this with several trial tantalum and niobium solutions, it was found that the oxide dissolved within about 15 minutes. Since small amounts of zirconium (IV), titanium(IV), and tin(1V) are present in niobium and tantalum minerals, a study was made to determine the extent of interference of these metal ions in the gravimetric method used for niobium and tantalum. Precipitations were carried out in which equimolar amounts of niobium and other metal ion were present. Results for niobium were high in all trials. I n the case of zirconium results ranged from about O.3Y0 to 1.7% high when 0.1mmole and 0.5-mmole amounts of zirconium were present, respectively. The degree of interference increased with titanium, results being from 2.5% to 10% high, while for tin the range was from 21% to 10070 high. For column separations of zirconium or titanium from niobium, the total volume of solution added to the column was increased to 40 ml. to decrease the sul-
fate ion concentration (from 1M to about 0.25.V), so that titanium and zirconium would not be eluted with niobium. I n order to compensate for dilution the solution was made about 2% in hydrogen peroxide and 0.3 11.I in nitric acid by addition of appropriate amounts of each. A total of about 150 ml. of eluting solution was used. Quantitative separations for both 0.5-mmole and 0.1-mmole amounts were achieved only for titanium. The separation of tin from niobium failed, but niobium can be separated from 0.1mmole amounts of zirconium. LITERATURE CITED
(1) Alimarin, I. P., Medvedeva, A. M., Khromatog., ee Teoriya i Primenen., Akad. Nank. S.S.S.R., Trudy Vsesoyuz. Soveshchaniya, Moscov 1958, 379; C.A. 55, 17360d(1961).
(2) Alimarin, I. P., hfedvedeva, A. M., Trudy Komissii Anal. Khim., Akad. Nank, S.S.S.R., Inst. Geokham. i Anal. Khim. 6 , 351-64 (1955).
(3) Balanescu, G., Z. Anal. Chem. 83, 470
(1931). (4) Carey, M. A., Raby, B. A., Banks, C. V. ANAL.CHEM.36, 1166 (1964). ( 5 ) Dams, R., Hoste, J., Talanta 8, 664 (1961). ( 6 ) Dams, R., Hoste, J., Ibid., 9, 86 (1962). (7) Fritz, J. S., Abbink, J. E., ANAL. CHEM.34, l0S0 (1962). (8) Pavlenko, L. I., Zhur. Anal. Khim. 15, 716 (1960). (9) lZaby, B. A., Ames Laboratory, Ames, Iowa, private communication, 1963. (10) Ryabchikov, D. I., Bukhtiarov, V. E., Zhur. Anal. Khim. 15, 242 (1960); C.A. 54, 19261s (1960). (11) Sano, H., Shiomi, It., J. Inorg. Nucl. Chem. 5, 251 (1958). (12) Strelow, F. W. E., ANAL.CHEM.35, 1279 (1963). JAMES S. FRITZ LIONELH. DAHMER Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Iowa
WORKperformed in Ames Laboratory of U. S. Atomic Energy Commission.
Cation Exchange Separation of Small Amounts of Metal Ions from Cadrnium(ll), Zinc(ll), and Iron(lll) SIR: Little has been reported to date on ion exchange separations of a small quantity of one element (or group of elements) from a very large excess of another element. Many studies have been made concerning anion exchange separations of metallic elements, where elements that form chloride complexes (or form complexes a t lower hydrochloric acid concentration) are retained by the column and the others pass through. It should be possible to use this method to separate traces of elemepts retained by a small anion exchange column from large amounts of elements that are not retained. If a trace constituent must be retained by an ion exchange column and a major constituent not be retained, a cation exchange procedure based on complex formation would supplement the above mentioned anion eschange method. Cation exchange separations based on the formation of metal-chloride complexes are limited to a very few elements, because low hydrochloric acid concentrations must be used to avoid unselective elution of metal ions by the mass action effect of hydrogen ions. However, Fritz and Rettig (2) have shown that many metal ions can be separated from each other on a hydrogenform cation exchange column using dilute hydrochloric acid in aqueousacetone solutions. The presence of acetone greatly strengthens metal1274
ANALYTICAL CHEMISTRY
chloride complexes and makes it possible to work at low hydrochloric acid concentrations so that that elution of metals by hydrogen ions is not a problem. Other authors have used acetone or other organic solvents in various separations based on this same principle. (3, 4,5 ) I n this paper is demonstrated the separation of two elements present in mole ratios of 1000 to 1 or 10,000 to 1 using a small cation exchange column and dilute hydrochloric acid in aqueousacetone solvent as the liquid phase. After the separation, the minor constituent can be stripped from the column and determined by a semimicro titration with EDTA or by standard spectrophotometric procedures. EXPERIMENTAL
Purified Dowex 50W-X8, 100 to 200 mesh, was used in the hydrogen form. It was placed in a large column and backwashed with distilled water to remove fine particles, then washed with 3 liters of 10% ammonium citrate, 3 liters of 3J9 hydrochloric acid, and finally distilled water until a negative test was obtained with silver nitrate. Excess water was removed by suction filtration, and the resin was allowed to air-dry. X 0.0139 solution of EDTA was prepared from analyzed reagent grade material. It was standardized at pH 6 against a solution prepared from pure zinc metal, using XXS as indicator. X 1.OM cadmium ion solution was made b y dissolving 99.99970 pure
metal in enough hydrochloric acid to give a concentration of 0.5X upon dilution. For high concentrations, a solution of iron(II1) was made 3.0M by dissolving 99.9470 metal in hydrochloric acid. A% 5.0M solution of ZnClz in 0.5JI hydrochloric acid was used. For low concentrations, copper and manganese were 0.01Ji solutions of 99.999yo pure metal in 0 . 5 X hydrochloric acid. Kickel and cobalt were 0.01X solutions of the chloride salt in 0.531 hydrochloric acid. Iron(II1) was a 0.005.19 solution of 99.94y0 pure metal in 0.5M hydrochloric acid. Eluents were prepared as volume percentages, while the concentration of hydrochloric acid is expressed in molarity. Volume changes due to mixing were disregarded. Ion eschange columns with 1.2-cm. i.d. and stopcocks of Teflon were used. The resin was equilibrated by passing acetone (20 ml.) through a t maximum flow rate. Samples were prepared in the following manner: Appropriate amounts of metal ion solutions were pipetted into small beakers; the solutions were evaporated to 1 or 2 ml., and a few drops of concentrated HXOI were added when necessary to prevent hydrolysis. The cooled samples were diluted with 20 ml. of the acetone eluent. The prepared samples were transferred to a 125-m1. cylindrical separatory funnel which was attached to the top of the ion exchange column. ‘I he beaker was rinsed several times with a total of 10 ml. of the acetone eluent, which was then added to the sample.
1.000
,
1
I
,I
,
I
I
.
a00
termined by titration with 0.01J1 E D T A or by spectrophotometric determination. Analysis. Manganese was determined colorimetrically as permanganate a t 525 mp. Nickel was determined by differential spectrophotometry with dimethylglyoxime. Iron was determined spectrophotometrically by a 1,lO-phenanthroline procedure.
Table 1. Elution Curve for 10 mmoles Cd+2 from a 6-cm. Column of Dowex
50W-X8 MI. of eluent (80yoacetone0.8M HC1) 111. 0.3M EDTA Load-20 2&30 30-40 40-50 50-60
THEORY
30
+I
w
m
60
ao
w
$00
PERCEMT ACETWE
Figure 1. Cation exchange distribution coefficients of iron(ll1) and iron (11) in 0.5M hydrochloric acid as a function of acetone concentration
The sample was placed onto the column dropwise a t a flow rate of 0.3 to 0.5 ml. per minute. The metal ion, which is in high concentration was washed from the column first by adding 10-ml. frsctions of the appropriate acetone eluent. Ten-milliliter fractions were collected until negative qualitative tests were obtained with two 10-ml. fractions. The column was washed with 20 nil. of distilled nater at a maximum flow rate. The metal ion in lorn concentration was stripped with 352 hydrochloric or nitric acid, also a t maximum flow rate. The amount of metal was de-
Table II.
High concn. metal ion Cd +2 Fe + 3
Low concn.
metal ion c u +2 S i+ 2 S i+2 3In + 2
Co +2 S i +2
S i+a Zn+2
Fe +3 Fe +a
cu
pl'i
T2
+2
S i+Z One determination. b A v . of two detns. Av. of three detns. hv. of four detns.
Ratio 1,000:1 1,000:l 10,000 : 1 1,OOO:l 1,OOO:l 1,000:l 10,000: 1 1,000:l 10,000: 1 1,000:1 1,OOO:l 10,000: 1
Work by Fritz and Rettig ( 2 ) showed that a t a low, fixed concentration of hydrochloric acid (0.2MJ0.5M, or 1.0M) increasing proportions of acetone in the water-acetone mixed solvent causes the distribution coefficient to decrease markedly owing to the formation of neutral or anionic metal-chloric complexes. Higher concentration of hydrochloric acid, as well as a high proportion of acetone, increases the complexing tendency and lowers the distribution coefficients. The order of chloride complexing in acetone-water-hydrochloric acid, as evidenced by distribution coefficients on a hydrogen form cation exchange resin, is as follows: Bi+3 > Cd+2 > Zn+2 > Fe+3 > C U + > ~ CO+~ > ?vIn+2. Xickel(II), magnesium(II), and several other elements show no evidence of chloride complexing in this medium. Using the batch distribution coefficients determined by Fritz and Rettig, conditions for separations are easily predicted. An eluent is chosen so t h a t the distribution coefficient of the major constituent is approximately zero and that of the minor constituent is quite large, preferably several hundred. Under these conditions the minor constituent should remain on the column and be separated quantitatively from the major sample component. However, certain effects might cause difficulty. For example, the presence of a rather high concentration of a complexed metal might have an adverse effect on the distribution coefficient of
30.69 2.85 0.01 0.00 0.00
an uncomplexed metal present in a much lower concentration or on the rate a t which either the major or minor element attains equilibrium between the solution and resin phases. Also in this type of separation the ratio of sample solution to the ion exchange column volume is much larger than usual. This means that the minor constituents on the column will have to withstand much more washing than usual without appreciable displacement down the column. RESULTS AND DISCUSSION
A few fractions of cadmium were titrated with EDTA (Table I) but in most cases only qualtitative tests, such as NAS indicator at p H 6 for cadmium and zinc and thiocyanide for iron, were used. The amount of eluent required to elute completely the major constituent (cadmium, iron, or zinc) was considerably larger than the amount indicated by Fritz and Rettig. However, in that work the usual amount of metal cation used was only 0.2 mmole. I n this work, amounts of metal cation ranging from 10 to 50 mmoles were used. When iron was used as a major constituent, it exhibited a great tendency to tail. At times there were evidences of iron even after 400 ml. of elution. This tailing is possibly caused by the reduction of iron(II1) to iron(I1). It is known that cation exchange resin can reduce iron (1). Iron(II), as shown
Quantitative Separations of M e t a l Ions in High Ratio
Amount acetone eluent 50 ml. 507' acetone-0 8 M HC1 80 ml. 687, acetone-0 5.V HC1 80 ml. 657, acetone-0 5.11 HC1 250 ml. 85% acetone-0 5M HC1 150 ml 807, acetone-0 5M HC1 190 ml. 907, acetone-0 5 X HC1 160 ml S57c acetone-0 5M HCl 100 ml. 55% acetone-0 5M HCl 100 ml. 55Yc acetone-0 8 M HC1 100 ml. 557c acetone-0 5M HC1 80 ml. 657, acetone-0 5M HC1 SO ml. 65% acetone-0 Llf HC1
Column ht. cm. 6 6 6 6 6 6 12 12, 18 18 12 6, 12 6, 12
L O W concn. ion, mg. Taken Found
0 697 0 234 0 238 1 533 1 975 1 908 0 235 0 279 0 288 1 582 0 236 0 237
0 670b 0 233b 0 23jb 1 522" 1 954b 1 917b 0 2335 0 280d 0 287c 1 886; 0 236 0 23gb
VOL. 37, NO. 10, SEPTEMBER 1965
Recovery,
70 100 0 99 6 100 0 99 3 98 9 loo 5 99 2 100 4 99 7 100 3 100 0 100 8
1275
by a plot of distribution coefficient us. per cent acetone in Figure 1, will be eluted more slowly by eluents of 85% or less acetone. A list of the successful separations obtained is given in Table 11. Only cadmium, iron(III), and zinc were used as major constituents. Some separations involved a ratio of 10,000 to 1 of major constituent to minor constituent, but more were done in the 1000 to 1 range. From the Cd+2-Cu+2separation where C U +has ~ a distribution coefficient of about 190 a t 50% acetone-0.5M HC1, separations of Cd+2 from C O + ~ , Fe+3, Ga+3, Xln+2, or COz+2as minor constituent are suggested. Likewise, the Zn+2-Fe+3 separations, where Fe+3 has a distribution coefficient of 134 in 55% acetone, suggests separations of large amounts of Zn+2from C O + ~Gat3, , Mn+*, U O t 2 , or VO+2 as minor constituent. However, the problem of tailing when iron is the major constituent requires the minor constituent to have a
distribution coefficient of around 300 or more. An attempted separation of Fe+3 from VO+2 as minor constituent failed using 75y0 acetone-0.5M HCl, where VO+2has a distribution coefficient of 163. Lowering the eluent composition much below 75% acetone-0.5M HCl was impossible, because tailing then occurred even at 500 ml. of eluent. At 50% acetone-0.5M HC1 or higher concentrations of acetone, Bi+3 and In+3 are other possibilities for major constituents, because their distribution coefficients are essentially zero. At 85% acetone-0.5M HCl, Ga+3, C U + ~ , or UOz+2may be used as a major constituent to be separated from either Ni+2 or Mn+2. Only the minor constituent was determined quantitatively. It was felt that qualitative tests were sufficient for the major constituent. Eluent fractions were analyzed qualitatively in the manner previously described and, when two succeeding negative tests were
obtained, i t was assumed that the major constituent was completely eluted. The average recovery for the minor constituent was 99.9%. The relative standard deviation for all 26 results was *0.9%. LITERATURE CITED
(1) Fritz, J. S., Karraker, S. K., ANAL. CHEM.32, 957 (1960). (2) Fritz, J. S.. Rettia, T. A., Ibid.. 34. I
I
,
1562 (1962). (3) ~, Kember. N. F.. Macdonald. P. S.. Wells, R.'A., J . Chem. SOC.1955, 2273: (4) Kojima, M.,Bunseki Kagaku 7, 177 (1958). (5) Van Erkelens, P. C., Anal. Chim. Acta 25, 42 (1961). Division of Analytical Chemistry, 148th Meeting, ACS, Chicago, September 1964. Work performed in Ames Laboratory of U. S. Atomic Energy Commission. JAMES S. FRITZ JANET E. ABBINK Institute for Atomic Research and Dept. of Chem. Iowa State University Ames, Iowa
Microdetermination of Fluoride Using an Improved Distillation Procedure SIR: The interference of many cations and anions in available colorimetric methods for fluoride necessitates a preliminary separation of the fluoride. The classical Willard-Winter distillation procedure (6) has been in use since 1933, but its usefulness has been limited because the large volume of distillate required for the quantitative recovery of fluoride complicates the subsequent measurement of small amounts of fluoride and because quantitative distillation of fluoride is not obtained readily in the presence of metal ions that complex fluoride. These shortcomings were remedied recently by Singer and Armstrong ( 5 ) who developed a microstill that gives quantitative distillation with only 20 ml. of distillate, and by Blake (3)and Grimaldi, Ingram, and Cuttitta (4) who overcame the adverse effect of fluoride-complexing metal ions by the use of metal-complexing phosphoric acid in the distillation medium. This paper describes a versatile microdistillation procedure that is applicable to the separation of microgram levels of fluoride from anions and cations that interfere in colorimetric fluoride 1x0cedures. The distillation is carried out from a phosphoric acid medium with a micro still, similar to that of Singer and Armstrong, which has been improved to minimize the carryover of phosphate and sulfate. The fluoride in the distillate is determinable by a number of colorimetric methods. I n our study, the 1276
ANALYTICAL CHEMISTRY
INNER TUBING
Figure 1,
Modified distillation flask
improved distillation-separation procedure was coupled with an Alizarin Complexone spectrophotometric measurement of the fluoride ( I , 2, 7 ) . This resulted in a versatile, reliable method that appears to be suitable for a wide variety of samples. I n fact, of 42 diverse ions tested, most of them at the 500 to 1 diverse ion to fluoride molar ratio, only borate and silicate interfered seriously. EXPERIMENTAL
Apparatus. T h e distillation apparatus is similar to t h a t of Singer and ilrmstrong with t h e exception of t h e distillation flask which has been modified as shown in Figure 1. Teflon standard taper 14/35 sleeves (Arthur E. Smith, Inc., Pompano
Beach, Fla.) were used on all j o i n s to prevent freezing. Polyethylene 50ml. graduated cylinders were used to collect the distillate and a Cary Model 14 spectrophotometer with 1-em. optical cells was used for the absorbance measurements. Reagents. Concentrated perchloric acid and 850/, phosphoric acid were both purged of lower boiling point components including fluoride b y heating t h e acid to 160' C. in a roundbottom flask and bubbling steam through the acid for 4 hours. T h e fluoride solution was prepared from reagent grade sodium fluoride. T h e composite lanthanum(II1) - Alizarin Complexone (La-.1C) reagent was prepared as described earlier ( 7 ) . Lanthanum was substituted for cerium used in the earlier procedure because the sensitivity of the lanthanum reagent is constant, whereas the sensitivity of the cerium reagent tends to vary with different lots of cerium nitrate reagent. Also, the La-AC reagent seems to be slightly more sensitive to fluoride than the corresponding cerium reagent. Procedure. Clean all glassware in hot sulfuric acid and rinse thoroughly with distilled water. Heat a glycerol bath to 165' C. Add 0.5 ml. of 70% perchloric acid to the t r a p through t h e arm of t h e distillation flask and assemble t h e apparatus. By means of a polyethylene pipet, transfer 4 ml. of 0.5,11 sodium hydroxide to t h e 50-ml. polyethylene graduated cylinder and immerse the polyethylene tip of the condenser in the sodium hydroxide. Remove the bubbler tube and add an aliquot (5 ml. or less) of the
