A polarographic method is proposed for determination of small amounts of +6 uranium. An aqueous solution of salicylic acid (1.6 g /l), sulphuric acid (0.4% v/ v) and thymol (0.009 %) is used as supporting electrolyte. This procedure is applicable in the concentration range from 5 to 50 μg of uranium per ml. of the solution used for polarographic measurement. Total amounts of 10 μg of uranium per sample can be determined with a standard error of ± 0.8 μg

M. Branica

O.A. Weber

V. B. Vouk

English

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AR HIV Z A KEM I JU 25 (1953) Note on Polarographic Determination of +6 Uranium V. B. Vouk, M. Branica and 0. A. Weber In9titute f or IviedicaL Research, YugosLav Academy of Sciences a'f!d Arts, · Zagreb, Croatia Received No vember D, 19:;3 A polarographic method is ·proposed for determination of small amounts of +6 uranium. An aqueous solution of salicylic acid (1.6 g/l), sulphuric acid (0.40/o v/v) and thymol (0.0090/o) is used as supporting electrolyte. This procedure is applicable in the concentration range from 5 to 50 µg of uranium per ml. of the solution used for polarographic mea·surement. Total amounts of 10 ,ug of uranium per sample· can be determined with a standard error of ± 0.8 µg. 2.25· Colorimetric and photometric methods for determination of uranium are not sensitive enough for determination of very small amounts of this metal. On the other hand, fluorometric methods in liquid or solid medium are suitable for detection and determination of minute amounts of uranium, but the smallest amounts of impurities interfere by quenching the fluorescence. Polaro-graphic methods are both sensitive and specific enough, and if a suitable sup-porting electrolyte is used several other metal ions can be determined simul-taneously with uranium.1 Supporting electrolyte recommended by Lewis and Griffiths2 seemed to be particularly suitable for this purpose. We tried to make use of this electrolyte in determination of very small amounts of ura-nium, but the results were not satisfactory. Therefore, an investigation was undertaken to find out the optimum concentrations of various constituents of this electrolyte, and to select the most suitable suppressor of polarographic maxima. APPARATUS AND REAGEN'l'S A Cambridge Polarograph, Pen Recording Type, and a standard polarographic cell of the same firm with saturated calomel electrode were used. The cell was kept in a constant temperature water bath at 25.0 ± 0.1°C. Dissolved oxygen was displaced by bubbling hydrogen through the solution for 10 minutes p ·rior to the polarographic determination. The hydrogen was purified by passing it through a sat.urated solution of pyrogallol in sodium hydroxide. The capillary had m and t values of 1.42 mg of mercury per second (at 0 V) and 3.08 seconds (at 0.2 V), respectively, at 54 cm mercury column height in the sup-porting electrolyte used in the analysis. The wave heights were measured according to the point method, i. e. the in-crement in current between the extrapolation lines for the residual current -and the diffusion current was measured at the half-wave potential. Uranyl-nitrate, reagent grade (Schering-Kahlbaum, A. G. Berlin). Standard solution: A solution containing approximately 1 mg of uranium per ml. was first prepared and the exact concentration subsequently determined gravimetrically.3 The standard solution was kept in darkness. . Salicylic acid, .reagent grade (Erba, Milano). Sulphuric acid, reagent grade (»filr forensi sche Zwecke«, Merck, Darmstadt). Perchloric acid, reagent grade, 71 O/o (Mallincrodt, Saint Louis) Thymol, recrystallised. 226 V. B. VOUK, M. BRANICA AND 0. A. WEBER THE COMPOSITION OF THE SUP PORTING ELECTROLYTE The supporting electrolyte used by Lewis and Griffiths2 consisted of a half-saturated solution of salicylic acid which was at the same .time saturated with alizarin and contained 10 per cent v/v of sulphuric acid. The salicylic acid is a complexing agent facilitating simultaneous determination of metal ions having similar half-wave potentials; sulphuric acid increases the acidity (pH value of the supporting electrolyte should be ~ 2) and the conductivity of the solution. When we tried to use this electrolyte for determination of very small amounts of uranium, and performed the polarographic measurements with the maximum sensitivity of the recording equipment (which is of the order of 10-9 A per mm.), we found that the relation between the wave height and the concentration of uranium was not linear. Further experiments showed that the lack of linearity was due to the deformation of the current-volt(age curve of the supporting electrolyte. We decided, therefore, to investigate the influence of the concentration of salicylic acid and sulphuric acid on the form of the residual current of the supporting electrolyte. Experiments showed that an aqueous solution containing 1.6 g/l of salicylic acid and 0.40/o v/v of sulphu-r ic acid gave a residual current which increased linearly with the applied potential in the desired voltage range. This composition of the supporting electrolyte satisfied also the condition2 that the pH value should be in the neighbourhood of 2. By applying the compensation arrangement it was possible to obtain a residual current curve that was almost parallel to the abscissa I 2 I - -, I L ~/_L----------~- ': . ----' __j . 1 0 0 -100 - .30 0 -J. oo - S o o mV vs. S. C.E Fig. 1. Curve 1: Residual cur ren t of the support ing <ilectrolyle (l.60 g· salicylic aci d , 4 m l sulphur ic, acid [d ~ 1.84] and 0.090 g th ymol pe r liter of soiut ion) . Curve 2: Curren t-vo'ita.ge cu rve of +G ura nium (39.4 µg /ml.) in the s a.m e supporting elec t rolyte. Poten t ia ls arc given with r espect 1o the saturated calomel electrode ; •s<insi tivity of the recorder: 7.90 X 10-0 A/mm.; da mping 6; rat e of recording : 3.li'i mm. per m in. (fi'ig. 1) . The necessary counter current amounted to -0.12 µ A per volt. Fi-gure 1 shows also a typical polarographic wave of + 6 uranium in this electro-lyte and the position of the half-wave potential with respect to the saturated calomel electrode. Thymol in a concentration of 0.009°/o was chosen as suppressor of polaro-graphic maxima, because it had no influence on the height of the waves in a w ide concentration r ange. POLAROGRAPHIC DETERMINATION OF +6 URANIUM ·221· PROCEDURE The solution to be analysed (containing at least 10 µg of uranium) is eva-porated to dryness with .addition of 1 ml. of perchloric acid. (Small amounts of perchloric acid which may remain after evaporation have no influence on the height and form of the polarographic wave.) The residue, containing + 6 uranium, is dissolved in the supporting electrolyte. The electrolyte shc;mld be ;added in small amounts and the mixture gently heated. When a clear solut_i_on is obtained, it is transferred quantitatively to a volumetric flask and, after addition of t he necessary amount of thymol solution, diluted fo the mark with supporting electrolyte. The concentration of uranium in the solution prepared for polarographic measurement should be between 5 a nd 50 µg/ml. If neces-$ary, the volume of this solution can be limited to 2 ml. It is essential that the dissolved oxygen is completely removed from the solution prior to the polarographic measurement as the half-wave potentials of oxygen and + 6 uranium have very similar values. Besides, even traces of oxygen 1tend to distort the residual current curve of the supporting electro-lyte .. With regard to the interfering metal ions our experiences are in agre-ement with data given by other authors.2 ANALYSIS OF THE CALIBRATION CURVE As it is well known that the diffusion current is proportional to the concen-t ration of the electroreducible or electrooxidizable substances4• 5• 6, a linear regression analysis was performed for the calibration data presented in Table 1. TABLE 1 Concentration I wave height Mean wave height Sample No. c h* h (µg/ml) j (mm) ·(mm) -. I 1 1.9'1 I 4.3 4.4 2 I 4.5 . .. --·:··+· - --···· -··· 3 3.94 lli.6 11.8 4 12.0' . 5 5.92 d .8 i 17.3 6 I 16.8 7 9.86 .. 27.8 . .. . . . . .. 27.8 8 27.8 9 13.8 40.0 40.6 10 41.2 11 13~8 40,8 41.1 12 41.4 . 13 19.7 60.0 I 59.4 14 58.8 I 15 29.6 88.5 89.4 16 90.0 17 39.4 116.4 116.7 18 I I 117,0 19 I 49.3 149.4 I 149.7 20 150.0 I I ·• h-values for sam:ples N~. 1· to 10 were obtained u.sitj_g maximum sensitivity oi the recorder; the sensitivity was reduced to 1/s for samples No. 11 to 20. 228 V. B. VOUK, M. BRANICA AND 0 . A. WEBER subject to the condition that the straight line passes through the origin. The following regression equations were obtained: h = 3.003 c and c = 0.333 h, where h is the height of the polarographic wave in mm. and C is the concen·-ti ation of uranium in the solution prepared for polarographic measurement. The standard errors of the slopes in the above equations were ± 0.015 and ~ 2 ~ ~ URANIUM CONCENTRATION -?<glml/ Fig. 2. Regression of C upon h a.nd 95' /o-confidence limits for predictions of C from measured values of h . Uranium concentration in 1>g/ml of the solution used in polarographic measurements. Wave-heights in mm. and A X JO-•. The plotted points a re mean values of h of two repeated mcasuremen ts. ± 0.002, respectively. The regression of C upon h and the 950/o-confidence limit,;f (i. e the limits of .. error---for P-= 0.05) for predictions. of C from the measured value of h (h is the mean of two repeat determinations) are shown in Fig. 2. , v / \ v v ~ \ ~ -~ ~ l 1() lO 30 "° '° URANIUM CONCENTRATION ~l/ Fig. 3. Standard error (1) and r elative s ta ndard enoi· (2) of concentration v. uranium concentration in the solution used for polarographic measu.remcnts. h-values us~d for predicting concent!l'ation are arithmetic means of two repeaited meaisurements. ·. POLAROGRAPHIC DETERMINATION OF + 6 URANIUM 229 The standard errors and the relative standard errors of the concentration are given in Fig. 3 for .the concentration range from 0-50 µg of uranium per ml. of the . solution used for polarographic measurements. This analysis of the calibration curve shows that the proposed method is reliable and precise even when very small amounts of uranium are involved. Thus, for instance, if the final volume of the solution is 2 ml., and the concen-tration of uranium is 5 µ g/ml., the total amount of 10 µ g uranium per sample can be determined with a standard error of ± 0.8 µg. REFERENCES 1. N. H. Furman and J . I. Watters, Electrometric methods, Chapter 25, p. 596 -610, in C. J . Rodden's Analytical Chemistry of the Manhattan Project: New York 1950. 2. J . A. Lewis and J.M. Griff i ths, Analyst 76 (1951) 277. 3. A. A. Sm a I e s and H. N. W i 1 s on, Report B. R. - 150, Feb. 22, 1943, p. 47, in C. J . Rodden's Analytical Chemistry of the Manhattan Project. New York 1950. 4. D. I 1 k o v i c, Collection Czechoslov. Chem. Communs. 6 (1934) 498. 5. J. J. Ling an e and B. A. Loveridge, J. Am. Chem. Soc. 72 (1950) 438. 6. H. St re h 1 ow and M. von S tack e lb erg, Z. Elektrochem. 54 (1950) 51. 7. See, for instance,, 0. L. Davies, Statistical Methods in Research and Producti on, London 1949, ox 0. A. Weber, K. Voloder and V. B. Vouk, Arh. h i g. rada 3 (1952) 296. IZVOD Prilog polarografskom odreclivanju + 6 uraoa V. B . Vouk, M. Branica i 0. A. Weber Opisana je polarografska metoda za odredivanje malih koliCina urana u osnov-nom elektrolitu, koji sadrfava sumpornu j salicilnu kiselinu. Utvrden je najpovoljniji odnos tih komponenata u osnovnom elektrolitu, a kao kriterij je uzet najpravilniji tok rezidualne struje. Za potiskivanje polarografskih maksimuma upotrebljen je timol. Rezultati pokazuju, da je u koncentracijskom podrucju do 50 µg/ml predlozena metoda pouzdana, osjetljiva i precizna. Vrsi li se odredivanje s dva paralelna uzorka, standardna pogreska odredivanja koncentracije iznosi manje od 0.5 µg/ml. Ukupne kolicine od 10 µg mogu se odredi·ti s pogreskom od ± 0.8 µg. INSTITUT ZA MEDIOINSKA ISTRA:ZIVANJA JUGOSLA VENSKE AKADEMIJE ZNANOSTI 1 UMJETNOSTI, ZAGREB P r imljeno 9. novembra 1953. 

Note on Polarographic Determination of +6 Uranium

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