Analyst, December, 1966, Vol. 91, pp. 775-778 775 The Colorimetric Determination of Boron in Soils, Sediments and Rocks with Methylene Blue BY R. E. STANTON AND ALISON J. McDONALD (Defiartment of Geology, Imperial College of Science and Technology, London, S . W.7) A method is presented for the determination of boron in soils, sediments and rocks. The sample is decomposed by a mixture of hydrofluoric and sulphuric acids, and the complex formed between fluoroborate ions and methylene blue is extracted with 1,S-dichloroethane. Boron is determined either by visual colour of the blue complex, or by spectrophotometry. The method is rapid and sensitive. THE reaction of the fluoroborate ion with methylene blue has been applied to the deter- mination of boron in soils, sediments and rocks, following its use in the analysis of iron and steel,1,2 and the procedure described below has been developed.METHOD REAGENTS- Sulphuric acid, 10 N-Prepare from analytical-reagent grade acid. Hydrojuoric acid, 40 per cent. w/w, analytical-reagent grade. Methylene blue, 0.08 per cent. w/v, aqueous. 1,2-Dichl oroet hane . Sodium tetraborate-Decahydrate, analytical-reagent grade. Standard boron solutions-Dissolve 0.4408 g of sodium tetraborate in 10 N sulphuric acid, and dilute with this acid to 500 ml in a calibrated flask to give a solution containing 100 pg of boron per ml. Dilute further with 10 N sulphuric acid to give solutions containing 2, 5 and 1Opg of boron per ml. PROCEDURE- Weigh 0.1 g of sample into a polythene beaker, and add 2-5 ml of 10 N sulphuric acid and 0.5 ml of 40 per cent.hydrofluoric acid. Stir the solution with a polythene rod, cover and leave to stand at room temperature for 2 hours. Add 2 ml of water, mix and leave t o stand for 15 minutes. Transfer 1 ml of the clear solution by pipette into a test-tube calibrated at 15 ml. Add 1 ml of 0.08 per cent. methylene blue and dilute to 15 ml with water. Add 5 ml of 1,2-dichloroethane, stopper the tube and shake it vigorously for 30 seconds. Allow the phases to separate and compare the intensity of colour in the organic layer with a standard series. A reagent blank must be determined. PREPARATION OF THE STAKDARD SERIES- Into eleven polythene beakers transfer by pipette, from the dilute standard solutions, 0, 1.0, 2.0, 3.0, 4.0, 5-0, 7.5, 10.0, 15-0, 20.0 and 25.0 pg of boron, respectively.Add sufficient 10 N sulphuric acid to each beaker to give a total volume of 2-5 ml. Add 0.5 ml of 40 per cent. hydrofluoric acid, mix well and leave to stand for 2 hours. Then add 2 ml of water and mix. Remove 1 ml from each solution with a pipette and treat as described for a sample solution in Procedure. DISCUSSION OF THE METHOD This method was developed for geochemical research studies in which the boron was derived from the mineral colemanite, and was readily attacked in the cold by a mixture of dilute hydrofluoric and sulphuric acids. Consequently, application of the method to other materials may be restricted by this method of sample decomposition.776 STANTON AND MCDONALD : COLORIMETRIC DETERMINATION [Analyst, VOl.91 TABLE I COMPARISON BETWEEN CRUSHED AND UNCRUSHED SAMPLES Sample - Sample +- Boron, p.p.m. Boron, p.p.m. No. - 20 mesh - 80 mesh No. -20 mesh -80 mesh 1 38 45 2 105 100 3 220 240 4 320 330 5 340 340 6 280 200 7 200 180 8 180 190 Formation of fluoroborate takes place as the sample is being attacked. The acid con- centrations are not critical, but must be kept at the same constant level for both samples and standards, and 10 N sulphuric acid is used so that alkaline samples produce no significant variation in acidity. The volume of 40 per cent. hydrofluoric acid is kept low to minimise the colour extracted from the zero standard, The time allowed for this stage is not critical; some samples went completely into solution within 1 hour, whereas others appeared unaltered, even after standing overnight.Nevertheless, boron was always completely converted into the soluble fluoroborate within 2 hours. Samples have been left in contact with the acids for up to 3 days without ill effect, although there is great danger of volume loss by evaporation. Complete recovery was achieved when boron was added to samples as sodium tetraborate ; it was also found possible to increase the sample weight to 250 mg without any other alteration to the procedure. As it was necessary for geological reasons to analyse the -20-mesh fraction of many samples, a comparison was made between results obtained when this fraction was crushed to pass 80 mesh and on the uncrushed material. From the results given in Table I it will be seen that, in general, there is no significant difference, although analysis of the coarser material may introduce greater sampling errors.There is always a slight colour extracted from the zero standard that is dependent upon the final aqueous phase acidity, decreasing with decreasing acidity while the efficiency of TABLE I1 EFFECTS OF VARIOUS ELEMENTS ON THE DETERMINATION OF BORON Amount Boron added, present, Element mg CLg Aluminium . . . . 5.0 0 5.0 1-0 Arsenic (as 0.1 0 Na2HAs0,.7H.01 0.1 1.0 Barium . . Calcium . . Chloride . . Chromium (as Chromium (as CrC1,) K2Crd-3,) Cobalt . . Copper . . Iron(I1) . . Iron(I1J) . . Magnesium . . - I . . 0.1 0.1 . . 5-0 5.0 . * 1.0 1.0 0.04 0.04 1.0 1.0 0.1 0.1 . 0.1 0.1 . . 0.1 0.1 . . 5-0 5.0 . . 5.0 5.0 . . 5.0 5.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 Boron found, CLP (0.1 1.0 3.5 4.5 <om1 1.0 0.1 1.0 0.2 1.0 <o-1 1-0 >5*0 B5.0 >5*0 >5-0 0.2 1.0 (0.1 1.0 <0.1 1.0 <o-1 0.9 <0*1 1.0 Element Manganese .. .. Mercury( 11) .. Molybdenum (as Na2M00,.2H20) Nickel . . . . Nitrate . . . . Potassium , . .. Silicon (as SiO,) Silver . . Sodium . . Titanium . . Tungsten (as Na,W0,.2H20) Vanadium .. ,. Zinc .. .. Amount Boron added, mg 1.0 1.0 0.1 0.1 0.1 0.1 0.1 0.1 1.0 1.0 1.0 1.0 2.5 2.5 0.1 0.1 5.0 5.0 1.0 1.0 0.1 0.1 0-1 0.1 0.1 0.1 present, CLg 0 1-0 0 1.0 0 1-0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 0 1.0 Boron found, CLg <0.1 1.0 0.4 1.2 t 0 . 1 1.0 <0.1 1.0 3.0 4.0 < O . l 1.0 <0*1 1.0 <o-1 1.0 <0*1 1.0 (0.1 1.0 tom1 1.0 <0*1 1.0 < O * l 1.0December, 19661 OF BORON IN SOILS, SEDIMENTS AND ROCKS WITH METHYLENE BLUE 777 extraction of the fluoroborate complex increases.It is therefore important that the final acid concentration should be constant, and if more than 1 ml of sample solution is used for analysis, the preparation of the standard series must be adjusted to obtain similar conditions. Likewise, when less than 1 ml is used, compensating amounts of hydrofluoric and sulphuric acids must be added with the aliquot. The standard series shows an increasing intensity of blue from a slightly blue zero. The molarity of methylene blue in the final aqueous phase must not be less than four times that of the boron, an increase of 50 per cent. having no effect upon the intensity of colour in the organic phase.The complex is readily extractable, agitation for 15 seconds is probably adequate, and it is stable for 2 days. Benzene, carbon tetrachloride, chloroform, isopentyl acetate, isopentanol, toluene, various petroleum spirits and white spirit were tried unsuccessfully as alternative solvents. I t was found convenient to use polythene ice-cube trays for the acid treatment of the sample, a second tray being used as a cover; pipettes were made from quartz glass or poly- thene. Borosilicate glassware must, of course, be avoided, and even soft glass can contain a few per cent. of boron and so cause contamination. Quartz glass test-tubes were used for the colorimetry, although lead glass is also satisfactory and such test-tubes are much cheaper.Bark corks were unsatisfactory as they absorbed a considerable amount of methylene blue which did not wash out and could be liberated during a later test to give high results; the use of silicone rubber stoppers eliminated this source of error. A spectrophotometric finish could be adopted, the boron - methylene blue complex exhibiting an absorption maximum a t 640 mp. A blank determination should be used as reference. INTERFERENCE FROM OTHER ELEMENTS- Dichromate, nitrate, arsenate and mercury (11) ions have adverse effects, but the concentrations at which interference occurs are unlikely to be obtained in normal samples. RE s u LTS The effects of various ions are shown in Table 11. The reproducibility of the colorimetric stage was tested by using several aliquots from one sample solution.A mean value of 177.5 p.p.m. was obtained, with a standard deviation of k6.1 p.p.m. Replicate analyses on three samples gave mean values of 32-5, 177-5 and 233 p.p.m., with standard deviations of -I 2.8, & 6-1 and I_t 12.1 p.p,m., respectively. Several samples, including the standard rocks G1, W1 and Syl, were analysed by the proposed method and by the official A.O.A.C. m e t h ~ d . ~ The results are shown in Table 111, Sample No. G l w 1 SYl 1 2 3 4 6 6 7 8 9 10 11 TABLE I11 COMPARISON OF RESULTS FROM VARIOUS METHODS Boron, p.p.m. Proposed A.O.A.C. Mass procedure method Spectrography spectrometry 2.8 <2 < 10 12 23 17 18 72 69 70 - 184 173 80 80 92 91 122 137 134 137 166 160 188 182 186 182 182 182 188 182 44 46 r A - - - - - - - - - - - - - - - - - - - - - - - together with some spectrographic4 and mass spectrometric5 determinations on the standard rocks. The low values for G1 and W1 are likely to be caused by the inadequacy of the acid treatment of the particular mineral containing the boron.778 STANTON AND MCDONALD The work described forms part of the programme of the Applied Geochemistry Research Group under the direction of Professor J. S. Webb. REFERENCES 1. 2. 3. Pasztor, L., Bode, J. D., and Fernando, Q., Analyt. Chem., 1960, 32, 277. Goto, H., and Takeyama, S., Nippon Kink. Gakk., [ J . J a p . Inst. Metals], 1961, 25, 588. Horwitz, W., Editor, “Official Methods of Analysis of the Association of Official Agricultural Chem- ists,’’ Eighth Edition, The Association of Official Agricultural Chemists, Washington, D.C., 1955, p. 38. 4. Heier, K. S., Norsk Geol. Tidsskr., 1964, 44, 205. 5. Brown, R., and Wolstenholme, VT. -4., Nature, 1964, 201, 598. Received June 28th, 1966