as Table I shows, a hundredfold excess of a foreign metal does not cause a large error. Silver mill give a precipitate of silver chloride which must be removed since the solid will make the gold extraction difficult if not impossible. The small amount of silver ion remaining does not interfere with the resultant determination of gold. Tellurium is frequently recommended as a reagent to concentrate gold, and its possible interference is important; however, it was found that a tenfold excess of tellurium present as the chloride complex was not extracted and does not interfere with the gold determination. The serious interference of ferric ion can be completely inasked by the addition of an excess of fluoride ion. The gold may be recovered quantitatively in the presence of a large excess of iron, and the large excess of fluoride ion required does not interfere with the extraction. Osmium interferes as the tetraoxide is extracted into the chloroform and absorbs strongly in thc near-ultraviolet. The interference can be avoided by first extracting with carbon tetrachloride before making the gold determination. In the normal snalytical pro-
cedure the interference of osmium will not occur as it will be lost as the volatile tetraoxide during the preparation of the solution. Rhodium and ruthenium do not interfere when present in a tenfold excess. Palladium , platinum, and iridium interfere a t this level, but 1%hen the concentrations are reduced to the same concentration level as the gold the interferences are small, and when the concentrations are reduced to one tenth that of the gold the interferences are reduced to less than 1%. All of the interference studies were made on the oxidation states of the metals obtained when the chloride solutions were oxidized with concentrated nitric acid. Determination of Gold. Introduce a suitable aliquot of the solution to be analyzed into a small volumetric flask. Add hydrochloric acid so t h a t the final percentage of acid is between 0.3 and 0.5M and dilute. Place a suitable aliquot in a small separatory funnel. then remove a second aliquot and add a n excess of ammonium thiocyanate. Add 1M ammonium fluoride to this second aliquot until a n y red color due to a ferric thiocyanate complex is discharged. Then introduce
this amount plus 1 ml. in addition of the ammonium fluoride into the separatory funnel containing the solution to be analyzed. Solid ammonium fluoride may be used if desired. The solution is then extracted in the manner described above. The absorbance of the solution is measured at 323 mp using a chloroform extraction of the reagents as the reference solution, and the concentration of the gold is calculated. LITERATURE CITED
(1) Ayres, G. H., ANAL.CHEM.21, G52
(1949).
(2) Beamish, F. E., Ibid., 33, 1059 (1961). (3) Lenher, V., Kao, C. H., J . Phys. Chena. 30. 126 (1926). \----,-
(4)-ki/LcBiyde,W. -4. E., Yoe, H. €I.,ASAL. CHEX.20, 1094 (1948). ( 5 ) Rlylius, F., 2. anorg. C h e m 70, 203 (1911): Mvlius. F.. Huttner. C.. Ber. 44, 1315 (1611). ’ (6) Ringbom, .4.,2. anal. Cizem. 115, 332 (1939). ( 7 ) Sandell, E. B., A i x . 4 ~ . CHEJI. 20, 253 (1918). (8) Sandell, E. B., “Coloriinetric Determination of Traces of Rlctals,” Interscience, New York, 1950. RECEIVEDfor review April 24, 1‘361. Accepted August 2, 1961. Presented in part Southwest Regional Meeting, ACS, Oklahoma City, Okla., Decemher 1960. Work supported by the National Science Foundation.
Ultraviolet Determination of Uranium in Concentrated Hydrochloric Acid CLARENCE M. CALLAHAN
U. S. Naval Radiological Defense Laboratory, Sun Francisco 24, Calif.
b A spectrophotometric method for determining microgram quantities of uranium in concentrated hydrochloric acid is given. The method is based on the absorbance of a chloride complex of uranium at 246 mp. Beer’s law is obeyed in the uranium concentration range of less than 1 to 60 p.p.m. Interfering ions are removed b y one or more extractions of the uranium into ethyl acetate using aluminum nitrate as a salting agent. The extent of interference of 55 ionic species after a single extraction i s tabulated. The absorption spectra of uranium(V1) in concentrated hydrochloric, sulfuric, perchloric, and acetic acids are given.
D
a systematic investigation of the spectra of the various oxidation states of uranium i t was observed that trace amounts of uranium(V1) in concentrated hydrochloric acid showed a strong broad absorption URING
1660
ANALYTICAL CHEMISTRY
peak a t 246 mk. The peak presumably was due to a weak chloride complex of the uranyl ion since the position and intensity of the absorption peak was found to be a function of the hydrochloric acid concentration at a constant uranium concentration. However, Beer’s law was obeyed and the position of the peak mas stabilized if the hydrochloric acid concentration remained constant and the uranium concentration was varied. The molar absorptivity of this uranium complex in concentrated hydrochloric acid was 5056. The spectrophotometric behavior of uranium in hydrochloric acid media closely paralleled that of bismuth, lead, and thallium in the same milieu, as reported by Merritt, Hershmson, and Rogers (12). 4 t the time the experimental work for this paper was completed, methods available for the trace determination of uranium !\ere limited. The fluorimetric method was by far the most sensitive,
the detection level being froni lo-’ to gram of uranium ( I , I?, f8). However, the precision was low and a special type of fluorimeter was required which was not readily adapted to other fluorimetric procedures. Electroanalytical methods such as polsrography (9) and coulometry wcre not applicable without recourse to special techniques. Colorimetric met hods were uniformly insensitive (10), the usual concentration range for most procedures being from about 50 to 2000 p.11.m. of uranium. I n addition, none of the reagents used were specific for uranium and usually suffer in one way or another from such effects as slow rate of color development, color instability, lirecipitate formation necessitating miaed solvent systems, volatilization of organic solvents, critical control of ]”, rigid adherence to amount and order of addition of color developing reagents, etc. Since then a number of sensitive colorimetric methods for detcriiiiiiing
uranium have been published (2, 6, 7 , 10, 11, 15, 16) which eliminate many, and in one case nearly all (IO), of the objectionable features enumerated above. This paper describes a simple spectrophotometric method for determining uranium utilizing the commonest of laboratory reagents. I t is based on the 246-mp absorption peak of uranium (VI) in concentrated hydrochloric acid. Solutions containing from 4 to 40 p.p.ni. (absorbance readings between 0.1 and l ) of uranium(V1) represent the range of minimum error (3).
pared from the stock solution as required. The hydrochloric acid, ethyl acetate, and aluminum nitrate were analytical reagent grade chemicals. These chemicals were further checked for freedom from trace uranium contamination by the fused sodium fluoride fluorimetric method (4) using the Galvanek-Morrison fluorimeter. The salts used to prepare solutions of diverse ions are listed in Table I. These were reagent grade chemicals except that the purity was not specified for the arsenic, cesium, and zirconium sources, which were obtained from A. D. Mackay Co. of Xew York. I n most cases the diverse ion stock solutions contained 10 mg. of ion per ml.
DISCUSSION
The absorption spectra of the actinide and lanthanide elements in aqueous solutions are characterized by narrow, sharply defined bands in the ultraviolet, visible, and the near-infrared portions of the spectrum (8, I S , 25). These bands are in marked contrast t o the broad bands usually encountered in absorption work, and use has been made of them in the analytical determination of these elements, particularly in differentiating those of the rare earth series (14, 19). Unfortunately, their low molar absorptivities restrict their use to fairly concentrated solutions. The origin of these bands is
EXPERIMENTAL
Apparatus. All spectrophotometric measurements were made with a Beckman spectrophotometer, Model DU, using matched fused silica cells with a n optical p a t h of 0.998 cm. T h e wave length scale was checked against t h e 5461- a n d 3650-A. emission lines from a mercury vapor lamp. A tungsten lamp was employed for measurements above 3500 A. a n d a hydrogen discharge lamp for readings at shorter wave lengths. After each use t h e silica cells were thoroughly cleaned with a mild sulfuric acid solution, followed by several distilled water rinses. They u-ere then dipped in acetone several times, drained, and allowed to air-dry. Before reuse they were polished with lens paper. The sample cell and the blank cell were never interchanged. In all measurements the sensitivity control knob of the spectrophotometer initially was set a t two revolutions from the extreme clockwise position. Then the indicator needle was roughly balanced by adjusting the slit width. Final balancing was accomplished with the sensitivity control knob which, however, never was allowed to vary by more than one half revolution in either direction from the position indicated above. Thus in scanning the spectrum the slit width was continually varied. At 246 mp the slit width was set a t 0.46 mm . A Galvanek-Morrison fluorimeter obtained from the Jerrell-Ash Co. was used to check the reagents for trace uranium contamination. -4Beckman p H meter, Model G, was used for all p H measurements. Reagents. A standard uranium stock solution was prepared by igniting 4.22 grams of reagent grade UO2(NOs)2.6H2Oin a platinum crucible a t from 900' t o 1000" C. until constant weight was obtained. T h e amount of uranium present was 2.0032 grams calculated as U30~. The oxide was then dissolved in hydrochloric acid and diluted to 1 liter. A final check of the concentration was made by precipitating the uranium contained in several 25-ml. aliquots of this solution with carbonate-free ammonium hydroxide ( 6 ) . The results corresponded gram of uranium per to 2.002 X ml. >fore dilute solutions were pre-
Table I. Effect of Various Ions on Extraction and Determination of Uranium
Ion Ag
+
AS04-3 Ba+2 Be + 2 BrBr03Ca +2 Cd +z Ce +a
ce c1-
+4
co +z
Cr + 3 Crz07-2 c u +2 CuJZ
cs
FFe Fe +a Fe +3
%Iz I-
103-
K+ La + 3 Li + Mg+2 Mn +2 Mn04MOO,-' MOOi-' Na+ NH4+ Ni +2 NOaPb +* Sn +z
Added As AgN03 As in HN03
BaCl2 Be(NOd2
XaBr KBr03 CaClZ Cd(N0dz Ce(N 0 3 ) 3 ( NH4)zCe(KO3) 3N HCI CO(NOd 2 Cr(NOda KzCrz07 CU(CH,COO)z Cu in HXOa CSCl NaF Fe in HC1 FeCh Fe(NOJ HgNOa HdCzH30z)z KI KIOs KC1 La(NOd8 LiCl Mg(NOa)e MnCOa in HC1 KMn04 N~MoO~ NazMoOc NaCl NHiNO3 Ni(N03), 5N Hx03 Pb( KO312 2.2N
SnCl? in HC1
sod-'
2N
Th +4 Ti + 4 T1+
Th(NOd4
Sr +2
vo wo,
+2
-2
Zn +2 Zr +4
zr+4
Acetic acid Citrate Oxalate Tartrate
SrC12
TiO, in H2SOI T1 NO3 VOSO, Na2W04 Zn(CzH30z)z %(OH)( in HC1 ZrOCll
Ion Concn. P.P.M. 2500 2500 1250 2500 500 2500 2500 1250 2500 3500 2500
2020 2500 900 2500 250 2500 2500 140 2500 2500 2500 2500 2500 2500 2500 2500 2500 1250 2500 595 328 2500 2500 2500 2500
19.7 59.0 19.6 19.4 25.1 21.8 17.4 19.6 19.6 19.9 m
37.6 19 7 23.9 60.0 23.4 20.1 19.4 20.0 20.4 34.5 43.9 21.8 m
-0.3 39.0 -0.4 -0.6 5.1 1.8 -2.6 -0.4 -0.4 -0.1 m
17 6 -0.3 3.9 40.0 3.4 0.1 -0.6 0.0 0.4 14.5 23..9 1.8 m
21.8 -4.0 0.1
1250 5000 240 2500 3270 2500 2500 137 2500
41.8 16.0 20.1 19.6 20.8 19.6 40.4 57.6 20.1 41.3 20.1 19.1 20.1 21.0 19.0 14.9 38.6 20.3 19.6 20.4 20.0 28.4 19.9 21.1 19.9 39.7 34.6
2500 2500 2500
24.9 19.2 21.5 19.6
4.9 -0.8 1.5
2500 2500
4N Acetic acid
Sodium citrate Oxalic acid Tartaric acid
Uranium Determination P.P.M. 1st extraction 2nd extraction ReRecovered Error covered Error
19.7
-0.3
23.4 20.5
3.4 0.5
20.9
0.Y
25.1
5.1
-0.4 0.8 -0.4 20.4 37.6 0.1 21.3 0.1 -0.9 0.1 1.0 -1.0 -5.1 18.6 0.3 -0.4 0.4 0 8.4 -0.1 1.1 -0.1
19.7 14.6
-0.4
VOL. 33, NO. 12, NOVEMBER 1961
166
b Figure 2, Absorbance of uranium(V1) in acetic, SUIfuric, and perchloric acids I
;-
Figure 1. Absorbance of uranium (VI) in hydrochloric acid
now generally attributed to forbidden intra-shell transitions of the 4fn electrons (26) in the lanthanons and of the 5 f n electrons in the actinide elements (8, 21). The forbidden nature of these transitions accounts for the low molar absorptivity observed. In spite of the potential usefulness of the spectra of uranium in studying the solution chemistry of the various oxidation states of this element, there still are no definitive sets of absorption spectra in the literature (8). Probably the best spectra available are those of uranium(III), (IV), and (VI) in 1N hydrochloric acid extending from 400 to 1000 n ~ p(17) and those of uranium (111) and (IV) in perchloric acid extending from 200 to 1400 mp (22). A comparison of these spectra shows a general decrease both of absorption intensity and of the number of bands as the oxidation state increases from (111) to (VI). I n fact, the uranium(V1) spectra appear to be completely transparent except for a faint peak at about 420 m p (8). The molar absorptivity of this peak is about 8 or 10. The sharp rise in absorption below 350 mp appears to be the cut-off point of the solution. Actually, it was found to be a much more sensitive absorption region for uranium. Figure 1 shows the absorption curves of uranium in hydrochloric acid between 220 and 350 mp. Curves A , B , C, and E are the curves of a range of concentration of uranium(V1) in concentrated hydrochloric acid. The maxima for all are at 246 mp. I n this media Beer's law is obeyed over the concentration range of less than 1 t o over 60 p.p.m. The effect of decreasing the hydrochloric acid concentration on the intensity and position of the absorption maxima while holding the uranium concentration constant is 1662
ANALYTICAL CHEMISTRY
shown by comparing curves B, D, and F in Figure 1. The decrease in the absorbance and the shift of the peak to shorter wave lengths indicate that the absorbing species probably is a n extremely weak anionic complex of the uranyl ion such as U02Cls- or UO2Cl~-*. S o positive information concerning the actual composition of the complex could be obtained either by Job's method of continuous variation (23) or by the method of Yoe and Jones (24), since the mole ratio of the chloride ion to uranium is about 150,000:1. The absorbance curves of 20 p.p.m. of uranium in concentrated acetic, sulfuric, and perchloric acids are given in Figure 2. The maxima of these curves were not obtained. The absorption is least in perchloric acid as would be expected from the known weak tendency of the perchlorate ion to complex. The cutoff wave length for acetic acid solutions is 250 mp. Because of the strong absorption by nitric acid in the ultraviolet a corresponding curve of uranium in nitric acid could not be obtained. The spectra of 40 p.p.m. of uranium (VI) in the various acids discussed were scanned from 220 to 1000 mp, Aside from the peak already mentioned, there was no significant absorption elsewhere at this concentration level. Measurements were made at 4 m p intervals from 220 to 300 mp, at 5mp intervals from 300 to 400 mp, and a t correspondingly larger intervals from 400 up to 1000 mp. The uranium solutions used in obtaining these spectra were prepared by evaporating to dryness suitable aliquots of uranium stock solution followed by dissolution and dilution of the residue to the requisite volume with the desired acid. Effect of Diverse Ions. The spectral region around 246 mp has been shown to be a general absorption region of considerable sensitivity for many
inorganic materials in hydrochloric acid and other inorganic solvents (1). Therefore, any method for determining uranium spectrophotometrically in hydrochloric acid is nonspecific and a separation of possible interfering materials is indicated. Ion exchange and mercury cathode deposition are excellent means for removing interfering substances from uranium solutions. However, favorable results have been obtained in this laboratory in determining uranium fluorimetrically by extracting uranium nitrate into ethyl acetate using aluminum nitrate as a salting agent (4). Besides requiring little time and the simplest of equipment, the particular advantage of extraction is that the uranium is quickly obtained as a dry residue either by evaporation or direct combustion of the ethyl acetate. A stripping extraction following the principal extraction ensures a quantitative separation of the uranium. To determine the effectiveness of the ethyl acetate extraction in separating uranium from foreign materials, as well as to obtain a relative measure of the extent to which such materials interfere in absorbance measurements a t 246 mp, known aliquots of diverse ions and uranium were mixed, the uranium was extracted, and the absorbance determined. The difference between the absorbance expected and the absorbance actually measured indicated the effectiveness of the procedure for purifying the uranium. I n all cases the diverse ions were tested in individual experiments except that the magnesium, calcium, and barium ions were combined in one extraction. Potassium and sodium were also extracted together. The extraction procedure and subsequent preparation for spectrophotometric measurement were as follows: A 5-ml. aliquot of stock solution containing 200 p.p.m. of uranium was pipetted into a 60-ml. separatory funnel.
o -l
90
I-
/
h
i
2
O ?
I
I C 4
I
I Ob
I
I C 8
I
I
,
C
jQ4N15 O i A I IN0313 9 r i z O > E R N LLILITEC.
Figure 3. Dependence of uranium extraction on amount of salting agent
To this was added 5 ml. of the diverse ion solution and 10 ml. of 2N nitric acid, followed by 20 grams of Al(?JO&. 9H20. Then 10 ml. of ethyl acetate was added and the solution was agitated for 1 minute. The phases were separated, the aqueous phase being returned to the separatory funnel. The beaker that had contained the aqueous phase was rinsed with 5 ml. of fresh ethyl acetate and this was also added to the separatory funnel. The contents of the funnel were agitated and the separation made to ensure the removal of the last traces of uranium. The aqueous phase was discarded and the ethyl acetate portions were combined. The ethyl acetate was removed by evaporation on the hot plate. After cooling, 5 ml. of concentrated perchloric acid was added to the residue and fumed to dryness. The residue was transferred and made up to volume in a 50-ml. volumetric flask by using a number of portions of concentrated hydrochloric acid. The absorbance a t 246 nip was then determined using a hydrochloric acid blank. In a number of cases where interference was severe a further purification of the uranium was carried out by dissolving and transferring to a separatory funnel the residue left after the ethyl acetate evaporation. Four 5-ml. portions of 1-Y nitric acid solution were used for the transferral, the solution of the residue being hastened by gentle warming. Then 20 grams of aluminum nitrate and 10 nil. of ethyl acetate were added and the extraction was carried out as indicated above. The results for the extraction of 55 substances are listed in Table I. The third column gives the concentration of diverse ion in the solution extracted and the fourth column indicates the amount of uranium present as determined from a calibration curve. In most cases the weight ratio of diverse ion to uranium is 125. The positive or negative error of the value in column four from the known concentration of 20 p.p.m. of uranium is shown in the fifth column, and is an indication of the extent of interference of the ion. The
results show that A s O ~ - ~Ce+', , Cr20T-2, Fei2, Fe+3? H g +Z J I- J Mn+*, MnOa-a, MOO^-^, Sn+2, T1+, Zrf4, and hydrochloric acid cause very serious positive errors. The improvement obtainable with a second extraction is given in columns six and seven for several interfering cations and anions. The results show that multiple extractions, while cumbersome, is a feasible method for eliminating positive interference. A more detailed multiple extraction study was made on the strongly interfering mercury(I1) to determine the maximum number of cycles necessary for removal. Table I1 shows the results obtained using up to four extractions. In this case acceptable results were obtained with three extractions. The hydrochloric acid interference is due either to hydrogen ion or concentration effects, as reasonable amounts of chloride ion in a number of the diverse ion solutions are not harmful. Therefore hydrochloric acid should be avoided as a solvent for uranium samples or if unavoidably present should be removed by evaporation. When present as the anion of a salt the halogens show decreasing positive interference in the order I - > Br- > C1- > F- the fluoride ion showing little or no interference. The lack of fluoride interference probably is due t o its complex formation with aluminum. Thorium does not interfere. The nitrate ion adsorbs strongly in this region, but is destroyed by the perchloric acid fuming. Aluminum causes no interference. A number of ions were reponsible for negative interference, although in no case was this type of interference as marked as the positive type. Effects of pH, Salting Agent, Time, and Temperature. The p H of the solution during extraction is not critical except t h a t the solution should be acidic. Extraction is as complete from solutions 5.1- in nitric acid as from solutions adjusted t o p H 5 . At p H 10 extraction t m d s t o be incomplete. High hydrochloric and acetic acid concentrations should be avoided. The completeness of uranium extraction is critically dependent on the amount of aluminum nitrate present. This is illustrated in Figure 3, where the completeness of extraction is plotted against the amount of aluminum nitrate present in the solution. The amount recovered represents the combined recovery of a principal and a stripping extraction. About 98% of the uranium is extracted by the principal extraction and practically a quantitative recovery is ensured by the additional stripping extraction. One gram of aluminum nitrate per milliliter of solution represents a safe working concentration of salting agent.
Table 11. Removal of Interfering Mercury(1i) Ion (concn. 2500 p.p,m.) b y Multiple Extraction, Showing Reproducibility of Data
U
U Determined After
concn,Extraction Cycles, P.P.M. P.P.M. l 20.0 35.2 20.0 m 20.0 20.0 20.0
2
5.0 5.0
3
4
D~viation
5.5
0.28 20.11 0.28 20.3, 0.48 0.22 19 6},3 20.1, 0.28 19.5 0.32 19.41 0.42 19.5) 0.32 Std. Dev. -0.36
20.0 20.0
20.1)
20.0
20.0 20.0
20.0
20.0 20.0
Mean 19.82.
The chloride complex is developed instantaneously and readings made on a sample over a period of 3 months showed no change in absorbance. Temperature fluctuations between 20" and 30" C. are not critical. No difference in the absorbance was observed in measurements made with the spectrophotometer thermostated a t 30" C. and those made a t ambient temperatures. Sensitivity. Because the absoyption occurs in the ultraviolet region. the method given by Sandell (20) for reporting sensitivity a t an absorbance reading of 0.001 unit is used. The absorbance value of 1 p.p.m. of uranium (0.023) gives a sensitivity of 0.043 p.p.m. of uranium. A more realistic value for readings reproducible from 0.002 to 0.003 absorbance unit would be a sensitivity of 0.10 p.p.m. of uranium, or since the cell had a cross section of 1 sq. em., the sensitivity may be expressed as 0.10 pg. of uranium per sq. em. RECOMMENDED PROCEDURE
Select the sample weight so that by the judicious choice of a volumetric flask the final uranium concentration in hydrochloric acid is between 4 and 40 p.p.m. This represents the range of minimum error, Le., the range between absorbance readings of 1 and about 0.1 (3). For solutions more dilute than 4 p.p.m., use 2- or &em. cells. Dissolve the sample in as small a volume of nitric acid as is possible and transfer it to a separatory funnel. Add 1 gram of aluminum nitrate for each milliliter of solution and then 10 ml. of ethyl acetate. Agitate until the salt is dissolved and then 1 minute longer. Separate the layers into 150-ml. beakers and return the aqueous phase to the separatory funnel. Rinse the beaker with 5 ml. of fresh ethyl acetate and VOL. 33, NO. 12, NOVEMBER 1961
1663
add this to the separatory funnel. Agitate and discard the aqueous phase. Combine the acetate fractions and remove the ethyl acetate by evaporating on an asbestos-covered hot plate. If the presence of one or more interfering ions is known or suspected, and additional extractions (see Table I) are necessary, dissolve the uranium residue left from the acetate evaporation with four 5-ml. portions of 1N nitric acid and transfer to a 60-ml. separatory funnel. Add 20 grams of aluminum nitrate and 10 ml. of ethyl acetate and extract as described above, repeating the stripping extraction. Evaporate the ethyl acetate and rrcycle again if another extraction is indicated. After the final extraction and evaporation of ethyl acetate, add 5 ml. of 70% perchloric acid to the uranium residue and fume to dryness. When cool, dissolve and transfer the residue to a suitable volunietric flask with small portions of concentrated hydrochloric acid making the solution to volume with additional hydrochloric acid. Do not heat to dissolve the residue. The absorbance of the solution is mrasured a t 246 mp against a concentrated hydrochloric acid blank. Remove the cell holder promptly after each measurement to prevent corrosion of the cell compartment. RELIABILITY
The success of this procedure depends on the isolation of uranium in a
relatively pure form. For optimum results considerable care must be exercised in the extraction process. Reliability data for the recommended procedure are given in Table 11. The data show the effects of u p to four extraction cycles in removing the strongly interfering mercury(I1) ion. Acceptable results are obtained after three extractions. The standard deviation is less than 2% at the 20-p.p.m. level. LITERATURE CITED
(1) Buck, R. P., Singhadeja, S., Rogers, L. B., ANAL.CHEM.26, 1240 (1954). (2) Cheng, K. L., Ibid., 30, 1027 (1958). (3) Delahay, P., “Instrumental Analysis,” pp. 207-8, Macmillan, New York, 1957. (4) Grimaldi, F. S., May, I., Fletcher,
M. H., Titcomb, J., “Collected Papers on Methods of Analysis for Uranium and Thorium,” Geological Survey Bulletin 1006, pp. 125-31, U. S. Government Printing Offire, Washington, D. C.,
1954. (5) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A,, Huffman, J. I., “Applied Inorganic Analysis,” pp. 46&71, Wiley, New York, 1955. (6) Holcomb, H. P., Yoe, J. H., ANAL. CHEM.32, 612 (1960). (7) Horton, C. A., White, J. C., Ibid., 30, 1779 (1958). (8) Katz, J. J., Seaborg, G. T., “The Chemistry of the Actinide Elements,” pp. 171-203, Methuen, London, 1957. (9) Legge, D. I., ANAL.CHEM.26, 1617 (1954). (10) Maeck, W. J., Booman, G. L.,
Elliott, M. C., Rein, J. E., Zbid., 31,
1130 (1959). (11) Meloan, C. E., Holkeboer, P., Brandt, W. W., Ibid., 32, 791 (1960). (12) Merritt. C.. Hershenson. H. M..’ ’ Rogers, L.’B., Zbid., 25, 572 (1953). (13) Moeller, T., “Inorganic Chemistry,” pp. 598-901, Wiley, New York, 1952. (14) Moeller, T., Brantley, J. C., ANAL. CHEM.22, 433 (1950). (15) Motajima, K., Yoshida, H., Iyowa, K., Zbid., 32, 1083 (1960). (16) Paige, B. E., Elliott, M. C., Rein, J. E., Ibid., 29, 1029 (1957). (17) Rodden, C. J., editor, “Analytical
Chemistry of Manhattan Project,” pp. 122-35, McGraw-Hill, New York, 1950. (18) Rodden, C. J., ANAL. CHEM. 25,
1598 (1953). (19) Rodden, C. J., J. Research Natl. Bur. Standards 26. 557 (1941): , , 28. , 265 (1942). (20) Sandell, E. B., “Colorimetric,, Determination of Traces of Metals, 3rd ed., pp. 83, 900, Interscience, New York, 1959. (21) Seaborg, G. T., Nucleonics 5 (No. 5), 16 (1949). (22) Stewart, D. C., U . S. Atomic Energy Comm. Rept. ANL-4812, (February 1952). (23) Vosburgh, W. C., Cooper, G. R., J . Am. Chem. SOC.63, 437 (1941). (24) Yoe, J. H., Jones, A. L., IND.ENQ. CHEM.,ANAL.ED. 16, 111 (1944). (25) Yost, D. M., Russell, H., Garner, C. S., “The Rare-Earth Elements and ~
Their Compounds,” Wiley, New York, 1947.
RECEIVEDfor review April 6, 1961. Accepted July 10, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
Increased Selectivity in Chelometric Titrations through End Point Location by Linear Extrapolation Copper as a Photometric Indicator DAVID A. AIKENS, GABY SCHMUCKLER,’ FAWZY S. SADEK? and CHARLES N. REILLEY Department o f Chemistry, University of North Carolina, Chapel Hi//, N. C. b The inherent selectivity of end point location by linear extrapolation permits selective determinations to be carried out when the difference in effective chelonate stability constants of soughtafter and interfering ions is as small as 2 log K units. The necessary masking often can b e provided by careful choice of chelon and buffer systems. Photometric titrations with a Cu(ll) indicator are presented as an example. Ca is determined in the presence of Mg in 0.5M ammonium hydroxide a t pH 10 with ethylene glycol bis(&xninoethyl ether)-N,N’tetraacetic acid. Selective determinations of Cd and of Zn in the presence of the other are carried out with diethylenetriaminepentaacetic acid under the same conditions. Analytical and 1664
ANALYTICAL CHEMISTRY
empirical methods for the determination of appropriate solution conditions are presented.
HE problem of insufficient selecT t i v i t y in chelometric titrations stems in large measure from the method of end point detection generally employed. I n particular, this refers to end points determined potentiometrically or by means of visual indicators. For equal concentrations of soughtafter ion and interference, the use of a visual end point requires that the effective chelonate stability constant of the sought-after ion exceed that of the interfering ion by 5 to 6 log K units, while the difference necessary for a suecessful potentiometric end point is about
4 log K units. The degree of masking required to provide the necessary difference in chelonate stability is often difficult t o provide. I n contrast, end point location by linear extrapolation of points far from end point, such as the amperometric, conductometric, or photometric method, is applicable when the difference in effective chelonate stabilities is only 2 log K units. This difference corresponds to formation of 1% of the less stable chelonate a t the point of 50% formation of the more stable chelonate, so that extrapolation to the end point 1 Present address, Department of Chemistry, Technion, Haifa, Israel. * On leave from National Research Center, Cairo, U. A. R.
