liter.

(f) 0.15 N Nitric Acid solution. Dilute 9.5 ml of Reagent Grade HNO3 to 1 liter. Standardize against Reagent Grade NaOH or Na2CO3. The phosphorus equivalent of the solution is equal to 0.00135 x Normality.

(g) KNO3 wash solution. Dissolve 11 g Reagent Grade KNO3 in H2O and dilute to 1 liter. Procedure

If the phosphorus content of the material is less than 0.07%, weigh a 5 g sample; if more than 0.07% weigh a 1 g sample. Weigh the proper size sample to the nearest 0.1 mg and transfer to a nickel crucible. Add 0.5 g each of Na2O2 and ZnO, both Reagent Grade. Add the same reagents to another nickel crucible and carry through the test as a blank. Place the crucible on a hot plate and maintain at 90-110 °C for 30 min. Remove from the hot plate and allow to cool. Add 1 ml of benzene and ignite. Continue burning until all the oil is consumed, warming on the hot plate again if necessary. After the oil is completely burned, heat over a meeker burner until most of the carbon is oxidized and the mass begins to melt. Do not allow the mass to fuse completely.

Allow the crucible to cool and wash as much of the contents as possible into a 250 ml beaker. Rinse the crucible into the beaker several times with 1-1 HNO3 and hot H2O. The final volume should be about 150 ml, and should include 20 ml of concentrated HNO3. Filter through a No. 41 Whatman paper into a 500 ml flask. Heat the filtrate to boiling and add KMnO4 solution, drop by drop, until the solution retains the purple color. Destroy the excess KMnO4 with a few drops of 3% H2O2.

Cool the solution to 37-45°C and add 50 ml of the ammonium molybdate solution. Stopper the flask, shake vigorously for 3-5 min, and allow the precipitate to settle for about 1 hr. Filter through No. 42 Whatman paper and wash well with the KNO3 wash solution.

Place the filter paper and precipitate in the original flask and add 15 ml H2O and several drops of the phenolphtalein indicator solution. Pipette 10 ml of the 0.15 N NaOH solution into the flask and agitate to dissolve the precipitate. The solution should be alkaline at this point; if not, pipette another 10 ml NaOH into the flask. Add the same total amount of NaOH solution to the blank. Dilute to 150 ml with H2O and titrate the excess NaOH with 0.15 N HNO3. Calculations

Calculate the percent phosphorus in the sample as follows:

% P =


B = ml HNO3 to titrate the blank
S = ml HNO3 to titrate the sample
E = Phosphorus equivalent of the HNO3 solution.

Report as:

% Phosphorus (P), to the nearest 0.01%