AnaZyst, December, 1966, Vol. 91, $9. 771-774 77 1 The Determination of Fluorine or Phosphorus in Organic Compounds by a Micro-titrimetric Method BY F. H. OLIVER (Chemica2 Research Department, Parke, Davis G. Co., Staines Road, Hounslow, Middlesex) A method is described for determining fluorine or phosphorus in organic compounds synthesised for medical research. After combustion of the compound in an oxygen flask and absorption of thc decomposition products in water, the contents are diluted with isopropanol, and titrated with thorium nitrate solution with a Solochrome cyaninc R screened indicator. The addition of buffcrs is unnecessary. The removal of elements that interfere in the titration of fluorine is also described. IN the determination of the elements in organic compounds by micro-analytical techniques it is desirable that the methods used should be both accurate and simple.This is not achieved in the determination of fluorine or phosphorus; known methods are accurate, but there is a need for greater simplicity. With the oxygen-flask1 method, the decomposition of organic compounds has been simplified, but the final determination of the ionised fluorine or phosphorus is elaborate. The hydrofluoric acid formed by combustion of fluorine compounds that contain no other acid-forming elements can be titrated directly with 0.01 N sodium hydroxide.2 An alternative method that was found to give good results is to add potassium iodate and then potassium iodide to the acid solution and titrate the liberated iodine with a standard solution of sodium thiosulphate.Attempts made by the author, when using the oxygen flask for combustion, to find a titration finish applicable to organic compounds containing fluorine or phosphorus that have been synthesised for medical research are described in this paper. No reports in the literature could be found relating to the titration of orthophosphate with thorium, but, as thorium forms an insoluble phosphate, it was thought that a thorium solution could be used as a titrant for phosphorus and the same method used as that used for fluorine, especially as phosphate was found to interfere with the determination of fluorine. Sulphate also interfered, but as the interference was not quantitative it could not be used as a method for the determination of sulphur.EXPERIMENTAL Metals that form insoluble fluorides and phosphates were chosen for examination. These are : thorium, zirconium, lanthanum, cerium, yttrium, bismuth and iron. Indicators that form coloured complexes with one or more of them are: Alizarin red S, acid Alizarin black S, Solochrome (Eriochrome) cyanine R, * pyrocatechol violet , methylthymol blue, xylenol orange, thoron, PAN , SPADNS, Zincon, Tiron, phenylfluorone, arsenazo and purpurin. Complexes of these metals and indicators have all been used in the spectrophotometric determination of either the metal or fluorine, but reports on them in the literature are too numerous to quote. The majority of these failed in the present work for one or more of the following reasons: some required very close pH control as the indicators are also acid - base indicators; end-points with some were slow; and in many instances the colour change was gradual from one tone to another with no sharp change taking place.In nearly all of these combinations the metals formed highly insoluble lakes that separated out, and end-points were masked. The most promising results, however, wcre obtained by titrating fluoride and ortho- phosphate ions with thorium nitrate and Solochrome cyanine R as indicator. Willard and Horton3 state that the preferred order of indicators for the titration of fluoride with thorium nitrate is (i) purpurin sulphonate, (ii) Alizarin red S, and (iii) Solochrome cyanine K. * The indicator is referred to by the suppliers as “Solochrome cyaninc I<” and this designation will be used throughout.772 [Analyst, Vol.91 Alizarin red S is the most widely mentioned indicator used for the determination of fluorine. In no instance, however, when used on the micro-scale in the author's laboratory, was it possible to detect a reliable end-point with this indicator by straight titration, and at all times it was necessary to exercise strict pH control by the use of buffers. The recom- mended method was that of Dahle et aZ.,4 and is based on colour matching by comparison and described in full by Clark,5 and Milton and Waters.6 It was therefore decided to investigate more fully the use of Solochrome cyanine R. Supplies of this indicator were obtained from three sources, A, B and C. A 0.25 per cent. aqueous solution of each indicator was prepared and 3 drops of indicator and 1 drop of 2 N nitric acid were added to three separate flasks, each of which contained 25 ml of distilled water.The colour of each solution was golden brown and changed, for the solutions con- taining A and B, to red - purple on the addition of 1 drop of 0.01 N thorium nitrate, although with C, 0.1 ml of the thorium solution was required to give a colour change. Visual inspection of the solid indicators showed that a second compound was probably present in each. Any such compound was water-soluble as no residue was left when the solutions were filtered. A 0.25 per cent. solution of each indicator was then prepared in 96 per cent. ethanol and, on filtering, A and B left a small amount of a white organic crystalline material, while C left an inorganic residue amounting to about 66 per cent.of the dye. This was shown to be sodium sulphate. A and R were used for further investigations. A standard solution of 0.01 N sodium fluoride was used for the titration with thorium nitrate, but the colour change at the end-point was not considered satisfactory. Cheng' has shown that in the determination of chlorine and bromine, the colour of the end-point is greatly enhanced if the determination is carried out in a mixture of water and an organic solvent such as ethanol, methanol or isopropanol. Experiments on these lines were then conducted for the titration of fluoride with thorium nitrate, and, as isopropanol was the solvent chosen by Cheng for determining chlorine and bromine, this solvent was used in the present work.To each of two flasks, one of which contained 25 ml of isopropanol and water (4 + I), and the other, 25 ml of distilled water, were added 3 drops of indicator (A or B) and 1 drop of 2 N nitric acid. The isopropanol solution turned yellow, but the aqueous solution showed no change. Both solutions turned purple on adding 1 drop of thorium nitrate solution, but the colour of the propanol solution was much more intense. Screening the indicator with methylene blue made the end-point even more distinctive, and in the titration of fluoride the colour change was from green to blue - purple. Successive titrations of 2 ml of 0.01 N hydrofluoric acid in a mixture of 3 ml of water and 20 ml of isopropanol with 0.02 N thorium nitrate required 0.98, 0.99, 0.98, 0-98 and 0.99 ml.Similar experiments conducted with orthophosphoric acid solutions gave excellent end- points. In the titration of sulphate solutions, however, the end-points were poor, and as they were not quantitative this titration was not investigated further. The presence of phosphorus and sulphur interferes with the determination of fluorine and their removal is described later in this paper. METHOD OLIVER: DETERMINATION OF FLUORINE OR PHOSPHORUS IN All combustions were carried out in 250-ml silica oxygen flasks.8 Solochrome cyanine R-Prepare a 0.25 per cent. solution in 95 per cent. ethanol and filter illethylene blue solution, 0.05 per cent., w / v , aqueous. Thorium nitrate, 0-02 N (0.005 M)-Dissolve 2.7610 g of thorium nitrate tetrahydrate, Th(NOJ4.4H,O, in distilled water and make up to 1 litre.Isopropanol-Use AnalaR grade. DETERMINATION OF FLUORINE- Weigh accurately sufficient of the compound to give approximately 1 mg of ionised fluorine. Wrap it in a square of filter-paper and place it in the platinum spiral. Transfer 4 ml of distilled water into a 250-ml silica flask and flush it with oxygen, Light the tab and plunge the spiral into the flask. When the combustion is complete, shake the flask well and allow it to stand for 10 minutes. Wash down the spiral and stopper with the minimum amount of water (1 to 2 ml) and gently boil the contents of the flask for about 10 seconds REAGENTS- into a bottle. The solution is quite stable.December, 19661 ORGAKIC COMPOUNDS BY A MICRO-TITRIMETRIC METHOD 773 to expel carbon dioxide.Cool the flask and add 20 ml of isopropanol. Then add 0-3 ml of Solochrome cyanine R indicator and 3 drops of methylene blue. The colour should now be green; if not, add 1 drop of 2 N nitric acid. Titrate with 0-02 K thorium nitrate to a blue - purple end-point (when the green colour begins to darken, the thorium nitrate should be added slowly with vigorous shaking of the flask). The thorium nitrate is standardised empirically by combusting standard organic fluorine compounds. The results obtained by this method are shown in Table I. TABLE I DETERMINATION OF FLUORINE IN THE ABSENCE OF SULPHUR AND PHOSPHORUS Compound Trifluoroacetanilide M.A.S. . . . . . . p-Fluorobenzoic acid M.A.S. . , . . . . Trifluoromethylbenzoic acid M.A.S.. . . . 2,5-Di-(2-fluorophenyl)-oxadiazole 1,3,4 . . 2-(2-FIuorophenyl)-5-phenyloxadiazole 1,3,4 Fluoxymesterone . . . . . . . . C14H10N02F3 * * . . . . . . . . C21H2i"20F3 . . . . .. . . . . C21H29N08F3 - * . . . . . . . . CloHl2NO2F . . . . . . . . . . C,H,NO,F, . . . . * . . . .. . . . . . . . . . . . . . . . . . . .. Weight, mg 2.983 3.125 3.068 3.130 2.979 3.111 5.300 4.857 3.155 5.879 7.082 8.290 9.237 3.736 5.264 5.278 6.100 3.922 Fluorine r A - found, required, per cent. per cent. 30.25 30.14 30.33 30.14 29.92 30.14 30.04 30.14 30.20 30.14 29.95 30.14 13.64 13.56 13-62 13-56 29.93 29.98 14.92 14.72 7.56 7.92 7-72 7.92 5-76 5.65 20.32 20.25 15.0 15.0 14.38 14.2 18.9 18.9 9.62 9.64 Difference, per cent. so.11 +0*19 - 0.22 -0.10 + 0.06 -0.19 + 0.08 + 0.06 - 0.05 + 0.20 - 0.37 - 0-20 + 0.10 + 0.08 Nil f0.18 - 0.02 Nil REMOVAL OF PHOSPHORUS AND SULPHUR- Soluble salts of barium, lead, zinc, magnesium and silver were used for the attempted removal of phosphorus (assumed to be present as orthophosphate), and barium and lead for the removal of sulphur as sulphate.Only silver gave complete recovery of fluoride in the presence of phosphorus, and neither barium nor lead gave complete recovery of fluoride in the presence of sulphur. However, complete removal of sulphate was obtained by the use of benzidine. REMOVAL OF PHOSPHORUS- The method of Colsong for the removal of phosphorus in the determination of sulphur was used, except that the ion-exchange resin procedure was omitted as excess of silver ions has no effect on the titration with thorium nitrate.REAGENT- Silver oxide-Prepare as described by Colson. After the contents of the flask have been briefly boiled, add about 50 mg of silver oxide sludge and boil for about 1 minute. Cool the flask and filter through a small Hirsch funnel into a titration flask. Wash out the flask with the minimum amount of water and add isopropanol to give a final strength of 70 to 80 per cent. Carry out the titration as for fluorine. The addition of 1 drop of 2 N nitric acid is necessary to neutralise the slight solubility of the silver oxide. Determine the blank value. REMOVAL OF SULPHUR- Bring the contents of the flask to the boil and add 2 ml of 1 per cent. w/v solution of pure benzidine in 96 per cent. ethanol; boil for a further 15 seconds.Allow the flask to cool under running tap water for a minimum of 1 hour, and then filter into a titration flask as above and carry out the titration as for fluorine.774 OLIVER The excess benzidine will need to be neutralised with 2 N nitric acid. Table I1 shows the results obtained with some compounds that contain fluorine and either phosphorus or sulphur. TABLE I1 DETERMINATION OF FLUORINE WHEN SULPHUR OR PHOSPHORUS IS PRESENT Weight , Compound mg Trifluoroacetanilide . . . . . . . . 3.160 (+ phenylthiourea) . . . . . . . . 3.288 3.362 5.444 Cl,Hl,N,O,SF . . . . .. . . 8.317 C,,H,,NPF, . . . . . . . . . . 3.504 3.890 C1,Hl,F3SN3HCl.H,0 . . . . . . 4-848 C18H16N202SF3 . . . . .. . . 4.485 Fluorine =required: per cent. per cent. 29.7 30.14 30.65 30.14 30.0 30-14 18.16 17-94 18.0 17-94 15.78 16.00 6.18 6-20 25.1 24.6 24-88 24.6 Difference, per cent.+0.51 + 0.22 + 0.06 - 0.22 - 0.02 + 0*50 + 0.28 - 0.44 -0.14 DETERMINATION OF PHOSPHORUS- The method and reagents are the same as those described under Determination of Fluorine. Standardise the thorium nitrate empirically with standard organic phosphorus compounds. Owing to the lack of organic phosphorus standards only triphenylphosphine can be quoted, although research compounds have been successfully analysed. Table 111 shows the results obtained with triphen ylphosphine. TABLE I11 DETERMINATIOK OF PHOSPHORUS I N THE ABSENCE OF FLUORINE AND SULPHUR Phosphorus - Vl‘eight, found, required, Difference, Compound mg per cent. per cent. per cent. Triphenylphosphine . . .. .. . . 4.068 11-70 11.80 -0.10 4.614 11-85 11-80 + 0.05 3.977 12-06 11-80 + 0.26 4.893 11-70 11.80 -0.10 DISCUSSION The method described gives excellent results for fluorine or phosphorus when no inter- fering elements are present. It is not necessary to add buffers to maintain a definite pH. Although phosphorus and sulphur can be removed when the determination of fluorine is required, it has not been found possible to remove fluorine or sulphur when requiring to determine phosphorus. 1. 2. 3. 4. 5. 6. 7. 8. 9. REFERENCES Schoniger, W., Mikrochinz. Ada, 1955, 123. Wilson, C. T,. , and Wilson, D. W., Editors, “Comprehensive Analytical Chemistry,” Elsevier Willard, H. H., and Horton, C. A., Analyt. Chem., 1950, 22, 1190. Dahle, D., Honnar, K. V., and Wiechmann, H. J., J . Ass. Off. Agric. Chem., 1938, 21, 459. Clark, S. J., “Quantitative Methods of Organic Microanalysis,” Biitterworths Scientific Publica- tions, London, 1956, p. 132. Milton, R. F., and Waters, W. *4., Editors, “Methods of Quantitative Micro-Analysis,” Edward Arnold & C o , London, 1949, p. 191. Cheng, F. W., Microchem. J . , 1969, 3, 537. Johnson, C. A,, and Leonard, M. A., Analyst, 1961, 86, 101. Colson, A. F., Tbid., 1963, 88, 26. Piiblishing Co., Amsterdam, London, New York and Princeton, Volume lB, p. 558. Received November 17t8, 1965