388 LEWIS AND GRIFFITHS : DETERMINATION OF INORGANIC COMPOUNDS [Vol. 76 Inorganic Chromatography on Cellulose Part IV* Determination of Inorganic Compounds by Paper-Strip Separation and Polarography BY J. A. LEWIS AND J. M. GRIFFITHS (Presented at the meeting of the Society on Wednesday, February 7th, 1951) Separations are described in which problems of interference in quantita- tive analysis are overcome by using paper-strip chromatography. These separations can be divided into three type:; : (a) the separation of one individual from a number of metals, (b) the separation of a group of metals into individual metal salts and (c) the separation of a larger group into smaller groups of metal salts in which interference no longer occurs. Final estimations have been carried out by polarography.THE separation of inorganic salts by a chromatographic technique using absorbent paper in conjunction with organic solvents has already been the subject of a number of publications from this 1 a b o r a t 0 r y . l ~ ~ ~ ~ ~ ~ ~ ~ The work with paper strips has, however, mainly been con- cerned with qualitative analysis, although the application of the technique as a basis for quantitative estimation of metals has been indicated. This paper presents details of the separation and determination of a number of metals and mixtures of metal salts by means of chromatography on paper strips followed by polarography for the final determination. The separation3$4 is performed by placing an accurately measured volume of the test solution near one end of a strip of absorbent paper.The end of the strip nearest the test patchis then immersed in the organic solvent, which is allowed to diffuse through the paper and over the sample of metal salts; one or more of the metal salts dissolve and move down the paper to form well-defined zones. The regions containing the metal salts are then separated from the rest of the paper strip and the amount of metal in each region, after solution, is deter- mined by the polarograph. Three types of separation are described, ( a ) the separation of one metal from a mixture, exemplified by uraniuin in the presence of a large number of other metals, (b) the separation of several metal salts in a mixture, such as cobalt, nickel and copper in a sample of alloy steel and copper and cobalt in iron pyrites and (c) the separation of a mixture of metals into groups containing several metals that can be determined by polaro- graphy without further separation, represented by ten metals separable into two groups of five containing (2) vanadium, copper, uranium, lead and titanium and (ii) iron, molybdenum, bismuth, antimony and cadmium.A new supporting electrolyte for polarography that makes use of salicylic acid to form complex ions with certain metals has been found valuable for determination of several metals in a mixture. This combination of chromatographic separation and polarography has proved satis- factory, the procedure being both rapid and reasonably accurate when the smallness of the quantities of material required for test is borne in mind.Moreover, the method shows promise of wide application in micro-analytical work. EXPERIMENTAL The extension of paper chromatography to a quantitative method required rigorous attention to details, i.e., the acidity of the metal solution, the effects of interfering anions, the composition of the solvent and sometimes of the atmosphere in the gas-jar. Methods for the removal of metals or groups from the paper and subsequent treatment to produce a convenient solution for estimation have been investigated. Volumes were measured on to the paper (Whatman No. 1) with a micrometer syringe. * For particulars of earlier papers in this series (not in The Analyst), see reference list, p. 395.July, 19511 BY PAPER-STRIP SEPARATION AND POLAROGRAPHY 389 THE SEPARATION AND DETERMINATION OF ONE METAL FROM A GROUP OF METALS- The first type of paper strip separation investigated was the isolation of one element, and the first element so examined was uranium. The isolation of this element on a semi- quantitative basis has been described by Arden, Burstall and Lin~tead.~ The solvents used were tetrahydrosylvan and tetrahydropyran with additions of small amounts of nitric acid and saturated with water.For quantitative separation it was found necessary to use a t least 5 per cent. of nitric acid with tetrahydrosylvan and at least 7 per cent. of nitric acid with tetrahydropyran. Among simple solvents, nitromethane shows promise of being useful for the separation. The original metal solution was in nitric acid of 50 per cent. v/v concentration.I t was found that the whole of the uranium was always present in the leading 6 cm of the solvent run and that a run of 10 cm was sufficient to separate uranium from the common metals. To test the effects of other metals a run was carried out with trace amounts of the following present: Li, Re, Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Cs, Ba, Ce, La, Pr, Ta, W, Os, Ir, Pt, Au, Hg, T1, Pb, Bi, Th. The effect on the quantitative separation of uranium was negligible. With regard to anions, phosphate can be tolerated in amounts up to 0.7 M in the original metal solution provided there is an excess of ferric iron present and provided a run is continued for 16 cm in order to obtain a section of paper sufficiently free from phosphate to be ignited.Sulphate can be tolerated in amounts up t o 0.8 M . Above these concentrations, phosphate and sulphate cause the uranium to spread over the paper. These high concentrations, however, are unlikely to be encountered in a solution of a sample. Chloride ions promote the movement of other metallic ions, notably iron, on the paper. A solution of the uranium is made by removing the paper from the gas-jar, drying it, tearing out the last 6 cm of solvent run and burning this section of paper. It was found that heating the paper in a crucible to reduce it to ash led to low results, so the paper is held by one corner in glass forceps and burnt, the ash being dropped into a beaker. The unburnt corner, remote from the leading solvent edge, can be rejected.The ash is then twice taken to dryness with a few drops of diluted perchloric acid, 2 ml of acid (0-1 N hydrochloric acid saturated with alkali-soluble methyl cellulose) are added, the whole is stirred with a glass rod and 1 ml of the resulting solution is taken for polarographic estimation between 0 and - 0.4 volt. The results shown in Table I are for synthetic solutions used in the course of a study of the method. With synthetic solutions it was found necessary to add ammonium nitrate to the original solution as “salting-out” agent; this is unnecessary with sample solutions. TABLE I DETERMINATION OF u308 I N SYNTHETIC SOLUTIONS Quantity of U308 on absorbent paper, pg . . 20 70 100 150 200 24 68 101 141 21 1 68 101 142 193 Observed quantities of U308 determined by 72 101 142 197 99 145 204 93 147 20 1 polarograph, pg .. .. . . . .[ k% 69 24 72 21 67 102 147 204 Mean observed quantities of U,O,, pg . . 23 69 99 144 202 Solutions of ores and other products have given results of which a selection are shown The first five results on siliceous ores have been published by Arden, Burstall The method has been used with satisfactory results for routine estimations and exemplifies in Table 11. and Lin~tead.~ the isolation of one element for analysis. THE SEPARATION OF A GROUP OF METALS INTO INDIVIDUAL METAL SALTS- For some groups of metals, estimation in the presence of one another is difficult and the chemical separation tedious. As an example of the application of paper-strip separation to such a group, a mixture of the chlorides of nickel, manganese, cobalt, copper and iron can be separated into individual metal salts and taken into solution for estimation.The polarograph was used for final determination. In simple chloride solution, polarographic estimation390 LEWIS AND GRIFFITHS: DETERMINATION OF INORGANIC COMPOUNDS [VOl. 76 presents problems of interference within this group, but the metals, once separated, can be estimated in any desired supporting electrolyte. For the purpose of the extraction, the atmosphere in the gas-jar was brought to equilibrium with separate vessels, together in the same gas-j a]:, containing (a) saturated ammonium nitrate solution (giving 65 per cent. relative humidity) and (b) acetone. The paper strips were allowed time to “condition” in this atmosphere after placing the spot of solution 12 cm from the top of the paper.The solvent used for extraction was 50 parts by volume of acetone, 8 parts of concentrated hydrochloric acid and 42 parts of methyl n-propyl ketone, b.p. 98” to 100” C. This solvent was allowed to seep down through the spot for a distance of at least 30 cm. The separation was usually complete in about 6 hours at normal room temperature TABLE I1 COMPARISON BETWEEN DETERMINATIONS OF URANIUM BY CHEMICAL AND PAPER-STRIP METHODS Uranium as U,O, determined by Chemical Polarographic Type of ore analysis, analysis, f A 1 % Yo Pitchblende . . . . . . 26.46 26.4 Siliceous . . . . . . . . 1.33 1-25 1.26 1.16 0.31 0.30 0.69 0.70 0.91 0-94 0.045 0.050 1.28 1.53 0.19 0.21 Monazite .... .. 0.37 0.35 0.34 0-34 0.38 0.38 Phosphate . . .. .. 0.024 0-039 0.015 0.020 0.009 0.01 1 of about 20” to 25” C. By tapering the upper end of the paper to less than 1 cm to reduce “wick” action and using a strip 40 cm long and 3.5 cm wide, it was possible to allow the run to proceed overnight. When the run was completed, the strip was removed and dried and the positions of the various metals ascertained by exposing the strip to strong ammonia vapour and then spraying it with a mixture of equal volumes of rubeanic acid (a 0.25 per cent. solution in alcohol) and 1 per cent. of benzidine in a 10 per cent. aqueous solution of acetic acid. The two stock solutions were mixed just before use. Nickel, manganese, cobalt and copper, under this treatment, give deep blue, pale blue, yellow arid greyish-green stains, respectively, which appear in that order down the paper.Iron gives a very pale brown self-colour below all the foregoing metals. The manganese position is marked lightly with pencil as the colour is transient. The sections of the strip containing the various metals were then cut out and ashed in separate crucibles, the ash being taken just to dryness with the appropriate fuming agent, dissolved in 2ml of supporting electrolyte and a polarogram taken with the conditions as indicated in Table 111. In the fuming process, the use of industrial infra-red lamps over the vessels was found most valuable in avoiding spil:ting, baking and the necessity for constant supervision. In Table I11 mean results found by this procedure are shown, the maximum deviation from the mean being 5 per cent.in the region of 75 to 100 pg; the deviation increases as the amount of metal decreases. For iron, a blank of about 30 pg has been measured on a strip of filter-paper cut adjacent to that on which the estimation was carried out, and this figure has been subtracted from the observed result. .Because of this blank with Whatman No. 1 filter-paper, no determination of iron has been made below 30 pg. Filter-papers of lower ash content have been shown to give a lower blank, but do not give quite such good separations. An alloy steel and a sample of iron pyrites have been analysed for some of these elements by this technique. The alloy steel was digested with aqua regia, taken to dryness severalJuly, 19511 BY PAPER-STRIP SEPARATION AND POLAROGRAPHY TABLE I11 391 MEAN RESULTS OF POLAROGRAPHIC DETERMINATIONS BY THE PROCEDURE DESCRIBED Ion r Nickel Hydrochloric acid Manganese Hydrochloric acid Cobalt Aqua regia, then hydro- chloric acid potassium chloride + 0.02 per cent.of agar-agar 100 98 75 75 10 12.5 0.1 N Copper Nitric acid Remarks - Iron Nitric acid Fuming agent Supporting electrolyte 0.1 N potassium chloride + 0.01 per cent. of gelatin 100 96 75 71 10 10 10 per cent. sodium potassium tartrate M potassium oxalate Dissolution of copper is very slow N potassium chloride satu- rated with alkali-solu ble methyl cellulose 100 99.5 75 75 10 10 100 97.5 75 76 10 10 100 102 75 77 30 33.5 All from one strip of paper All from.one strip of paper Each extracted from 100 pg of each of the other metals times with hydrochloric acid and then digested with hydrochloric acid, diluted and filtered.The residue was dissolved in caustic soda and tungstic acid was precipitated by acidifying the solution with hydrochloric acid. The mixture was filtered and the bulked filtrates were made up to 50 ml; 0.10 ml of this solution, representing 2135 pg of sample, was taken for estimation. The iron pyrites was dissolved in aqua regia, evaporated to dryness and then taken to dryness several times with, and finally dissolved in, hydrochloric acid. The solution was made up to 25 ml, and 0.05 ml, representing 4956 pg of sample, was taken for estimation. Both experiments were performed in quadruplicate with the results shown in Table IV. TABLE IV ANALYSES FOR COBALT, NICKEL AND COPPER IN AN ALLOY STEEL AND A SAMPLE OF IRON PYRITES Element in alloy steel Amount present as stated by the British Chemical Standards, % ..4-35 (4-26 to 4.46) .. 0.43 (0.42 to 0.47) .. 0-05 approx. Found, % 4.40 0.46 0-04 approx. Remarks Cobalt .. Nickel .. Copper . . By comparison of standard stains Element in iron pyrites Copper . . Cobalt .. Amount present as stated by Bureau of Analysed Samples, Ltd., O/ Found, % 2-70 0.12 Remarks /O 2.69 0.10 .. .. Cobalt section taken up in base contain- ing some cobalt* Rubeanic acid stain Nickel .. .. Trace (one observer) Trace * This was found to aid consistent results for the very small amount of cobalt on the paper. Notes-Technical grades of methyl n-propyl ketone were tested for discolouration with concentrated hydrochloric acid and grades that did not discolour were chosen for this work.The ketone as obtained was distilled and the fraction boiling in the range 98" to 100" C was used. Occasional cleaning and recharging of gas-jars is advisable for the maintenance of good results. The elements gallium, zinc and uranium, which tend to interfere with the polarographic estimation of nickel and cobalt in chloride solution, move into the copper - iron region on the paper strip and do not interfere with the estimation of these in their respective supporting electrolytes.392 LEWIS AND GRIFFITHS : DETERMINATION OF INORGANIC COMPOUNDS [Vol. 76 The effect on the separation of anions other than chloride has been examined, and it was found that up to 20 per cent.of nitric acid or calcium nitrate and 10 per cent. of sulphuric acid or sodium sulphate (in the original metal solution) could be tolerated. Above these limits, the separations were progressively less satisfactory. THE SEPARATION OF A GROUP OF METALS INTO SMALLER GROUPS- In the course of a search for supporting electrolytes of wide application to the polaro- graphic determination of metals, salicylic acid in aqueous solution was found to be valuable in giving well-defined waves at convenient intervals for certain metals. This supporting electrolyte can be used, for example, in the estimation of commonly occurring groups of metals, such as (a) vanadium, molybdenum and titanium, ( b ) iron, copper and uranium and (c) bismuth, antimony and lead, in one operation. Other possible combinations can be derived from Table V on p.394. During the investigation of this supporting electrolyte containing salicylic acid, it was found that iron interfered with vanadium, molybdenum and bismuth with copper, antimony with uranium, and titanium with cadmium. A simple paper-strip separation was therefore used to divide these metals into two groups in which each metal could be directly estimated. Investigation showed that n-butyl alcohol in conjunction with hydrochloric acid* separated the chlorides of these ten elements into two group:; within which all interference was eliminated. The promise shown by the early work was mentioned by Burstall, Davies, Linstead and Wells4 in Part I1 of this series, but some modification has since been made to render the separation quantitative.Molybdenum is estimated after conversion on the paper strip to molybdenum blue, which reduces at a potential less negative than does molybdic acid. This fortunate discovery was useful in that the half-wave potential was thereby separated from that of bismuth in the same sub-group. SALICYLIC ACID AS A SUPPORTING ELECTROLYTE FOR POLAROGRAPHY Among a number of organic acids, salicylic acid showed promise of being useful in com- plexing certain metals so that waves were produc'ed on the polarograph at convenient intervals. The satisfactory use of salicylic acid appears to depend on the following factors- (a) Salicylic acid concentration is not critical, but 1 volume of saturated solution in 4 volumes of final solution gives good results. (b) Sulphuric acid (about 5 per cent.v/v) should be present so that the pH of the final solution is less than 2. (c) Chloride ions should be absent. The presence of chloride alters the anode potential and prevents the appearance of waves for vanadium, iron and molybdenum blue at the start of the polarogram. If these metals are to be estimated, chloride can be removed by shaking the solution with solid mercurous sulphaie and then filtering it, although an allowance must be made for a mercurous wave at 0 volts. (d) Phosphate ions should be absent. The presence of this anion causes coalescence of polarographic waves in salicylic acid. (e) Nitrate must be absent. In acid solution, attack on the anode by any appreciable amount of nitrate ion renders zero setting difficu1.t and vitiates readings for iron and vanadium.I t also has a slight effect on wave heights generally. (f) Alizarin may be present with advantage. A small amount of solid AnalaR alizarin, added to the final solution, prevents the appearance of maxima, e.g., on the copper wave. After a paper-strip separation, this addition of alizarin is not necessary, probably because the maxima are suppressed by cellulose dissolved when the metals are taken into solution for polarography. Routine estimations of iron have been performed with satisfactory results, in the absence of vanadium, by adding 1 volume of solution in sulphuric acid to 1 volume of an aqueous solution comprised of- Half-saturated salicylic acid.Half-saturated A.R. alizarin. 10 per cent. v/v concentrated sulphuric acid, A.R., sp.gr. 1-84. * Solvent suggested by Mr. N. F. Kember of this laboratory.July, 19511 BY PAPER-STRIP SEPARATION AND POLAROGRAPHY 393 A procedure found satisfactory for evaluation of waves in the region of zero e.m.f. (e.g., that for iron) is to measure wave heights from a line obtained by polarography of the supporting electrolyte only, before the metal solution is added. The other nine elements can be estimated to an accuracy of k3 per cent. (& 1 per cent. for copper) in concentrations of 30 to 1500 pg per ml in the final solution, estimations being made in triplicate. Between 5 and 30 pg per ml the accuracy of the determinations decreases to k20 per cent. Other common cations with the exception of tin do not interfere.PAPER-STRIP SEPARATION TO ELIMINATE INTERFERENCE Solutions were made up to contain the chlorides of copper, uranium, lead, iron, bismuth, antimony and cadmium together with titanium hydroxide (precipitated from sulphate) and ammonium vanadate and molybdate. The best separations in the initial experiments were attained with 20 per cent. of hydrochloric acid in the aqueous solution of metal salts, with the spot left wet on the paper, and with 5 per cent. v/v of concentrated hydrochloric acid in the butyl alcohol used as separating solvent. These conditions were therefore used in all subsequent investigations. Separations have been made on Whatman Nos. 1 and 3 filter- papers. The former gives a slower run of about 9 hours or, conveniently, overnight and will take a maximum of 0.1 ml of solution on a strip 2.5 cm wide.The No. 3 paper permits separation in 3 hours and 0.25 ml of solution can be taken up on the paper without impairing results. The groups into which the metals separate are: (a) vanadium, copper, uranium, lead and titanium in the upper section of the paper; (b) iron, molybdenum, bismuth, antimony and cadmium in the lower quarter of the solvent run. There is a metal-free space between these two groups which facilitates division. The line at which to part the paper is detected by producing molybdenum blue on the strip. This is visible only when at least 100 pg of molybdenum is present. With smaller amounts, the leading quarter of the solvent run is assumed to contain group (b) ; alternatively, a known amount of molybdenum is added to the sample solution.The colour of the reduced molyb- denum compound is developed by drying the strip under an infra-red lamp, followed by spraying with a saturated aqueous solution of butyl alcohol and again drying. The strip is then exposed simultaneously to air and the vapour from dilute ammonia. Infra-red radiation or sunlight and a pH of about 4-0 appear to be essential to the production of the colour. Should the colour appear before completion of the treatment, the treatment is discontinued. The strip is torn immediately above the blue spot and the metals are extracted for polarography. Some trouble was experienced in removing the groups quantitatively from the paper before the polarographic determination.The methods tried were (i) ashing, (ii) macerating with cold acid and (iii) digestion with hot acid. A limitation was imposed by the requirements for the final solution. The procedure finally adopted was to boil the section of the paper concerned for 1 minute, first with water, then with saturated aqueous salicylic acid, and then to heat with 2 N sulphuric acid. The last stage had the effect of pulping the paper, which was then filtered through sintered glass, together with added mercurous sulphate (see later) before adding the filtrate to the other extracts. It was also found necessary to cool the solutions before combining them in order to avoid decomposition of molybdenum blue. Chloride was removed from the combined water and salicylic acid extracts before adding the chloride-stripped sulphuric acid leach.Removal of chloride was achieved by shaking with a small amount of solid mercurous sulphate, 1 drop of which was added as a slurry with dilute sulphuric acid; the solution was then filtered through sintered glass, conveniently from a Schwarz - Bergkampf micro-beaker, into the sulphuric acid filtrate. On subsequent polarographic estimation, a mercury wave is produced at 0 volts, and a correction must be made when estimating vanadium or iron. THE EFFECT OF VARIOUS IONS ON THE SEPARATION Most common cations, tin being an exception, do not give waves in the salicylic acid medium and hence do not interfere in these determinations. Tin, however, prevents the development of molybdenum blue in the paper separation and also interferes by interaction with ferric iron.In addition, tin gives a wave that interferes with cadmium, but only if present in excess of the equivalent of any ferric iron that is present. Phosphate ions interfere with the paper-strip separation when present in concentrations394 LEWIS AND GRIFFITHS DETERMINATION OF INORGANIC COMPOUNDS [VOl. 76 above the equivalent of 0.03 M orthophosphoric acid in the sample solution. Phosphate also inhibits the formation of the molybdenum blue compound. Sulphuric acid in the sample solution can be tolerated up to 0.2 M , but interferes with the development of the molybdenum blue colour and slows down the movement of the metals in relation to that of the solvent front on the paper strip. Where phosphate or sulphate, or both, are present, two paper strips are run in parallel.One is then developed with potassium ferrocyanide to find the position of the molybdenum, which shows up as a dull brown stain. The other strip is then assumed to have the molybdenum in a corresponding position and is divided above this to give the two groups for estimation. HALF-WAVE POTENTIALS IN SALICYLIC ACID The half-wave potentials of the elements, determined after the separation and solution These potentials are determined against procedures already described, are given in Table V. an internal mercury anode. TABLE V HALF-WAVE POTENTIALS IN SA:LICYLIC - SULPHURIC ACID Element Ei, volts Group Vanadium . . . . .. .. .. - 0.025 Copper .. .. .. .. * . Uranium . . .. . . . . .. - 0.575 Lead .. -0.375 1 Upper section of .. .. .. .. .. - 0.725 1 ‘lter-paper Titanium . . .. .. .. .. - 1.026 J Iron . . .. .. .. .. .. -0.025 7 Lower section of filter-paper Molybdenum blue . . .. .. .. -0.20 Bismuth . . . . . I .. .. - 0.40 Antimony .. .. .. .. .. - 0.60 Cadmium . . .. .. .. .. - 1.126 REsuL:rs The quantities that can be estimated by the combined paper-strip - polarograph method are limited (a) by the amount of the metal chlorides that will dissolve in 0-25 ml of solution and (b) by the amounts that can be quantitatively removed from the paper and estimated on the polarograph. TABLE VI QUANTITATIVE RESULTS Observed amount A f I Calculated Vanadium, Copper, Uranium, Lead, Titanium, amount, Ilg Pg Pg Pg Pg Pg 995 1000 985 960 960 1000 96 102 97 97 92 100 28 29 29 30 29 30 Calculated Ferric iron, Molybdenum, Bismuth, Antimony, Cadmium, amount, Pg r g Pg r g PLg Pg 965 955 1020 1035 960 1000 94 99 100 104 98 100 27 28 31-5 31 30-5 30 For routine work, preliminary experiments indicate that the use of the method of stan- dard addition for one element can be followed by the use of that element as internal standard for the others.Disproportionate amounts can easily be separated and can be estimated within the usual limits of polarograph:y, i.e., a small wave for a metal can be esti- mated accurately when it occurs before, but not after, a large one. Means of three results are quoted in Table VI: the maximum deviations from the mean were 6 per cent. in the 1OOO-pg range and in the 100-pg range, becoming progressively greater as the amount of the metals decreased.The determinations were carried out by comparison with standard wave heights. Each measurement was made in the presence of a similar concentration of the other nine elements.July, 19511 BY PAPER-STRIP SEPARATION AND POLAROGRAPHY 395 A standard mercury wave height has been subtracted from the values for iron and vanadium. The solubility of lead in sulphuric acid is greatly increased by the prior addition of salicylic acid. The lead wave is always small, and it has been found that the use of a nearly neutral nitrate solution instead of sulphate is advisable where the estimation of lead is important. The original chloride solution of titanium must be freshly prepared to avoid hydrolysis. The tendency for high results to be obtained for bismuth and antimony may be due to molybdic acid regenerated by the ferric iron, and the estimation of this group must be carried out quickly after the metals have been removed from the paper.The polarograph has been shown to be a useful instrument for the final estimation of quantities separated by paper-strip extraction, and conversely the extractions are considered a valuable complement to polarography by extending the scope of the technique. These investigations have been carried out in part for the Ministry of Supply and in part for the Chemistry Research Board, D.S.I.R., and are published by permission of the Director of the Chemical Research Laboratory. REFERENCES 1. Report of the Chemistry Research Board, 1947. 2. 3. 4. 5. Arden, T. V., Burstall, F.H., Davies, G. R., Lewis, J. A., and LirIJcGau, R. P., Nature, 1948, Arden, T. V., Burstall, F. H., and Linstead, R. P., J. Chew. SOC., 1949, S 311. Burstall, F. H., Davies, G. R., Linstead, R. P., and Wells, R. A., Ibid., 1950, 616. Burstall, F. H., Davies, G. R., and Wells, R. A., Disc. Farad. SOC., 1949, No. 7, 179. 162, 691. NOTE-References 3, 4 and 5 are to Parts I, I1 and I11 of this series. CHEMICAL RESEARCH LABORATORY TEDDINGTON, MIDDLESEX DISCUSSION DR. H. LIEBMANN asked whether RF values individually determined were a reliable guide to the possibility of separating a given metal from others, and if the presence of other metals influenced the RF values. MR. LEWIS replied that, while other metals had some effect on the movement of a particular element, the RF value individually determined was a very strong indication of its behaviour in other circumstances.Anions, however, had great effects on RF values. MR. D. G. HIGGS asked whether it was possible to separate very small quantities of metals such as iron from relatively pure metals such as molybdenum. Would i t be possible, for example, to separate 0.01 per cent. of iron from more than 99.5 per cent. pure molybdenum? MR. LEWIS answered that the problem of separating trace amounts (and parts per million could be successfully dealt with) was best solved by the use of upward development on a paper cone. A large amount of sample could be put on the paper around the base and the solvent chosen so that the trace element was concentrated a t the apex.DR. J . H. HAMENCE asked the authors whether from their very wide experience of inorganic chromato- graphy they could give an opinion as to the best technique to apply in developing a chromatograph. Various workers had used different techniques, some with the paper in a vertical and others with it in a horizontal position. MR. LEWIS said that the workers a t the Chemical Research Laboratory had found downward displace- ment the best technique in most circumstances and held that the assistance of gravity was useful. MR. J. HASLAM asked whether, in the determination of the general metal content of foodstuffs after a preliminary wet digestion, i t was likely that addition of salicylic acid and subsequent polarography of the wet digestion product would give a useful general picture of the metal content of the foodstuff. MR. LEWIS replied that the salicylic acid supporting electrolyte would undoubtedly be useful for the estimation of small amounts of lead, antimony or copper in a sulphuric acid digestion product, but that the commonly occurring tin might cause complications. DR. LIEBMANN asked whether tin produced a curve in the salicylic acid medium, and if so, which metals would interfere. MR. LEWIS said that tin did produce an ill-defined wave in salicylic - sulphuric acid a t 1-1 volts, where it interfered with cadmium, but only if present in excess of its equivalent of ferric iron. It interfered with iron by reducing it, and was itself oxidised. MR. N. STRAFFORD asked whether arsenic would give a wave in the determination of poisonous metals in foodstuffs, medicinals, and so on, by polarography. If it did not, then polarography would not provide a universal method for the determination of poisonous metals. MR. LEWIS stated that arsenic did not give a wave in the salicylic medium, but that useful waves had been reported in other media. In a limited experience of this particular element, he had not found a wave suitable for analysis. Which did the authors consider to be the better?