516 POLLOCK: A PHOTOMETRIC METHOD FOR [Vol. 83 A Photometric Method for the Determination of Tungsten in Low-grade Mine Ore and Mineral-dressing Products BY J. B. PClLLOCK (The Geological Survey of Uganda, P.0. Box No. 9, Entebbe, Uganda) The sample is treated with hydrofluoric acid to remove silica and fused with sodium carbonate, the resulting melt being extracted with water. The solution thus obtained is made up to a known volume and filtered, without washing the precipitate. An appropriate aliquot is acidified to an exact pH, rhodamine B solution is added and the optical density is measured a t 6000 A. THE formation of a coloured complex between rhodamine B and tungsten in dilute acid solution was described by Eegriwel as a qualitative test for tungsten. De Boer,2 and later H e ~ n e , ~ made use of the reaction to determine amounts of tungsten of the order of 0.1 mg, comparison between sample and standard being made visually.The procedure was simple, but the accuracy was low. The photometric method described can be applied directly to samples with a tungsten content in the range 0.2 to 1.0 per cent. of tungstic oxide, and, by adjustment of the initial sample weight and size of aliquot, or both, the range 0.05 to 5 per cent. of tungstic oxide can be covered. The accuracy compares well with that of other methods. No extractions are required, in contrast to other recent applications of rhodamine B,4~6~8 and interference was negligible when low-grade Uganda tungsten ores and the mineral-dressing products derived therefrom were analysed. EXPERIMENTAL OPTIMUM WAVELENGTH FOR MEASURING TUNGSTEN - RHODAMINE B COMPLEX- Curve A, Fig.1, shows that the maximum optical density of the tungsten - rhodamine B complex occurs at 5975 A. Instrument response is, however, rather sluggish at this wave- length and since, in addition, the inevitable presence of aluminium causes a definite smoothing of the peak, as can be seen from curve B, Fig. 1, measurements are made at 6000 A. OPTICAL DENSITY OF TUNGSTEN - RHODAMINE B COMPLEX- factors- The optical density of the tungsten - rhodamme B complex is controlled by the following (i) the pH of the acidified sodium tungstate solution when rhodamine B solution (ii) the time of standing after adding rhodamine B solution, and (iii) the time of standing after adding gum arabic solution and diluting to 100ml.Acidity of the sodium tungstate solution-Fig. 2 shows the influence of the pH of the acidified sodium tungstate solution on the optical density of the tungsten - rhodamine B complex. The optical density is almost constant between pH 1.65 and 1.70, and, as solutions are more easily adjusted to pH 1.65, this value is used in practice. It can be seen that, although aluminium causes interference throughout the entire pH range, it does not alter the optimum pH. is added,Sept., 19581 THE DETERMINATION OF TUNGSTEN 517 Time of standing after addition of rhodamine B solution-The time of standing after rhodamine B solution has been added is critical and must be kept constant to within 3 or 4 minutes in order to ensure reproducible results. One hour has been chosen, after experiment, as this represents a reasonable compromise between incomplete development of the tungsten - rhodamine B complex, resulting from too short a time of standing, and precipitation of the complex after too long standing.If gum arabic solution is not added, precipitation of the tungsten complex will commence about 14 to 2 hours after the rhodamine B solution has been added. t I 0 . 4 L - L u - u A 5925 5975 6025 6075 Wavelength, A Fig. 1. Absorption spectra of tung- sten - rhodamine B complex: curve A, tungsten equivalent to 0.5 per cent. of tungstic oxide in a 0.5-g sample; curve B, as for curve A plus aluminium equivalent to 17 per cent. of aluminium oxide in a 0.5-g sample t o + ~ ~ I I b O I ~o I 1.50 I pH before adding rhodamine B Fig.2. Effect of the pH of the sodium tungstate solution on the optical density of the tungsten - rhodamine B complex: curve A, tungsten equivalent to 0.5 per cent. of tungstic oxide in a 0.5-g sample; curve B, as for curve A plus alu- minium equivalent to 17 per cent. of aluminium oxide in a 0.5-g sample Time of standing after addition of gum arabic solution-Although gum arabic tends to retard further development of the tungsten complex, the optical density increases slowly for some days after make up is complete. For practical purposes, however, solutions can be regarded as stable after standing for 3 hours. Curve B, Fig. 3, shows a typical calibration graph prepared by the procedure described on p. 519, the optical densities being measured after 3 hours; curve A represents the same solutions 24 hours later.It can be seen that hour to hour stability is attained after 3 hours and a delay of 1 or 2 hours when measuring the optical densities will not materially affect the result. The optical densities can even be measured after only 30 minutes' standing, for example, to counteract interference by tin or to obtain results urgently required, but, in such circum- stances, a special calibration graph should be constructed. If time or circumstances do not permit the construction of a special graph, standard solutions in duplicate, containing the equivalent of, say, 0.5 per cent. of tungstic oxide, may be analysed together with the samples, and the results can then be corrected by means of the values found for these standard solutions. U S E OF OTHER ACIDS- Hydrochloric acid may be replaced by sulphuric acid or hydrobromic acid for acidifying the sodium tungstate and blank solutions.Curves B and A, Fig. 4, are calibration graphs for the range 0.1 to 0.5 per cent. of tungstic oxide, obtained by using sulphuric and hydro- bromic acids, respectively. By comparison with curve C, which is a calibration graph for the same range obtained by using hydrochloric acid, it can be seen that sulphuric and hydro- bromic acids give higher optical densities. Measurements at the lower ends of the curves are also more likely to yield accurate results than when hydrochloric acid is used. Unfortunately, when either sulphuric or hydrobromic acid is used to acidify the sample aliquot, traces of tin cause considerable negative interference.More than a trace of tin results in complete suppression of the tungsten - rhodamine B complex. Therefore, unless tin is known to be wholly absent from the sample, only hydrochloric acid can be used for518 POLLOCK: A PHOTOMETRIC METHOD FOR [Vol. 83 acidification, the procedure described under "Volume of aliquot," p. 520, being adopted for samples containing 0.05 to 0.2 per cent. of tungstic oxide. '0 0.4 0.8 1.2 Tungstic oxide in a 0.5-g sample, % Fig. 3. Calibration curves for tungsten: curve A, optical densities measured after 27 hours; curve B, optical den- sities measured after 3 hours - 1.2- - 1.0- - x .- 2 0.8- 0 -0 - - m : O ' T 0.2 OO Tungstic oxide in a 0.5-g sample, yo Fig. 4. Calibration curves for tungstenwithdifferent acids being used for acidifying the sodium tungstate and blank solutions: curve A, hydro- bromic acid ; curve B, sulphuric acid ; curve C, hydrochloric acid O'\ METHOD REAGENTS- Standard sodium tungstate solution-Weigh 0.1778 g of hydrated sodium tungstate, Na2W0,.2H20, into a 600-ml beaker and dissolve in 200 to 300ml of water. Transfer to a 1-litre calibrated flask, wash the beaker three times, add 40 g of sodium carbonate dissolved in about 500 ml of water and dilute to the mark.Standard sodium aluminate solution-Weigh 0.6250 g of aluminium oxide into a 35-ml platinum crucible, add 10 g of anhydrous sodium carbonate and fuse for 3 hours. When cold, place the crucible and lid in a 600-ml beaker, add 200 ml of water and warm on a hot-plate until dissolution of the melt is complete. Transfer to a 250-ml calibrated flask, wash the crucible, lid and beaker three times each, add the washings to the flask, cool, and dilute to the mark.Filter the solution through a dry 15-cm Whatman No. 541 filter-paper into a bottle fitted with a ground-glass stopper. Use the first 10 ml or so of filtrate to rinse the bottle. If after 21 or 3 weeks an insoluble precipitate forms, discard the solution. If alumina purchased from chemical suppliers proves to be refractory, a suitable grade can be prepared by precipitation with ammonia from slightly acidified aluminium chloride solution. Before precipitation is commenced, add a generous amount of filter-paper pulp. This is best prepared by shaking torn-up Whatman No.41 filter-papers with water in a closed jar or bottle, an electric shaking machine being used. Wash the precipitated alu- minium hydroxide thoroughly with boiling 5 per cent. w/v ammonium nitrate solution and ignite cautiously in an electric furnace at a temperature of 700" C. High-temperature ignition will result in a refractory product. Sodium carbonate solution, 4 $er cent. w/v--Prepare from anhydrous sodium carbonate and store the solution in a polythene bottle. Do not wash the precipitate.Sept., 19581 THE DETERMINATION OF TUNGSTEN 519 Hydrochloric acid solution, 20 per cent. v/w. Rhodamine B solution-Weigh 0.2 g of rhodamine B into a 1600-ml beaker. Add 950 ml of water and stir with an electric stirrer for 1 hour. Transfer to a 1-litre calibrated flask, wash the beaker three times, add the washings to the flask and dilute to the mark.Filter the solution through a 9-cm No. 4 sintered-glass Buchner funnel and use the first few millilitres of filtrate to rinse the receiving flask. Prepare several litres a t one time and mix thoroughly together. The solution can be stored for several weeks, for preference in a dark bottle fitted with a ground-glass stopper. Gum arabic solution-Weigh 1 g of gum arabic into a 250-ml beaker, add 100 ml of boiling water, and immerse in a boiling-water bath until dissolution is complete. Filter the solution through a 15-cm Whatman No. 541 filter-paper and store in a covered beaker or stoppered bottle. PROCEDURE FOR PREPARING CALIBRATIOS GRAPH- To each of six 250-ml beakers add, from a 50-ml burette, a volume of sodium aluminate solution that corresponds to the average known or expected alumina content of the samples to be analysed, eg., if the average alumina content is expected to be 10 per cent., add 5 ml of sodium aluminate solution, if 20 per cent.add 10 ml and so on (see “Volume of aliquot,” p. 520). If the alumina content is completely unknown, assume it to be 17 per cent. and add 8 5 m l of sodium aluminate solution. Mark the beakers as containing blank solution, 0.2, 0.4, 0.6, 0.8 and 1.0 per cent. of tungstic oxide, respectively. To the blank solution add, from a 100-ml burette, 16.5ml of 4 per cent. sodium carbonate solution. Into the remaining beakers run 2, 4, 6, 8 and 10-ml portions of standard sodium tungstate solution from a burette of accuracy equivalent to SPL grade A, and then sufficient 4 per cent. sodium carbonate solution to make the total volume in each beaker up to 25 ml.Calibrate the instrument with buffer solution a t pH 4.0. Acidify the blank solution with 20 per cent. v/v hydrochloric acid, using a 3-ml Ostwald - Folin pipette fitted with a rubber bulb, until the aluminium hydroxide, which is a t first precipitated, just re-dissolves and the solution is clear. The sodium aluminate solution added when preparing solutions for the calibration graph, in addition to balancing the alumina in the sample solutions, serves to indicate when an approximate pH of 3 to 4 has been reached and greatly facilitates this stage of the procedure. Immerse the electrodes of the pH meter and, by careful dropwise addition of dilute hydrochloric acid, adjust the pH to 1.65.Remove and wash the electrodes so that the wash water runs into the beaker, and then carefully dry them with filter-paper. Check the calibration of the instrument with pH 4.0 buffer solution and re-set if necessary. Acidify and adjust each of the remaining solutions in turn, completing the whole operation on one solution before proceeding to the next. Check the calibration of the pH meter after the first two solutions and again after the fifth, or otherwise as experience dictates. Exact adjustment of pH is essential to the method, since the pH of the acidified tungsten solution controls the optical density of the complex formed when rhodamine B solution is added, hence the emphasis on careful calibration of the pH meter.Xote that adjustment of the pH of the tungsten solutions must be made only by adding acid; if the correct pH is accidentally passed, it is useless to attempt to restore the status quo by adding sodium carbonate or other alkaline solutions; the state of the tungsten has been fixed unalterably and the standard tungsten solution or sample aliquot must be discarded and the procedure repeated with a fresh solution. When all solutions have been adjusted to pH 1-85, add to each, by pipette, 25ml of rhodamine B solution. Keep the solution in motion while the rhodamine B solution is added, to ensure proper mixing. Set the solutions aside for exactly 1 hour after the last has been treated with rhodamine B solution and then add 5 ml of gum arabic solution to each beaker.Great precision in measuring the volume of gum arabic solution is not critical; a 5-ml Ostwald - Folin pipette fitted with a rubber bulb may conveniently be used. Note, however, that the gum arabic solution must be added before the solutions are diluted in any way; failure to observe this point will result in partial precipitation of the tungsten complex. After adding gum arabic solution, transfer each solution to a 100-ml calibrated flask, rinse each beaker three times, add the washings to the flask, make up to the mark, shake thoroughly and loosen the stopper to allow escape of carbon dioxide. Set the solutions aside for 3 hours and then measure the optical densities in 4-cm cells against the blank solution Do not wash the precipitate.Discard the solution after 4 days. Set up a reliable pH meter of sensitivity not less than 0.05 units of pH. Swirl the beaker to assist evolution of carbon dioxide and wash down the sides.520 POLLOCK: A PHOTOMETRIC METHOD FOR [Vol. 83 at 6 0 0 0 ~ . Construct a calibration graph of optical density against tungsten content, the solutions representing, respectively, 0.2, 0.4, 0.6, 0.8 and 1.0 per cent. of tungstic oxide. PROCEDURE FOR BREAKING DOWN SAMPLES- Weigh exactly 0.5000 g of sample into a 35-ml platinum crucible, add 5 to 10 ml of hydro- fluoric acid and 4 drops of 50 per cent. v/v sulphuric acid and evaporate to dryness on a sand-bath, care being taken not to lose any sample by heating too rapidly. Repeat the addition of hydrofluoric acid and subsequent evaporation twice. Cover the crucible with a platinum lid and ignite, gently at first then more strongly, until red heat is attained, at which stage remove the lid to facilitate expulsion of the last traces of silicon tetrafluoride and sulphur trioxide. Allow to cool and grind up the residue with a smoothly rounded glass rod, a sheet of paper being first placed under the crucible to collect any particles that may jump out during this or the next stage.Add a weighed 4-g portion of anhydrous sodium carbonate, mix thoroughly with the same glass rod, which should finally be dusted with a camel-hair brush. Cover the crucible with the lid used during ignition, and fuse over a Meker burner at full heat for 2 to 3 hours. Allow to cool, place the crucible and lid in a 250-ml beaker and add 50 to 60 ml of water.Heat on a sand-bath or hot-plate at low heat until dissolution is complete, remove and wash the crucible and lid, the former being well scrubbed with a rubber-tipped glass rod. Transfer the solution to a 100-ml calibrated flask, cool, and make up to the mark. Filter through a dry 11-cm Whatman No. 540 filter-paper into a flask or bottle fitted with a ground- glass stopper. Use the first 10ml of filtrate t o rinse the container. PROCEDURE FOR DETERMINING TUNGSTIC OXIDE IN THE RANGE 0.2 TO 1.0 PER CEKT.- Take as many 250-ml beakers as are required for the samples to be analysed, plus one for the blank solution. To the last-named add sodium aluminate solution equivalent to the expected average alumina content of the samples; if this is unknown, add 8.5 ml of sodium aluminate solution, equivalent to 17 per cent.of alumina. Add sufficient 4 per cent. sodium carbonate solution to make the total volume 215 ml. In each of the sample beakers, place, by pipette, 25ml of sample solution. Adjust each solution in turn to pH 1.65, starting with the blank solution, by following the procedure described for preparing the calibration graph. Check the calibration of the pH meter with pH 4.0 buffer solution after adjusting the blank solution, and thereafter as frequently as experience has shown to be necessary. By pipette, place 25 ml of rhodamine B solution in each beaker, set aside for exactly 1 hour, and then add to each 5ml of gum arabic solution. Transfer each solution to a 100-ml calibrated flask, wash the beaker three times, make up to the mark, shake thoroughly, and loosen the stopper to permit escape of carbon dioxide.Set the solutions aside for 3 hours and then measure the optical densities in 4-cm cells against the blank solution at 6000 A ; read the tungstic oxide content from the calibration graph. Volume of aliquot-If the tungstic oxide Icontent is in the range 1.0 to 5.0 per cent., the sample can be analysed by taking an appropriately smaller aliquot. It may be necessary to add sodium aluminate solution to bring the alumina concentration of the sample solution into line with that of the solutions used to prepare the calibration graph, and the total volume before acidification must be made up to 25ml with sodium carbonate solution.If the tungstic oxide content lies in the range 0.05 to 0.2 per cent., either an initial sample of 1.0000 g can be taken or an aliquot of up to 50 ml can be used; both these courses may be followed if necessary. In either event it may be necessary to prepare a separate calibration graph to allow for the higher alumina concentration. INTERFERING ELEMENTS Table I shows the compositions of four typical low-grade Uganda tungsten ores, in which molybdenum and tin were not detected. It can be seen that many of the elements and anions stated by Eegriwel to form com- plexes with rhodamine B are absent. The remainder either separate during break-down of the sample or do not form complexes under the conditions of the analysis. Molybdenum, potentially the most serious interfering element, since it will be found with the tungsten in the final sample solution and will form a complex with rhodamine B, can be disregarded with the others.The molybdenum content of the usual low-grade tungsten ore is zero,Sept., 19881 THE DETERMINATION OF TUNGSTEN 521 so far as interference is concerned, as it amounts at the most to a few parts per million. Even in concentrates containing about 65 per cent. of tungstic oxide, in which molybdenum is often detectable in trace amounts, the maximum molybdenum content on record in Uganda is only 0.27 per cent., as molybdenum trioxide. In low-grade ore containing 1 per cent. of tungstic oxide, this maximum figure would be equivalent to 0.004 per cent. of molybdenum trioxide. The optical density of the molybdenum - rhodamine B complex formed under the conditions of analysis has been found to be approximately one-third that of the tungsten complex, and a maximum content of 0.004 per cent.of molybdenum trioxide would therefore cause negligible interference equivalent to 0.0013 per cent. of tungstic oxide. TABLE I COMPOSITION OF LOW-GRADE UGANDA TUNGSTEK ORES Amount of oxide in ore from Oxide, Bahati mine, 50, 70.65 .41,0, 13.12 FeO 0.69 Fez03 10.23 MnO 0.01 p,o, 0.16 TiO, 0.31 CaO 2.10 MgO 1.12 Na,O 0.09 K2O % Y O 0.74 0.94 0.23 Total 100.37 Amount of oxide in ore from Kirwa mine, 83.50 8.39 0.39 4.07 0.01 0.09 0.51 1.65 0.24 0.01 0.36 0.99 0.30 100.31 % Amount of oxide in ore from Nyamolilo mine, 64.47 25.09 0.30 2.30 0.03 0.09 0.74 0.95 0.83 2.42 0.45 2.06 0.29 100.02 0 1 i 0 Amount of oxide in ore from Ruhiza mine, % 75.01 19.90 0.65 5.40 0.01 0.14 0.45 0.85 0.25 1.05 0.26 1.53 0.22 99.72 Two elements, not stated by Eegriwe to form complexes with rhodamine B, are capable of interference, namely, aluminium and tin.Aluminium passes into solution with tungsten and causes negative interference, as can be seen in Figs. 1 and 2. This is probably due to formation of a complex of low optical density between aluminium and part of the rhodamine B, thus reducing the over-all absorption of the solution. However, compensation for inter- ference from aluminium can be effectively made by adding sodium aluminate to the solutions used to prepare the calibration graph, so that the alumina content of each corresponds to the average known or expected alumina content of the samples to be analysed.In any one mine or deposit there will not be a great variation in alumina content over a large number of samples, and even from mine to mine the difference is sufficiently small to be dealt with by this means. It can be seen from Table I that the preparation of the calibration graph from solutions containing the equivalent of 17 per cent. of alumina will leave a maximum alumina inter- ference of 8.61 per cent. for the sample from Kirwa mine and less with the other three samples. This, of course, represents an extreme variation from the average, for, in the exploration of a single mine or deposit, a calibration graph would be prepared from solutions with an alumina content corresponding to the average alumina content of the particular country rock in that area, and the variation of individual samples from the average alumina content would then be of the order of 2 to 3 per cent.of alumina. The position is the same for mineral-dressing products. In thirty-four samples of slimes, tailings and middlings analysed during the investigation, the variation in alumina content of individual samples from the over-all average was not sufficient t o cause serious error. Maximum deviation from the average alumina content in products from any one mine was of the order of 5 per cent. of alumina when slimes, tailings and middlings were included together, and less when they were assessed separately. Interference by tin in the analysis of low-grade Uganda tungsten ores can be ignored.Examination of tungsten concentrates over many years has shown that tin is often wholly absent or detectable only in trace amounts. In the few instances when more than a trace was present, it did not exceed 0.3 per cent. of tin in a concentrate containing 65 per cent. of tungstic oxide. Scaling down this maximum example to a concentration of 1 per cent.,522 HIGH AND PLACITO : THE SPECTROPHOTOBfETRIC DETERMINATION [Vol. 83 of tungstic oxide (a permissible calculation, since cassiterite in a low-grade ore will be concen- trated with the tungsten) gives 0.0046 per cent. of tin. Experiments have shown, however, that decomposition of cassiterite by the procedure described is incomplete and that only about one-third of the tin present in a sample passes into solution.If, therefore, a sample contains 0.0046 per cent. of tin, approximately 0.0016 per cent. will be found in the final solution. Such an amount will cause no error in the determination of tungsten. For the sake of completeness, experiments were conducted to ascertain the effect of larger amounts of tin, such as would be presen.t in a sample where the cassiterite was equal in quantity to the tungsten ore. It was found that in these circumstances interference could be prevented by measuring the optical density 30 to 40 minutes after making up the solution, instead of 3 hours later. The reason is, apparently, that the optical density of the tin - rhodamine B complex develops more slowly than that of the tungsten complex formed under the same conditions, and indeed interference from tin up to about 30 minutes after adding the rhodamine B solution is negative.From 30 to 40 minutes after preparing the solution there is no interference from tin; thereafter, interference increases progressively for 4 or 5 days and then decreases again. COMPARISON OF RESULTS Synthetic samples, prepared by adding different amounts of sodium tungstate solution The results, as to 0.4-g portions of mica schist, were analyseld by the proposed method. tungstic oxide, were as follows- Tungstic oxide added, yo . . 0.30 0.50 0.54 0.58 0.75 0.95 Tungstic oxide found, yo . . 0.31 0.51 0.58 0.55 0.74 0.96 Samples of mineral-dressing products were also analysed by the proposed method, by The results were a modified version of Jeffery's dithiol method' and gravimetrically. as follows- Tungstic oxide by proposed method, yo . . 0.23 0.18 2.18 1.65 1.09 2.12 2.14 1.88 Tungstic oxide by dithiol method, yo . . 0.24 0.14 - 1.60 1.18 2.26 2.14 1.83 Tungstic oxide by gravimetric method, % - - 2.37 1.54 - 2.19 - - I thank the Director of Geological Survey, IEntebbe, for permission to publish this paper. I also thank Mr. P. G. Jeffery (Geological Survey of Uganda) and Mr. R. Pickup (Mineral Resources Division, Colonial Geological Surveys) for helpful discussion and Dr. A. E. 0. Marzys (World Health Organisation, Teheran) for translating papers by Eegriwe and Heyne from the original German. REFEREKES 1 . 2. 3. 4. 5. 6. 7. Eegriwe. E., 2. anal. Chew., 1927, 70, 400. De Boer, J . H., Rec. Trav. Chim. Pays-Bas, 1929, 48, 979. Heyne, G., 2'. angew. Chenz., 1931, 44, 237. MacSulty, B. J., and Woollard, L. D., Anal. (;him. Acta, 1955, 13, 64. __ __ , Ibid., 1955, 13, 154. Onishi, H., and Sandell, E. B., Ibid., 1955, 13, 159. Jeffery, P. G., Analyst, 1956, 81, 104. Received March 3rd, 19%