AnaZyst, December, 1968, Vol. 93, f@. 797-798 797 The Determination of Molybdenum in Mixtures Containing Molybdenum Disulphide by Atomic-absorption Spectrophotometry BY R. J. JULIETTI AND J. A. E. WILKINSON (Morganite Research and Development L t d . , Battersea Church Road, Battersea, London, S. W. 11) A rapid method is described for determining the total molybdenum content of mixtures containing molybdenum disulphide, graphite and a resin. The novel feature of the technique is the decomposition of the mixture by fusion with sodium hydroxide. Dissolution of the melt in sulphuric acid permits the determination of molybdenum by atomic-absorption spectro- photometry without interference. Results at the 8 per cent. molybdenum level are accurate to within &0.1 per cent. MOLYBDENUM disulphide is finding increasing use in industry as a solid lubricant. It is widely used as an additive to oils and greases, particularly for high temperature lubrication, in the form of a dispersion for “dry-film” impregnation, and in bearing components and electrical contacts.Rapid methods for its determination at the percentage level are, there- fore, of value. EXPERIMENTAL Molybdenum disulphide is particularly difficult to decompose ; the usual methods involve dissolution in strong acids, e.g., as in Defence Specification DEF-2304,l in which fuming perchloric acid is used. In this laboratory we have also used mixtures of nitric and hydro- bromic acids and of nitric and sulphuric acids for dissolution. A combustion methodof decomposition has also been reported.2 The samples normally received for analysis contain molybdenum disulphide, graphite and a resin, and the determination of molybdenum is sufficient to define the molybdenum disulphide content.In earlier methods if the sample contained a resin, it was decomposed by heating at 450” C, a t which temperature there is no loss of molybdenum. The material was then heated with a mixture of acids, usually containing nitric acid to accelerate the attack. After filtration and removal of all nitrate by fuming with concentrated sulphuric acid, the solution was passed through a Jones’ reductor and titrated in the usual way. This method, although sufficiently accurate for process control analysis (k0.1 per cent.), was time con- suming. The rapid finish afforded by atomic-absorption spectrophotometry suggested a means of shortening the procedure.If an acid decomposition were used, the interference from sulphate and nitrate could be overcome as suggested by David.3 However, we found that decomposition by fusion with sodium hydroxide is much more rapid than by any other method. The molybdenum disulphide dissolves at just above the fusion temperature of the alkali, and further, by heating for a little longer at a higher tempera- ture, the bulk of the graphite can also be decomposed. The fusion was carried out in a nickel crucible and the melt dissolved in a standard amount of dilute sulphuric acid (1 + 1). No interference was found from either sodium or nickel. Interference from the variable amount of sulphate resulting from the oxidation of molybdenum disulphide is swamped by the presence of the large excess of sulphuric acid.The following procedure has been found satisfactory. PROCEDURE- 50mm diameter) and fuse over a bunsen burner. walls with a layer of flux. Weigh 5 g of sodium hydroxide pellets into a nickel crucible (about 40mm tall and On cooling, swirl the crucible to coat the 0 SAC and the authors.798 JULIETTI AND WILKINSON Weigh 0.5 g of sample, containing about 8 per cent. of molybdenum, and spread it over the top of the melt by tapping. Fuse gently, without a lid, over a small bunsen flame until the effervescence subsides (about 15 minutes). Do not swirl. Transfer the crucible to a muffle furnace at 650” C and leave for 30 minutes. Allow to cool, place the crucible upright in a 250-ml beaker containing 50 ml of distilled water and cover the beaker with a watch-glass. Cautiously add 20 ml of dilute sulphuric acid (1 + 1)* to the crucible, with the watch-glass slightly displaced.Heat, if necessary, to complete the dissolution of the melt. Remove and wash the crucible, collecting the washings in the beaker, and boil the solution for 5 minutes to remove any traces of hydrogen sulphide. Allow the solution to cool and filter it through a No. 40 filter-paper into a 250-ml cali- brated flask. Dilute to the mark and compare the absorbance with that of the standard solution by atomic-absorption spectrophotometry. STANDARD SOLUTION OF MOLYBDENUM- Weigh accurately about 0.6 g of analytical-reagent grade molybdenum trioxide, previously dried for 1 hour at 150” C.Dissolve it in 50 ml of 10 per cent. w/v sodium hydroxide solution. Add 20 ml of dilute sulphuric acid (1 + 1). Cool and make up to 250 ml with distilled water. This solution is stable for at least 1 month. ATOMIC-ABSORPTION MEASUREMENTS- A Techtron A.A.4 atomic-absorption spectrophotometer was used, and the conditions were as follows: wavelength, 313.3 nm; slit width, 50 pm; flame, air - acetylene; burner, 50-mm slot A.B.40, high temperature burner, which was found to give much less variation in absorbance readings for molybdenum than the 100-mm, A.B.41, burner supplied as a standard fitting with the instrument; air pressure, 1 bar (15 p s i . ) ; acetylene flow, approximately on position 4 of the flow meter, but adjusted to give maximum absorbance when a solution containing molybdenum is aspirated; height of light path above burner, about lOmm, but also adjusted for maximum absorbance after the flame conditions have been set ; and aspiration rate, about 2 ml per minute.RESULTS Two samples were analysed for molybdenum by the above procedure. Fourteen results on the first sample, which contained a resin, had a mean value of 7-71 per cent. of molyb- denum, compared with values of 7-79 and 7.84 per cent. by our previous chemical method. The mean value of eight results on the second sample (not containing a resin) was 9.11 per cent. Values of 9.08 and 9.13 per cent. of molybdenum were obtained on this sample by the gravimetric benzoin a-oxime method. The coefficient of variation on the two sets of results by the atomic-absorption procedure described was less than 1 per cent. in each instance. CONCLUSION This method has been used successfully in this laboratory and, compared with our previous method, has the following advantages. (i) There is no need for a preliminary decomposition of the resin. (ii) The dissolution of molybdenum disulphide is more rapid. (iii) No nitrate ions are introduced. (iv) The total working time is much shorter. The authors thank the directors of Morganite Research and Development Limited for permission to publish this paper. REFERENCES 1. 2. 3. Received Jzrly 4th, 1968 * This reagent should be prepared and added as accurately as possible with a measuring cylinder, as Defence Specification DEF-2304, January 1966, “Molybdenum Disulphide (Powdered) ” ZX-35, H.M. Stationery Office, London, 1966. Kalnin, I. L., Analyf. Chem., 1964, 36, 886. David, D. J.. Analyst, 1968, 93, 79. the amount of sulphuric acid in the standard and sample solutions must be the same.