pulses and interference with the Ka24 measurement. Inasmuch as Na24 itself as well as some trace elements has low energy y-rays which may be recorded as high energy pulses by random coincidences a t high counting rates, the proportionality between disintegration rate and counting rate was tested for pure Naz4 radioactivity. Figure 2 shows the result of counting a series of graded sources of pure NaZ4produced by 15m.e.v. cyclotron deuteron bombardment of NaZCO3. An integral scintillation counter was used to measure y activity above 2.6 m.e.v. to 1% statistical error. Below about 15,000 c.p.m. proportionality is strict, but a t higher activity levels the counting rate increases faster than the disintegration rate, owing to random coincidences of lower energy “az4?-rays. For best results, counting rates should not exceed about 15.000 c.p.m., and preferably sample and standard counting rates should be approximately the same if counting rates near 15,000 c.p.m. are used.
I n reactors where the ratio of fast to slow neutrons is high or where the Na content of the sample is very low compared to A1 or Mg, measurable Na24 radioactivity may be produced by the interfering reactions A124(n,a)Na24 (& = -3.14 m.e.v.) or MgZ4(n,p)Na24 (Q = -4.73 m.e.v.). The possibility of this interference may be tested by measuring induced NaZ4 activity with and Lvithout Cd shielding during neutron irradiation. CONCLUSIONS
The procedure for determination of sodium in silicate minerals and rocks by neutron activation is free from interference, even a t the low content of O.Oti7, K a tested, and does not require chemical processing. If counting and calculations are carried out using an automatic sample changer and a digital computer, the procedure requires a minimum of handling and may be made very rapid. Potassium may be determined in the same samples, using a procedure reported previously (9).
ACKNOWLEDGMENT
Support of this work by a grant from the National Science Foundation is gratefully acknowledged, and helpful discussions with Jose Catoggio are appreciated. LITERATURE CITED
(1) Ahrens, L. H., Phys. Chem. Earth 2,30
(1957).
( 2 ) Fairbairn, H. W., Geochim. et Cosmochim. Acta 4, 143 (1953). (3) Hughes, D. J., Magurno, B. $., Brussel, 1vI. K., U. S. At. Energy Comm., Suppl. 1 to “Seutron Cross Sections,”
BNL-325 (Jan. 1, 1960). (4) Hughes, D. J., Schwartz, R. B., U. S. At. Energy Comm., “Neutron Cross Sections,” BXL-325 (July 1, 1958). (5) Natl. Bur. Standards, Circ. 552, 2nd ed. (1957). (6) Ibid., 3rd ed. (1959). ( 7 ) Stevens, R. E., et al., U. S . Geol. Survey Bull. 1113 (1960). (8) Strominger, D., Hollander, J. M., Seaborg, G. T., Revs. Modern Phys. 30, 585 (1958). (9) Winchester, J. R., ANAL. CHEM.33, 1007 (1961). RECEIVED for review August 9, 1961. Accepted October 30, 1961.
Titration of Uranium with Potassium Dichromate Determ inat ion of Disproportiona lity Effects JORGE E. A. TONI‘ Atomic Energy Commission, Buenos Aires, Argentina
b The indicator, diphenylaminesulfonic acid, is shown to be responsible for the disproportionality observed between milligrams of uranium titrated and milliliters of potassium dichromate used, when the titration end point i s determined visually. Determining the end point potentiometrically, the discrepancy was eliminated within certain limits. The effect of some variables on the indicator was studied and optimum conditions were found for the potentiometric titration of as little as 10 mg. of uranium.
W
Rodden (9) and De Sesa (3) determined uranium volumetrically by an osidation-reduction procedure. Rodden used a reduction column containing zinc, titrated with potassium dichromate and cerium(1T’) sulfate, and observed that a disproportionality existed between milligrams of uranium and milliliters of reagent used. De Sesa also found this anomaly using the same reducing agent and potassium dichromate but did not, find it when the reduction n-as made ORKIKG SEPARATELY,
using lead in a perchloric acid medium and subsequent titration with cerium sulfate. This variation in titer occurred when the amount of uranium was less than 140 mg. Stvinehart ( 1 2 ) successfully used a lead reductor in hydrochloric-sulfuric acid medium and titrated with cerium(IV) sulfate, but the experiments were carried out with amounts of uranium from 189.8 to 554.7 mg. as uranium oxide (Cs08). All of these authors detected the end point visually using the classical indicators [ferroin for cerium(1V) and diphenylaminesulfonic acid for potassium dichromate 1. I n this work potassium dichromate was chosen as titrant because of its high purity and the stability of its solutions. The titration of uranium with potassium dichromate is based on the oxidation of uranium previously reduced by a n excess of iron(II1) and the final titration of the iron(I1) by dichromate. As shown by Kolthoff and Sarver (5, 10) either diphenylamine or diphenylaminesulfonic acid can be used as the visual indicator but the
latter is widely used because of its sharper color change (4). The same authors studied the reaction mechanism and considered the existence of intermediates a t the titration end point. They analyzed different factors influencing the system such as the time required for oxidant addition, temperature, acidity, and excess of oxidant. They remark ( 5 ) : “The results of the analytical study are not very satisfactory from the stoichiometric point of view, because side reactions may occur.” Having in mind the determination of the observed disproportionality and considering the observations of Kolthoff and Sarver, diphenylaminesulfonic acid was chosen as indicator and its behavior received attention in this work in vhich the redox process was studied by using potentiometry as the referent method. Potentiometry a t constant current (7) was also used to study the possibility of titrating dilute solutions of uranium which cannot be Present address, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina. VOL. 34, NO. 1, JANUARY 1962
99
Table 1.
Acidity Effect on Behavior of Indicator (Diphenylaminesulfonic Acid)
(Double line separates experiments performed on solutions having different uranium concentrations) Zinc Column Lead Column Scidity, N 0 . 03A\r, 0,03N, (HzSOa) KZCr207, ml. Acidity KZCrZO7, ml. 2 20.28 3N HC1 10.09 3AVHC1 - 3N H,SO4 10.02 5 20.28 9.7 20.01 3W HCl - 62' H2SOd 9.91 9.83 12 19.60a 3N HCl - 9N H2S04 9.68 3N HCl - 4 N 9.97 10.12 3N HCl - 2N HzSO4 a Doubtful end point.
evaluated by direct potentiometry. However, the main purpose was the determination of the disproportionality effects in uranium dichromate titrations and the development of a method which would avoid them. EXPERIMENTAL
Potentiometric determinations were made with a p H meter from W. C. Pye & Co., Ltd., Cambridge, England, using calomel as indicator electrode. The platinum electrode \vas periodically treated with sulfuric-hydrochloric acid solutions, solutions of sodium hydroxide, and occasional heating in a n alcohol flame to remove impurities. The method of the second derivative (6) was used for determinations of the end point. For the constant current potentiometry, two equal plate platinum electrodes were used, 48 sq. mm. each, and a small current was passed through them when they were immersed in the solution. These electrodes were connected to the potentiometer which recorded the variation between potentials caused by reagent addition. The intensity of the current was 1.8 pa., using a 45-volt battery and 25megohm resistance. Reducing Columns. They were of conventional design. The reducing agent occupied 17 X 350 mm. in t h e visual determination and 12 X 130 mm. in the potentiometric determination. Reagents. Indicator F Solution, 0.01M. T o 0.32 gram of the barium salt of diphenylaminesulfonic acid (Fisher reagent grade) were added 80 ml. of water, 0.5 gram of sodium sulfate (anhydrous), and the solution was let stand overnight. The resulting soluInstrumentation.
Table II. Effect of Time of Indicator System Contact Using Diphenylaminesulfonic Acid
(2N HZSOa) Indicator Added 1 M1. before Before titration end point 9.64 9.51 4.86 4.72
100
0
ANALYTICAL CHEMISTRY
tion was filtered and made up to 100 mlwith distilled water. Indicator RII Solution, 0.01M. Exactly 0.27 gram of the sodium salt of diphenylaminesulfonic acid (prepared in the Atomic Energy Commission, Argentina, by sulfonation of diphenylamine made in Argentina) was dissolved and diluted to 100 ml. with distilled water. The salt was purified by extraction with methanol. Indicator A Solution, 0.01M. Exactly 0.27 gram of the sodium salt of diphenylaminesulfonic acid (prepared in the Atomic Energy Commission, Argentina, by sulfonation of diphenylamine Merck, made in Germany) was dissolved and diluted t o 100 ml. with distilled water. The salt was purified by extraction with methanol. Uranium Oxide (U&). Primary standard (prepared by the analytical division of the iltomic Energv Commission, Argentina). Potassium Dichromate. 0.03N Solution. This solution Gas potentiometrically standardized against primary uranium oxide (UaO?). Sulfuric-Phosphoric Acid Mixture. To 500 ml. of distilled water, 350 ml. of phosphoric acid (85%) and 150 ml. of sulfuric acid (sp. gr. 1.84) were added. The mixture was allowed to cool before use. A solution having half the concentration of the commonly used mixture was adopted to decrease attack on the glass container. I "
the same weight on the different results. These results are comparative, disregarding the actual amount of uranium in each solution. The range of uranium used was between 20 and 70 mg. RECOMMENDED POTENTIOMETRIC METHODS. Zinc as Reducing Agent. Allow 10 ml. of uranium solution in 5% (v./v.) sulfuric acid to flow through the reducing column a t a rate of 2 ml. per minute. Wash with small portions of 5% (v./v.) sulfuric acid totaling 30 ml. After 15 minutes of aeration add 4 ml. of 10% (w./v.) ferric chloride hexahydrate and 6 ml. of the sulfuricphosphoric acid mixture. Titrate with 0.03iL' potassium dichromate. Lead as Reducing Agent. Pass 10 ml. of the uranium solution 3N in sulfuric acid and 3N in hydrochloric acid through the reducing column at rate of 2 ml. per minute. Wash with small portions of 1N hydrochloric acid totaling 30 ml. Add 4 ml. of ferric chloride 10% (w./v.) and 6 ml. of the sulfuricphosphoric acid mixture and titrate with 0.03N potassium dichromate. RESULTS A N D DISCUSSION Effect of Variables on the Indicators. EFFECT OF ACIDITY. The
acidity of t h e medium had a marked effect on t h e behavior of t h e indicator. When t h e acidity was increased, color change became difficult t o see a n d t h e end point appeared earlier (Table I ) . (In this, as for other tables, t h e value for acidity corresponds t o t h e solution passing through t h e reducing column.) EFFECT OF TIMEOF INDICATOR CONTACT. Values were lower R-hen the indicator was added immediately before the end point (Table 11).
Table Ill. Back-Titration ( 2 N HzSO4)
Direct Titration 10.10
Back-Titration Indicator Indicator added added after before excess of titration reagent 10.15 10.19
PROCEDURES
The behavior of the dichromate titration using a n indicator and zinc column as reducing agent was studied by application of the method of U. S. Atomic Energy Commission (12). The procedure recommended by Cooke, Hazel, and JIch'abb ( 1 ) was used to study the behavior when a lead column was employed as reducing agent. To obtain reliable results, solutions having different amounts of uranium were reduced in one of the columns and after reduction were made up t o a fixed volume with distilled water. The determinations were performed on aliquot portions which reproduced the conditions of acidity and volume corresponding t o the visual methods of zinc or lead columns (1, 12) by addition of the appropriate solution. In this way the possible sources of error had
Table IV. Back-Titration after Addition of Excess Reagent Indicator added after excess. Double line
separates experiments performed on solutions having different uranium concentrations Titration 0.03N, Bcidity Technique K2Cr207, 311. 2N H2S04 Direct 10.10 4.60 10.10 4.65 Back 9 , i N H2SOa Direct 9.46" Back 10.21 2 , 5 5 HC1 - Direct 9.83= 7N HzS04 Back 10.21 2N HzSOc Direct 10.21 20.13 Back 10.21 20.03 a Doubtful end point.
l
1
IO
20
30
u)
m i C-2G,h'2
,
I
50
, 4
2
,
l
,
l
,
,
8
10
I2
74
f6
l l4
' 20
l
l
22
l
24
403iY
Figure 1. Visual dichromate titration of uranium
l
26
l
28
l
30
m i Cr,O, Kz
32
.
0.03N
Potentiometric dichromate titration of uranium
Figure 2.
Zinc column
Changing the shape and dimensions of the platinum electrode and additional cleaning with different acidic and basic EFFECTOF BACK-TITRATIOK. The solutions as well as heating in an alcohol use of a back-titration method showed flame did not improve the results. that it was necessary to allow an excess Apparently the low oxidation-reduction of potassium dichromate to react for potential of potassium dichromate (1.36 15 minutes. It also indicated the desirvolts) was responsible and this was corability of addition of indicator after roborated by the use of other oxidants the dichromate titrant (Tables I11 and with higher potentials such as ceriumIV). Moreover, variations were ob(IV) sulfate (1.60 volts) or potassium served on increasing the acidity, but permanganate (1.52 volts) for which a net change of color was seen even in significant potential increases were cases with a doubtful end point by observed. Thus, as the potentiometric direct titration (Table IV). increase will depend on the excess conEFFECTOF METHODOF PREPARATION centration of dichromate] a final volume OF INDICATORS. The results obtained of 50 to 60 ml. was enough to cause an from the use of three indicator solutions, appreciable increase under the experwith the indicators prepared differently, imental conditions used here (recomare shown in Table V. I n these exmended potentiometric methods). periments, the same amounts of inProportionality Study. This study dicator (0.3 ml. of 0,OlM solution) were confirmed t h a t the anomalies were used but the end points were different. Effect of Diphenylaminesulfonic
1w
II
Figure 3. Potentiometric dichromate titration of uranium in low concentration a t constant current
Acid on the Potentiometric E n d Point.
Checking a visual titration by potentiometry, the visual end point appeared earlier t h a n the potentiometric one. The difference depends on the working conditions (aridity, temperature, addition of reagents, etc.). It is interesting t o note the effect of the indicator on the potentiometric end point. If two solutions, one with and the other without indicators, are titrated potentiometrically, different end points are obtained. The difference may amount to up to 2Oj, (Table VI). By titrating blanks potentiometrically, it was established that these differences were not due to consumption of potassium dichromate by the indicator. These results seem to show once again the existence of interactions among the different components of the system a t the end point when diphenylaminesulfonic acid is used as indicator. POTENTIOMETRIC PROPORTIONALITY STUDIES
Because of the variable results obtained when a visual indicator was used, potentiometric titrations were tried (8). Dubious end points were obtained during preliminary tests when the same conditions as those used for the usual titrations were employed (1, l a ) . The corresponding potentiometric increase was small (15 to 20 mv.) and inconstant.
3.40
I
1
I
,
2
I 4
I 6
I 8
I IO
I 12
I I4
JA L - 1 I I - L - L 76 20 2 2 24 26 28 30 32
16
m i Cr,0,K2
OOJN
Figure 4. Potentiometric dichromate titration of uranium a t constant current
due t o the indicator. Also the conditions for uranium potentiometry with potassium dichromate and the potentiometric relationship between milligrams of uranium titrated/milliliters of potassium dichromate used were considered. The anomalies observed earlier by Rodden (9) and De Sesa (3) for the visual dicromate titration were confirmed (Figure 1). These anomalies were experimentally confirmed to be independent of the reducing column used. The curve in Figure 1 and those obtained by the mentioned authors have the same characteristics but they do not coincide] undoubtedly because of experimental details. Thus while Rodden (9) did not get constant values for milligrams of uranium per milliliter of dichromate, De Sesa (3) obtained results which were close for amounts
Table V. Results Expressed in Milliliters of 0.03N K2Cr207
Acidity 2N H&Oi 3iV HC1 3N HC1 3N H2S0, Table
VI.
Indicator F
In&cator M
Indicator A
10.13 10.20
10.05
9.99
10.01 9.99
9.99
10.06
10.00
Effect on Potentiometric Response
Results expressed in ml. of 0.03.V K2Cr20T IndiIndiWithout cator cator Acidity Indicator F M 2 N HsS04 3 N HC1
9 96 9.95
10.14
10.16
VOL. 34, NO. 1, JANUARY 1962
10.06 10.15
101
greater than 140 mg. of uranium. I n this work constancy is attained for 70 mg. of uranium. Figure 2 shows that proportionality is obtained from 10 mg. of uranium (the smallest amount tested) to higher values in the area where the visual titration showed anomalies. It must be emphasized that if the visual titration is carried out with a n indicator and under the same concentration conditions used in the potentiometric titration, the disproportionalities are observed equally and are not related to differences in concentration. Constant
Current Potentiometry.
This study was made t o apply potentiometric dichromate titrations t o dilute solutions of uranium (2, 7 ) . Preliminary tests showed a great sensitivity in dilute solutions equal or similar to that for the visual titrations. At such dilutions classic potentiometry cannot be applied as shown before. The values mentioned earlier for intensity of current and electrode surface (see Instrumentation) were adopted after varying the intensity (1 to 45 pa.) and the surface (1 to 96 sq. mm.) within
wide ranges. Those values were adopted as the best because they yielded appreciable potentiometric increases (300 to 350 mv.) and well defined characteristic peaks for this type of potentiometry (Figure 3). The reproducibility and proportionality of the method for conditions equal to those for the visual titrations of uranium using a lead column as reducing agent ( 1 ) were studied. The standard deviation was 10.017 for eight deProterminations with : 16.79 ml. portionality was attained from about 20 mg. of uranium (Figure 4); with lower amounts the values were erratic. ACKNOWLEDGMENT
The author is deeply indebted to Reinaldo Vanossi for his counsel and advice during the present work.
(3) De Sesa, R l , First Conf. Anal. Chem.
Nuclear Reactor Technology, Gatlinburg, Tenn. (1957); U. S. Atomic Energy Comm. Rept. TID-7555, p. 58 (4) Kolthoff, I., Lingane, J., J. Am. Chem. SOC.55, 1871 (1933). (5) Kolthoff, I., Sarver, L., Zbrd., 5 2 , 4179 (1930). (6) Lingane, J., “Electroanalytical Chemistry,” p. 70, Interscience, New York, 1953. (7) Reilley, C., Cooke, W., Furman, S . ANAL. CHEM.23, 1223 (1951). (8) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” p. 582, McGraw-Hill, New York, 1950. (9) Rodden, C. J., ‘(FirBt Conf. Anal.
Nuclear Reactor Technology, Gatlinburg, Tenn. (1957); U. S. Atomic Energy Comm. Rept. TID-7555, p. 25 (10) Sarver. L., Kolthoff, I., J . Am. Chem. SOC.53,2902, 2906 (1931). (11) Swinehart, B. A., Second Conf.
Anal. Chem. Nuclear Reactor Technology, Gatlinburg, Tenn. (1958); E. S. Atomic Energy Comm. Rept.
LITERATURE CITED
TID-7568, p. 117. (12) U. S. Atomic Energy Comm, New Brunswick Laboratory, Manual of Analytical Methods for the Determination
(1) Cooke, W.,Hazel, F., McNabb, \V , ANAL.CHEM.22, 654 (1950). (2) Delahay, P., “New Instrumental Methods in Electrochemistry,” p. 256, Interscience, New York, 1954.
RECEIVEDfor review March 1, 1961. Accepted September 26, 1961. Sesiones Qufmicas Argentinas, Tucumh, Argentina, September 21, 1960.
of Uranium and Thorium in their Ores.
Cation Exchange Separation of Metal Ions with Hydrobromic Acid JAMES S. FRITZ and BARBARA 8. GARRALDA Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa
b Mercury(ll), bismuth(lll), and cadmium(l1) can be separated from most other metal cations by elution from a 16-cm. cation exchange column with 0.3 to 0.5M hydrobromic acid. Using different concentrations of hydrobromic acid, eluents ranging from 0.1 to 0.6M, mercury(ll), bismuth(lll), cadmium(ll), and lead(ll) can b e separated from each other and from other metal ions.
I
instances, the complexing effect of halogen acids has been used to elute metal ions selectively from a cation exchange column. A comprehensive paper on elution of metal ions with hydrofluoric acid has appeared (2). Kallmann, Oberthin, and Liu (3) have developed a very selective method for cadmium(I1) by elution with hydriodic acid. Yoshino and Kojima (5) and later Strelow (4) separated cadmium(I1) from zinc(I1) and other metal ions using a cation exchange column with 0.5M hydrochloric acid as the eluting agent. This separation depends largely N SEVERAL
102
ANALYTICAL CHEMISTRY
on the formationof cadmium(I1) chloride complexes, because a 0.5Af hydrogen ion concentration is not sufficient to elute a divalent metal ion in a reasonable time by the mass action effect. I n the present work, mercury(II), bismuth (111), cadmium( 11), tin(1V) , and lead(I1) are eluted from a cation exchange column with dilute solutions of hydrobromic acid. These elements can be separated from most other metal ions. Furthermore, by varying the concentration of hydrobromic acid eluent, i t is possible to separate mercury( 11), bismuth (111), cadmium (11), and lead(I1) from each other. EXPERIMENTAL
I o n Exchange Resin. Dowex 5OVX8 cation exchange resin of 100- to
200-mesh is used. The commercial resin must be purified before using. This is done by placing t h e resin in a large column, backwashing with water t o remove any fine particles, and washing with 10% ammonium citrate (pH 3.0 to 3.3, 3 N HCl, and finally water until a negative chloride test is obtained with silver nitrate. The purified resin
is in the hydrogen form. It is removed from the column and air-dried. I o n Exchange Column. Conventional, 12-mm. i.d. glass columns are used. T o prepare the column, pour a slurry of resin and water into t h e column until the bed has a height of 16 em. Add t h e eluent dropwise with a flow rate of about 2 ml. per minute from a 125-ml. cylindrical separatory funnel inserted in the top of the column through a one-holed rubber stopper. The dropwise addition prevents disturbance of the resin bed. PROCEDURE
Make 0.05M solutions of the metal salts. The salts used are the nitrates, perchlorates, or chlorides, with the exception of vanadium(IV), which is the sulfate. Take aliquots of the salt solutions containing the desired amount of each metal ion. The column load should usually not exceed 0.5 mmole of metal ions. Adjust the sample volume to 25 ml. with water. Load the sample onto the column dropwise. Rinse the column with 20 ml. of water. Elute with the hydrobromic acid, collecting the eluent. Use slightly more than the minimum volume of
