Analyst, January, 1978, Vol. 103, pp. 93-100 93 Analytical Methods Committee REPORT PREPARED BY THE COMPhEXlMETRlC STANDARDS PANEL OF THE ANALYTICAL STANDARDS SUB-COM M ITTEE Precise Standardisation of Disodium Dihydrogen Ethylenediaminetetraacetate Dihydrate by Spectrophotometric Titration Against Pure Zinc Metal Keywords EDTA standardisation ; zinc ; dithizone indicator; spectrophoto- metric tityatiow The Analytical Methods Committee has received and approved for publication the following Report from its Analytical Standards Sub-committee. Report The constitution of the Panel responsible for the preparation of this Report was: Professor E. Bishop (Chairman), Mr. D. J. Bucknell, Mr. A. G. Hill, Dr. R. G. Monk, Mr. H. N. Redman, Dr. J. M. Skinner, Dr. W. I. Stephen, Dr. P.Whitehead and Dr. W. J. Williams, with Mr. P. W. Shallis as Secretary. Introduction A previous Report1 from the Compleximetric Standards Panel described the development of a method for the precise standardisation of disodium dihydrogen ethylenediaminetetra- acetate dihydrate (EDTA) against pure bismuth metal using spectrophotometric titration and catechol violet as indicator. The background to the Panel’s work on compleximetric standards was given and also the conclusions of a survey carried out on metallochromic indi- cators to the effect that only two, catechol violet and dithizone, were available in a sufficiently pure state for use in high-precision work. Accordingly, it was decided to concentrate work on the titrations of bismuth and zinc with EDTA using catechol violet and dithizone, res- pectively, as indicators.This Report deals with the precise standardisation of EDTA using pure zinc metal as the standard and dithizone as the indicator. Precise Assay of EDTA Using Zinc Metal The advantages of bismuth metal as a standard, which were outlined in the previous Report,l are not shared by zinc. Zinc is a multi-nuclidic element and its relative atomic mass on the carbon-12 scale of 65.38 is given as being reliable to &0.01.2 Also, the zinc - EDTA complex is much less stable than is the bismuth compound; the titration of zinc therefore requires a considerably higher pH than the 2.4 adopted for the titration of bismuth. As a consequence the number of interfering elements in the titration of zinc with EDTA is muchgreater than in the titration of bismuth.Owing to the low solubility of dithizone in water, EDTA titrations using this indicator cannot be carried out in a completely aqueous medium. Although two-phase systems involving a liquid immiscible with water as a dithizone solvent, e.g., chloroform, have been used3 for the titration of cadmium and lead, the use of a mixed aqueous - acetone or aqueous - ethanol medium as described by Wanninen and Ringbom4 is to be preferred, particularly as it is more adaptable to spectrophotometric operation. These authors recommended an ethanol concentration of 40-50% V/V and a pH of 4.5 for the visual titration, and the initial work of the Panel was based on their procedure.94 AMC: PRECISE STANDARDISATION OF EDTA BY Analyst, Vol.103 Dithizone The sample of dithizone used in the early stages of the work was later examined by paper chromatography and found to contain four coloured constituents. Another sample was purified by a newly published method5 and was then shown to contain only a single coloured constituent; there was insufficient of this material for the Panel's work, but another com- mercially available sample was found to be equally pure and was used in all subsequent work. Standard Zinc Metal The sample of zinc rod used was analysed by solid-source mass spectrometry for all elements except hydrogen, polonium, actinium, radium and the noble gases. Figures for lead, cad- mium, iron, calcium, potassium, chlorine, aluminium, oxygen, nitrogen, carbon and boron were obtained totalling 5 pg g-l.The sum of the upper limits for the remaining elements was 9 pg gt. The maximum total for the impurities determined was, therefore, 14 pg gl, giving a purity of 99.998-99.999% for the metal. As had been found for the bismuth sample, occasional blow-holes were found in the zinc rods. Metal in the neighbourhood of these may contain more oxygen than the 0.5 pg g-1 found by mass spectrometry. Similar precautions to those taken with bismuth were therefore taken in the preparation of the zinc samples used for the assay of EDTA. EDTA The same batch of EDTA was used as had earlier been used for the work with bismuth. Owing to the presence of a small amount of a white insoluble substance that was initially distributed heterogeneously the material had been thoroughly homogenised.The sample contained not more than 60 pg g1 of nitrilotriacetic acid (NTA), which has been shown6 to affect the titration when present at a level of 250 pg g-l or above. Initial Investigations As noted above the initial work was based on the procedure of Wanninen and Ringbom4; titrations were carried out with visual observation of the end-point and the optimum con- ditions of pH, ethanol and indicator concentrations, and type of buffer to be used were established. The concentrations of zinc and EDTA were, however, very low, the final rnolarity (M)* being only 0.025. The recommended conditions were: ethanol, 40-50% V/V; dithizone, 10-20 p ~ ; pH, 6 -+ 0.5; and buffer, hexamine or acetate (pyridine - sulphuric acid and phthalate buffers were unsatisfactory).Several inter-laboratory comparative titrations were carried out using various adaptations of a British Standards Institution method for the determination of alumina, which uses the zinc - EDTA - dithizone procedure. It was found that, although good reproducibility was often obtained within a laboratory, different laboratories disagreed widely. This was considered to be due to the difficult visual detection of the end-point, which involved a change from grey-green to pink, the final titration being made with a dilute zinc solution. Accordingly, it was decided to change to spectrophotometry for the end-point detection, absorbance readings being made at 505 and 586 nm, these wavelengths corresponding to the maxima for zinc dithizonate and dithizone, respectively.It was clear from the shapes of the titration curves that decomposition of indicator was occurring and this observation led to protracted investigations aimed at minimising this effect. Stability of Indicator Stability of the indicator here refers to the resistance of the organic structure of the indi- cator, whether free or complexed with metal, to destruction by oxidation or other process. It does not refer to the stability of the zinc dithizonate in equilibrium with zinc and dithi- zonate ions, although the latter undoubtedly influences the former. A paper by Budesinsky and Sagat' recommends hydroxylammonium chloride as a st abiliser for stock solutions of 0.001 M dithizone in 0.1 M sodium hydroxide and their indicator solution was adopted as the basis for all subsequent wor:k.The use of this stabiliser in the titration Throughout this Report the symbol M denotes concentration in moll-1.January, 1978 SPECTROPHOTOMETRIC TITRATION AGAINST PURE ZINC METAL 95 solutions was also found to be of value, although the sulphate is preferred to the chloride as it gives less stable zinc complexes. It was not found possible to prevent destruction of the indicator completely; even titration under nitrogen gave no significant improvement. As it was thought possible that nitrate ion derived from nitric acid used to dissolve the zinc might be responsible for oxidation of the dithizone it was decided to try other methods of dissolution. Highly pure zinc dissolves only very slowly in non-oxidising acids, but experi- ments to dissolve it in dilute sulphuric acid were carried out using platinum gauze wrapped round the metal or in the presence of a trace of chloroplatinic acid as a catalyst.The pro- cedure was tedious and dissolution of the metal was incomplete even after 30-h boiling under reflux. Titration of zinc sulphate solutions prepared by such methods showed no less destruction of dithizone than occurred in nitrate solutions and a return to nitric acid in which to dissolve the zinc was decided on with relief. The following observations have been made on the stability of solutions of dithizone and zinc dithizonate a t pH values of 4.5-5.5 and containing 0.02 M hydroxylamine sulphate : (a) Stability is very poor in hexamine buffer solutions but acetate has no harmful effect.(b) In the presence of a large excess of zinc (more than 20 times the dithizone molarity) the zinc dithizonate (ZnDz,) produced is stable for at least 1 week. (c) Dithizone alone is less stable than zinc dithizonate is in (b). (a) Solutions containing both zinc and dithizone in comparable molarities are much less stable than those in ( b ) and (c). In particular, solutions containing zinc and dithizone in stoicheiometric proportions are the most unstable that were observed, instability increasing with concentration; this is illustrated in Table I. The use of anodic dissolution in dilute sulphuric acid was also tried. TABLE I DECOMPOSITION OF DITHIZONE IN SOLUTIONS OF STOICHEIOMETRIC ZNDZ, COMPOSITION AT ROOM TEMPERATURE Initial composition of solution: hydroxylammonium sulphate 0.02 M, ammonium nitrate 0.10 M and acetic acid 0.05 M. The pH was adjusted with ammonia to 6.50, zinc and dithizone were added to give required concentrations at 1 : 2 molar ratio and ethanol was added to the solution to give a concentration of 50% V/V.Total dit hizone / p~ Dithizone decomposed 7zzTzZz after 18 h, yo 6.35 4.9 22 12.7 6.7 47 25.4 6.5 74 In view of the greater stability of indicator in the presence of excess of zinc it was con- sidered desirable to begin a titration with a solution containing an excess of zinc and to back- titrate with standard dilute EDTA solution. Maximum rate of decomposition of indicator occurs during the colour change, but, because of the observations in (b) and (c) above, less total loss of indicator would be expected to occur when titrating the excess of zinc with EDTA than when titrating EDTA with standard zinc solution.Indicator decomposition was assessed by adding a large excess of zinc a t the end of a titration and measuring the absorbance at 505 nm in order to estimate the residual dithizone. It was found that increasing the dithizone concentration reduced the proportion of indicator decomposed. In the recommended method for the assay of EDTAagainst zinc (see Appendix) a dithizone concentration of about 25 p~ is used and 20-25% of this may be lost during the titration. When a dithizone concentration of about 6 p~ is used the loss can be as high as 4045%. Development of Spectrophotometric Titration Procedure The use of spectrophotometric titration was decided on owing to the poor agreement obtained in inter-laboratory trials using visual end-point detection.It was, however, also intended to follow a similar procedure to that used in the bismuth titrations and to do a least-squares fit of the theoretical titration curve to the experimental results. Thus it was hoped to calculate the true equivalence point and the conditional formation constants of the complexes of zinc with EDTA and dithizone under titration conditions.96 AMC: PRECISE STANDARDISATION OF EDTA BY Analyst, Vol. 103 The first few titrations, as well as drawing: attention to the decomposition of dithizone, showed very poor sensitivity, as measured by the slope of the steepest part of the titration curve and expressed as the change in absorbance for a 1% change in titrant concentration.The total titrant molarity was only 0.04-0.10 and slopes were between 2 and 3 as compared with 30 for the bismuth titration. As the precision attainable in a spectrophotometric titration depends on the slope of the curve and as the latter should increase with reactant concen- trations, the effects of zinc and EDTA concentrations on the titration were examined. For bismuth the maximum reactant concentration was limited to about 0 . 0 8 ~ by the solubility of the bismuth - EDTA complex. In the present instance it was found that the maximum concentration of the zinc - EDTA complex in a 50% V/V ethanol - water solution was 0.45 to 0.50 M, the solution containing additionally about 1 M concentrations of NH,+, Naf and NO3-- ions.In 40% ethanol - water solution the corresponding range was 0.55- 0.60 M. At higher reactant molarities a second liquid phase appears; this situation can be remedied simply by careful dilution, which rapidly re-establishes a single liquid phase. A concentration of zinc and EDTA of 0.34.4 M has been found to afford convenient con- ditions for titration and give satisfactory titration curves at pH values of 5.0-6.0, pH 5.50 being about the optimum. At a dithizone concentration of about 6 p ~ the slope of the steepest portion of the curve was 9-10, considerably better than at the much lower reactant concentrations initially employed. As noted above, the indicator concentration was in- creased to about 25 p~ and this change slightly reduced the slope to between 6 and 8.Attempts to fit the theoretical titration curve to the experimental results by least squares were unsuccessful. Values of formation constants for the zinc complexes with EDTA and dithizone were obtained which varied widely from one experiment to another and the curves calculated from such figures gave poor fits to the data. Although indicator decomposition may have contributed to this effect it did not appear that the mathematical model employed was adequate and it may be that kinetic effects were complicating matters. As a consequence of the above-mentioned difficulties it was not possible to determine the true equivalence point of the titration and it was therefore necessary to employ an empirical end-point. It was decided to take this as the point at which 50% of the dithizone was combined as zinc dithizonate.This occurs on the steepest part of the curve and is probably not far from the true equivalence point which, from such formation constants as are available, is more likely to be above rather than below the 50% value. Taking 60 or 70% of dithizone combined as ZnDz, gives an assay for EDTA 0.01 or 0.025% above that given by the 50% value. The true figure is probably within these limits if the NTA content of the EDTA is low enough, as in the present sample. The presence of NTA is known to blur the visual end-point in this titration, part only of the NTA titrating as EDTA. In the event of EDTA containing greater amounts of NTA being used for determining zinc it is clear that the stan- dardisation of the EDTA and the analysis of the samples for zinc must be done by exactly the same procedure.The use of the spectrophotometric procedure will minimise errors, but the recommended procedure is for the startdardisation of EDTA to be used only for the determination of zinc. Errors Due to Changes in Reactant Concentrations In order to assess the effect of the concentrations of zinc and EDTA on the assay results for EDTA an experiment was carried out in which the recommended procedure was applied using about double amounts of reactants, the solutions being made up to 100 ml in a calibrated flask without addition of ethanol. Aliquots were removed by pipette, diluted to 50ml if necessary, 50 ml of ethanol added and the titration was completed as described; the results are given in Table 11.The increase in the EDTA assay with dilution occurs as expected (see Table II), but the effect is negligible for small changes in reactant concentrations within, or even well outside, the limits prescribed in the method. The precision falls off considerably at higher dilutions (see results for 0.019 M solutions in Table 11) because of the increased rate of indicator decomposition. The recommended method is detailed in the Appendix and in Fig. 1 is shown a typical titration curve together with a bismuth - EDTA titration curve for comparison.January, 1978 SPECTROPHOTOMETRIC TITRATION AGAINST PURE ZINC METAL TABLE I1 97 EFFECT OF THE REACTANT CONCENTRATIONS ON THE ASSAY OF EDTA Molarity of zinc and EDTA EDTA, yo 0.382 99.909 0.191 99.907 0.038 99.916 (0.019 99.9 1 5) (0.019 99.93 1) Practical Notes The method is designed to give a final volume of about 100 ml of 50% V/V ethanol - water solution and a concentration of 0.30-0.38 M with respect to zinc and EDTA. Using a glass - calomel electrode system calibrated in an aqueous pH standard the pH may be adjusted to 5.50 before, or to 6.10 after, the addition of ethanol. After dissolution of the zinc in nitric acid and just before the addition of the EDTA the solution is about 30 ml in volume and about 1 M with respect to nitric acid.The solid EDTA is completely soluble in this solution but the addition of the prescribed amount of ammonia hastens dissolution of any solid that remains after swirling the beaker for about 1 min. What must be avoided is the addition of only part of the ammonia, either before or after the addition of the EDTA, as in this instance precipitation may occur, possibly to the extent of the liquid setting almost solid.This effect appears to be due to the precipitation of NaHEDTA, NH,HEDTA or both. 9G.95 100.00 100.05 EDTA to metal ratio, mol % Fig. 1. Spectrophotometric titra- tion of EDTA against zinc and bismuth. A, Titration of zinc: total concentration of zinc 0.368 M; concentration of dithizone 24 p ~ ; absorbance measured at 505 nm in 10-mm cells. B, Titration of bis- muth : total concentration of bis- muth 0.080 M ; concentration of catechol violet 30 p~ ; absorbance measured at 560 nm in 40-mm cells. Results of Collaborative Assays The results of collaborative assays of the sample of EDTA carried out in five laboratories by the recommended method are shown in Table 111.An analysis of variance of all the results gives a relative standard deviation of o.016y0 within a laboratory and 0.016% between laboratories.98 AMC: PRECISE STANDARDISATION OF EDTA BY TABLE I11 Analyst, Vol. 103 RESULTS OF COLLABORATIVE ASSAYS EDTA content as [CH ,N(CH ,COOH) (CH ,COONa) J ,.f!H ,O, yo Laboratory Mean, % A 99.928, 99.939, 99.907 99.925 B 99.903, 99.892, 99.904, 99.918, 99.918 99.907 C 99.925, 99.916, 99.927 99.923 D 99.940, 99.889, 99.904, 99.91 1, 99.930, 99.909, 99.919, 99.909, 99.905, 99.886, 99.901 99.909 E 99.914, 99.905, 99.927 99.915 All results: 99.913 Relative standard deviation, yo 0.016 0.01 1 0.006 0.017 0.011 0.015 Recommendation Zinc metal of purity not less than 99.999% is recommended as a primary standard for the standardisation of EDTA for use in the determination of zinc.The results obtained are in good agreement with those obtained with bismuth,l but because of the empirical nature of the end-point, the procedure cannot be recommended for the standardisation of EDTA to be used for other metal determinations at very high precision without prior investigation. APPENDIX Titration with Zinc Metal Recommended Method for the Precise Assay of EDTA by Spectrophotometric Reagents With a small hacksaw fitted with a clean, fine-toothed blade make a 1-2-cm longitudinal cut down the axis of the rod. Then saw across the rod to give hemicylindrical pieces of metal 2-5 mm thick and of mass 0.3-0.7 g. (This method of cutting gives a good chance of exposing blow-holes.) Wash the metal pieces with re-distilled acetone to remove grease and then immerse them for 2 min in dilute nitric acid (1 + 9).Wash twice with water then twice with re-distilled acetone and dry in a current of air. Dissolve 16.4 g of hydroxylammonium sul- phate in water and dilute to 100 ml. Zinc metal rod, purity 99.999%. Hydroxylammonium sulphate solution, 1 M. Nitric acid, 16 M . Analytical-reagent grade nitric acid, sp. gr. 1.42. Ammonia solution, 14 M. Analytical-reagent grade ammonia solution, sp. gr. 0.88. Zinc acetate solution, 0.5 M. Dissolve 11.0 g of analytical-reagent grade zinc acetate dihydrate in water and dilute to 100 ml. Bufer soluutioa, pH 5.50. Add 10 ml of 1 M hydroxylammonium sulphate solution and 5 ml of analytical-reagent grade glacial acetic acid to 200 ml of water and adjust the pH to 5.50 with 14 M ammonia solution using a glass - calomel electrode system.Dilute to 250 ml. Weigh 0.016 g of chromatographically pure dithizone and transfer it into a 25-ml calibrated flask. Add. a few drops of absolute ethanol and, when all the dithizone has been thoroughly wetted, add 0.5 ml of 1 M hydroxylammonium sulphate solution and 5ml of 0.5 M sodium hydroxide solution. Dilute to about 10ml and shake until all the solid has dissolved; dilute to the mark with water. Dithizone solution, 0.002 5 M. Ethanol, absolute. Standard Solution The standard solution should be made up by volume or mass, according to the method of titration to be used. If mass titration is used a suitable burette can be made by drawing off a hexagonal polyethylene ampoule of 5-7 ml capacity to a jet delivering 0.01-0.02-ml drops.EDTA solution, about 0.02 M. Weigh, to the nearest 0.01 mg, about 0.75 g of the EDTA being assayed, transfer it into a 100-ml volumetric flask, and dissolve it in about 20ml of water containing 0.3 ml of 14 M ammonia solution; dilute to the mark with water. For mass titration weigh the flask both empty and containing the solution.Janzcary, 1978 SPECTROPHOTOMETRIC TITRATION AGAINST PURE ZINC METAL 99 Apparatus been previously calibrated. Balance and weights. These should be capable of weighing to 0.01 mg and should have fiH meter. Spectrophotometer. Transfer $i+ette. A polyethylene ampoule holding 20-30m1, drawn off to a jet about 10 cm 1ong.l Beakers.Tall-form, lipless, 250-ml beakers with cover-glasses. Each beaker should be graduated at 50-ml intervals. Weighing bottle. 60 mm high, 30 mm diameter. Magnetic stirrer. With follower to fit a 250-ml beaker. Microbwette. For mass titration use the burette described above under Standard Solution. Hot-plate. Electric, thermostatically controlled. Tongs. Fisher-type pick-up tongs. (i) Transfer 45 ml of pH 5.50 buffer solution to a 100-ml volumetric flask, add 50 ml of Add 1 ml of 0.02 M EDTA solution, 1.00 ml Measure the absorbance (ii) Repeat (i), but with 2 ml of 0.5 M zinc acetate solution in place of the 0.02 M EDTA The meter should be equipped with glass and calomel electrodes. To be used with 10-mm cells. For volumetric titration use a 5-ml burette graduated in 0.02-ml divisions.Absorptiometric Standards absolute ethanol and cool to room temperature. of 0.002 5 M dithizone solution from a pipette and dilute to the mark. of this solution at 505 nm in 10-mm cells. solution. Procedure Weigh, to the nearest 0.01 mg, 2.0-2.5 g of the zinc metal rod and, by using tweezers, transfer it to a tall-form, lipless, 250-ml beaker. Then check the balance reading without the sample in case any detached particles of metal remain on the pan. To the metal in the beaker add 10 ml of water. Measure out, to the nearest 0.1 ml, a volume of 16 M nitric acid in millilitres equal to 3.5 times the mass of zinc in grams, transfer 4 ml of this to the beaker and cover with a clock-glass. Allow to stand for 5 min and then transfer the beaker to a hot-plate at a suitably low setting (see Note 1) to speed up the reaction. When the rate of reaction slackens (5-10 min) add a further 2 ml of acid and continue heating until the rate of dissolution of the metal again slows down. Add the remaining acid and continue heating for 15 min after the last trace of metal has dissolved.Dissolution should take 3045 min, making a total time to this stage of 45-60 min. Allow the solution to cool to room temperature, wash down the cover-glass and sides of the beaker with 10 ml of water from the transfer pipette and then add 2 ml of 1 M hydroxyl- ammonium sulphate solution. Weigh, to the nearest 0.01 mg, an amount of the EDTA sample calculated to be equivalent to 99.90-99.95% of the zinc present into an unstoppered weighing bottle. Support the beaker in a clamp at an angle of about 45" to the horizontal, and carefully add the EDTA to the zinc solution, using pick-up tongs to hold the weighing bottle.Before inverting the bottle lower it well inside the beaker in order to minimise the risk of loss of fine powder by air-flotation. Tap the bottle several times to transfer as much of the EDTA as possible to the zinc solution. Return the weighing bottle to the balance case and re-weigh it later. Remove the beaker from the clamp, replace the cover-glass, and swirl the beaker gently for about 1 min, during which period most of the solid will dissolve. Measure out, to the nearest 0.1 ml, a volume of 14 M ammonia solution equal to that of the nitric acid used, add it to the solution in the beaker and swirl for a further minute.Transfer the beaker to the hot- plate and swirl it occasionally until all of the EDTA has dissolved. Cool the beaker and contents to room temperature. Add 50ml of absolute ethanol, cool the solution to room temperature, insert glass and calomel electrodes and add 14 M ammonia solution (see Note 2) dropwise to bring the apparent pH to 6.10, corresponding to a pH of 5.50 for the solution before adding ethanol. (Alternatively, if sufficiently long electrodes are available, the pH can be adjusted to 5.50 before the addition of ethanol). Let this absorbance be A,. Let this absorbance be A,. Add 1 ml of glacial acetic acid, insert a stirrer bar and stir the solution.100 ANALYTICAL METHODS COMMITTEE Raise the electrodes from the solution and, if necessary, add water to bring the solution to the 100 ml mark on the beaker. Replace the electrodes and wash down the sides of the beaker with the solution by means of the transfer pipette to ensure complete homogeneity.Transfer a portion of the solution to a 10-mm cell, measure the absorbance (AB) against water a t 505 nm and return the sample to the beaker. Add 1.00 ml of 0.0025 M dithizone solution and again measure the absorbance at 505 nm. Titrate with 0.02 M EDTA solution until the absorbance has fallen to below 0.50; about six readings between 0.8 and 0.5 will be sufficient. Measure the final volume of the solution to 1-2%; let this volume be Vs ml. Plot the titration curve and read off the vollume of 0.02 M EDTA corresponding to an absorbance of NOTES- an equilibrium temperature of 75-80 "C. should be made with glacial acetic acid. 1. The hot-plate control setting should be such that 20 ml of water in a similar covered beaker reaches 2. In the unlikely event of the pH being higher than that required, adjustment to the correct value Calculation All experimental masses are the apparent masses in air. Relative atomic mass of zinc = 65.38 Relative molecular mass of EDTA Air-buoyancy correction of EDTA relative to zinc = 1.000 52 = 372.241 " " Mass of EDTA equivalent to 1 g of zinc 372.241 = 5.69054g - - 1.000 52 x 65.38 Symbol or Specimen formula jigures Mass of zinc taken zg Concentration of 0.02 M EDTA Mass or volume of 0.02 M EDTA used in e g g1 or g ml-l titration V' g or ml Mass of EDTA sample used in titration eV g Mass of EDTA sample weighed out E g Total mass of EDTA equivalent to zinc Mass of EDTA equivalent to zinc taken Percentage of EDTA in sample taken E + e V g 5.690 54 Z g 569.054 2 213 + eV _____ 2.491 37 g 0.007 505 2 g g-l 1.698 g 0.007 505 2 x 1.698 g 14.178 05 g = 0.012 74 g 14.190 79 g 5.69054 x 2.491 37 g 569.054 x 2.491 37 14.190 79 = 99.905 References 1. 2. 3. 4. 5. 6. 7. Analytical Methods Committee, Analyst, 1975, 100, 675. Greenwood, N. N., Chemy. Brit., 1973, 9, 52. Bovalini, E., and Casini, A,, Annuli Chim., 1953, 43, 287. Wanninen, E., and Ringbom, A., Analytica Chim. Acta, 1955, 12, 308. King, H. G. C., and Pruden, G., Analyst, 1971, 96, 146. Farrow, R. N. P., and Hill, A. G., Analyst, 1965, 90, 210. Budesinsky, B. W., and Sagat, M., Talanta, 1973, 20, 228.