Table I. Analysis of l'hSrteen Niobium Solutions Treated as Unknown Solutions Niobium, pg Foreign ions present, pg Present Found Error, 3.34 4.84 5.96 8.92 9.68 11.90 1.86 2.98 5.20 7.44 7.44 9.28 11.90
3.48 4.84 6.02 8.70 9.80 12.00 1.90 2.86 4.98 7.48 7.66 9.06 11.54
+4.0
...
+1.0 -2.5 +1.5 +I .o +2.0 -4.0 -4.5 +O. 6 +2.8 -2.4 -3.0
...
... ... ... Ta(3 60) Pb(1040)
+ CN+ CNEDTA + CNEDTA
co(270), Ni(445) EDTA Cu(510)
W400), Mg(70) Fe(780) prior extraction. Al(140) triethanolamine
+
errors in the phosphoresence intensity given in parentheses : Al( - 35 %), Cd( -25 Z), CO( -75 CU(- 30 %), Fe(II1) (- 62 %), Hg( - 50 %>, W I I X - 90 %I, W- 75 %I, Pb (+35 %>, W V X - 26 %>, V(V)(- 50 %>, and zn( - 50 %). The interference of Cd, Co, Cu, Hg, Mn(II), Ni, Pb, and Zn can be completely removed at this level by the addition of 1 ml of 1 % solution of EDTA and potassium cyanide to the solution to be analyzed. The interfetence of 50-fold molar excesses of Ti(1V) and A1 is removed by the addition of 0.2 ml of triethanolamine to the sample solution before extraction. The interference of iron(II1) may not be suppressed by the use of any common masking agent, so that even when relatively small amounts of iron (greater than equimolar with niobium) are present, it must be separated before the niobium determination. This may easily be accomplished, however, by modifying the extraction procedure when iron is present so that iron(II1) is first extracted as its oxinate before the niobium extraction. In the presence of up to a 100-fold molar excess of iron(III), 5 ml of 4 x citric acid should be added to the sample solution and the pH adjusted to 2.0. The solution should then be extracted twice using a 5% solution of oxine in chloroform. The aqueous phase may then be washed with one 5-ml aliquot of chloroform and its pH adjusted to 9.4 with fM ammonia solution. The niobium oxinate may then be extracted using only chloroform
x),
rather than 1 % oxine in chloroform, as sufficient free oxine remains behind in the aqueous phase after the extraction of the iron with 5 % oxine in chloroform to ensure efficient extraction. When this procedure is adopted in the presence of much iron, however, a blank and standard should be subjected to the same procedure simultaneously when high precision is required. Accuracy. Table I shows the results of the analysis of solutions for niobium by the recommended procedure. Phosphorescence Life-Time. The phosphorescence lifetime of niobium oxinate in EPA-chloroform at -196 "C was measured by presentation of the photomultiplier output of the spectrofluorometer directly at an oscilloscope and observation of the decay curve of the phosphorescence, Le., the Aminco photomultiplier microphotometer unit was not used. Repetitive experiments gave a value for the phosphorescence lifetime, T , of 7.5 i 0.3 milliseconds. The method described here for the determination of traces of niobium, based on our observation of the phosphorescence of its complex with 8-hydroxyquinoline in EPA at - 196 "C, is more sensitive than the corresponding absorptiometric method with 8-hydroxyquinoline and most other absorptiometric methods which employ organic reagents. The determination can be made very selective when cyanide and EDTA, or triethanolamine, are used as masking agents. Unlike spectrofluorometry, in the spectrophosphorimetric method employed here, a wide band-pass may be employed for the excitation and no problems are encountered with scattered radiation from the dewar flask, sample tube, or small amounts of particulate material in the solution. The determination is only slightly slower than a conventional spectrophotometric determination. Some additional care must be taken in spectrophosphorimetry compared to spectrophotometry, however, to observe cleanliness when filling and cooling the sample tubes. ACKNOWLEDGMENT
The authors thank J. J. Warren for his assistance with some of the early experimental work.
RECEIVED for review January 9, 1970. Accepted March 6, 1970. Spectrofluorometer purchased through a grant from the Science Research Council.
Spectrofluorimetric Determination of Submicrogram Amounts of Zirconium with Calcein Blue R. V. Hems, G . F . Kirkbright, and T.S . West Chemistry Department, Imperial College, London S. W.7., U.K.
SEVERALREAGENTS have been shown to provide sensitive methods for the spectrofluorimetric determination of zirconium. Flavanol (I), quercetin (2), and morin (3),where vicinal hydroxyl groups on an aromatic nucleus are responsible for the zirconium chelate formation, have been commonly recommended. The method described by Geiger and Sandell (3), (1) W. C. Alford, L. Shapiro, and C . E. White, ANAL.CHEM., 23,1149 (1951). (2) D. M. Hercules, Tuluntu, 8,485 (1961). (3) R. A. Geiger and E. B. Sandell, Anal. Chirn. Acfu., 16, 346 (1957).
784
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
for example, enables the determination of as little as 0.025 pg of zirconium in a sample volume of 25 ml. During the course of an extended study of the spectrophotometric and spectrofluorimetric determination of zirconium, several reagents in which the aromatic nucleus of the molecule is substituted with a hydroxyl group adjacent to a methyliminodiacetic group (e.g., methylthymol blue and xylenol orange) were examined. These reagents form colored complexes with zirconium. Calcein Blue (3-aminomethyl-4-methylumbelliferone-N,Ndiacetic acid), which bears a similar functional group, also reacted with zirconium to form a fluorescent complex.
Since its introduction by Wilkins ( 4 ) and Eggers ( 4 4 , Calcein Blue has been used as a metallofluorescent indicator for the compleximetric titration of several metal ions with EDTA. The indicator itself is fluorescent in the range pH 410 but forms nonfluorescent complexes with several metal ions. Above pH 12, the reagent itself does not fluoresce, but forms fluorescent complexes with calcium, strontium, and barium. We observe that the blue fluorescence of the Calcein Blue in weakly acid medium is apparently similarly quenched in the presence of a large excess of zirconium. When dilute solutions are employed, however, and equimolar or excess Calcein Blue is present, a fluorescent zirconium-Calcein Blue complex of different spectral characteristics to those of the reagent alone is formed. This paper describes the application of the measurement of the fluorescence of this complex to the determination of submicrogram amounts of zirconium.
1
EXCITATION
EMISSION
EXPERIMENTAL Apparatus. A double monochromator spectrofluorimeter (Aminco-Bowman, American Instrument Co., catalogue No. 4-8202) fitted with a high intensity xenon arc lamp and an RCA 1P28 photomultiplier was used in conjunction with an X-Y recorder (Bryans, England, Model 21001). Fused quartz cells (10 x 10 x 48 mm) were employed. In order to obtain maximum sensitivity, 5-mm slits corresponding to ca. 50 nm bandpass were used (Aminco slit arrangement No. 5.) in both the excitation and analyzing monochromators. pH measurements were made with a Vibron pH meter, Model 39A (E. I. L. Ltd., Richmond, Surrey, England). Reagents. ZIRCONIUM SOLUTION.A 10-3M stock solution of zirconium was prepared by dissolving 0.1611 gram of zirconyl chloride (ZrOC12.8H20; purified for use in fluoride determinations; British Drug Houses Ltd., Poole, England) in 3M hydrochloric acid and diluting to 500 ml using the same acid. This stock solution was diluted daily to give a working solution containing 0.1 pg/ml of zirconium (10-6M). CALCEINBLUE SoLurIoN. A 5 X 10-4M solution of Calcein Blue was prepared by dissolving 0.0161 gram of Calcein Blue (Hopkin and Williams Ltd., Chadwell Heath, Essex, England) in several drops of 0.1M potassium hydroxide and diluting to 100 ml with freshly distilled water. Suitable aliquots were diluted to 10-6M as required. Fresh stock solutions were prepared every 3 days. pH 5.5 SOLUTION.Thirty ml of glacial acetic acid (analytical reagent grade) were diluted to approximately 800 ml with distilled water and concentrated ammonia solution (Specific Gravity 0.88, analytical reagent grade) was added until the pH was 5.5. The solution was then diluted to 1 liter with distilled water. All volumetric flasks were treated with silicone “Repelcote,” dimethyldichlorosilane in CCl, (Hopkin and Williams 2 Ltd.,) to minimize absorption of zirconium onto the glass. Preparation of Calibration Graph. Transfer 0.1 to 1.0 ml of 10-6M zirconium solution to a series of 100-ml volumetric flasks, each containing 1.0 ml of 10-6M Calcein Blue solution. Wash down the sides of the flasks with distilled water, allow the solutions to mix for 5 minutes, and add 5 ml of pH 5.5 solution to each flask. Dilute to volume with distilled water. Measure the intensity of the fluorescence at 405 nm with an excitation wavelength of 350 nm. After subtraction of the reagent blank fluorescence, obtained simultaneously, the calibration graph is linear from 10 to 100 ng of zirconium (0.0001-0.001 ppm). The upper limit of the calibration graph can be extended by the use of larger quantities of Calcein Blue reagent. Thus linear calibration extends over the range 50 to 200 ng of zirconium (0.5 to 2 ppb) when 2 ml of 10-6M Calcein Blue reagent solution are employed. (4) D. H. Wilkins, Tuluntu, 4, 182 (1960). (4A) J. H. Eggers, ;bid., p 38.
300
400
500
WAVELENGTH, N M Figure 1. Excitation and emission spectra of reagent and zirconium complex at pH 5.5
+ +
A . Zr reagent. Excitation measured at 342 nm B. Zr reagent. Emission measured at 406 nm C. Reagent alone. Excitation measured at 332 nm D. Reagent alone. Emission measured at 442 nm
RESULTS AND DISCUSSION In order to allow for variation in the output intensity of the xenon arc source, all fluorescence intensities obtained were compared with that produced by a standard lO+M solution of quinine bisulfate in 0.1M sulfuric acid measured immediately afterward. Preparation of Zirconium Solutions. Zirconium in 3M hydrochloric acid solution does not polymerize on standing (5), and therefore stock solutions were prepared at this acidity. Dilute (10+M) solutions of zirconium, where polymerization is possible were, therefore, prepared fresh every day from the stock solution. No observable decrease in sensitivity was noted during the course of each day with these solutions. Spectral Characteristics. Figure 1 shows the excitation and emission spectra for the reagent and its zirconium complex in aqueous solution at pH 5.5. These spectra are uncorrected for variations in the emission characteristics of the lamp and the response characteristics of the photomultiplier. The relevant correction curves have been given by Chen (6). The blue fluorescence of Calcein Blue exhibits its excitation maximum at 332 nm and a fluorescence emission maximum at 442 nm. The zirconium complex shows its excitation maximum at 342 nm and fluorescence emission maximum at 406 nm. The presence of zirconium thus produces a large increase in the fluorescence emission at 406 mn. In order to obtain the largest difference in fluorescence emission between the zirconium complex and the Calcein Blue blank fluorescence at pH 5.5, it is necessary that the fluorescence emission be measured at 405 nm with excitation at 350 nm. Effect of pH. The intensity of fluorescence of the reagent alone and the zirconium complex was measured over a range of pH values obtained by adjusting the amount of ammonia or acetic acid added to the solution. Fluorescence intensities were measured at 405 nm with excitation at 350 nm within (5) B. C. Sinha and S . Das Gupta, Analyst, 92, 558 (1967). (6) R. F. Chen, Anal. Biochem., 20, 339 (1967). ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, J U N E 1970
785
Table I. Effect of Foreign Ions on Determination of Zirconium Molar Interference, Molar Interference, Ion excess excess
x
J
I I
I
,
I
I
1
5
3
I
I
7
9
,
PH
Figure 2. Effect of pH on fluorescence intensity (350/405 nm) of reagent alone ( A ) and zirconium complex ( B )
500 50 5
+128 +5.8
500 50 5 500
$100 +82 +22
...
...
500 50 5
-9.2 -2.5
500 50 5
quenched quenched
...
500 500 50 5
Y
I
I
I
I
I
I
r
a
2 4 6 ' IO MOLAR EXCESS CALCEIN BLUE Figure 3. Effect on fluorescence intensity (350/405 nm) of varying molar excess reagent (corrected for reagent fluorescence) 0
15 minutes of mixing the solutions. Figure 2 shows that the fluorescence of the reagent alone increases slowly with increasing pH up to ca. pH 6, above which a greater rate of increase of the intensity is observed. The zirconium complex shows maximum fluorescence at cu. pH 7. The maximum difference in fluorescence intensity due to the zirconium complex is observed when the solutions are adjusted to between pH 5.5 and 6.0. In the initial studies a sodium acetate-acetic acid buffer solution of pH 5.8 was used, and results of reasonable sensitivity and reproducibility were obtained. It was later found, however, that higher sensitivity was obtainable using the acetic acid-ammonia solution of pH 5.5 described here. Effect of Reagent Concentration. The effect of variation of the Calcein Blue concentration was studied using 1 ml of lO-3M zirconium solution and various amounts of 10-6M reagent solution at pH 5.5. Figure 3 shows the effect of the reagent excess on the fluorescence emission of the zirconium complex at 405 nm after subtraction of the fluorescence of the reagent blank. A concentration of reagent at least equimolar with the zirconium present is required, while it is clear that the use of large excesses of Calcein Blue (greater than three-fold excess over the zirconium concentration) results in some decrease in the net fluorescence intensity due to the zirconium complex. For the determination of 10 to 100 ng of zirconium, 1 ml of 10WMreagent solution was, therefore, employed. 786
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
Mg
Mn
Na NHdC Ni Pb
Sn(1V)
-5.0
Sr
...
Te(IV)
+25 +12
...
Th
500 50 5
quenched quenched +5.0
Ti
500
-5.0
TI
500
quenched
50 5
-41 -4.0
500 50 5 500 50 5
quenched quenched
500
+5.0
500
+2.0
500
-3.0
-2.0
quenched quenched quenched
-78 -25 -5
... quenched quenched
500 50 5 500
500 500 500 50
-2.0 - 19 -5
500 50 500 50
+16 +3.0 - 13 -4
500 50 500 50 5 500 50 5
+25 -4.2 - 56 - 14 -5 +112 +83 +7.5
500 50 500 50 5
- 16
...
-4.5
++1147
+4.5
500 50
$6.5 +1.0
Zn
500
Br-
50 500
-4.0 -41.6 -4.0 -1.0
Cl-
500
...
c03'-
500
+l
F-
500
+15 +83 20 -2.0
~ ~ 0 2 500 -
I500 50 5 500
-9.0 -1.0 - 55 $1.0
uo*2+
...
500
500 50 500 50
VOawo12-
50 5 500 500 50 5 500 50 5
.o
+
quenched -18.3 -4.5
quenched quenched quenched
-62 +2.5 ~
Influence of Time. The variation in fluorescence intensity at 405 nm with time was investigated for dilute solutions of the zirconium complex developed under optimal conditions. A reduction in intensity of 10% was observed when the solutions were allowed to stand for two hours in darkness or in normal laboratory conditions-Le., under fluorescent tube lighting. Continuous irradiationof the solution at 350 nm by the Xenon arc in the spectrofluorimeter for a similar period caused a decrease in fluorescence intensity at 405 nm of 32%. All fluorescence measurements in this study were made within 15 minutes after mixing the solutions and reproducible results were obtained. Order of Addition of Reagents. The effect of the order of mixing of the solutions on the fluorescent intensity obtained
by the recommended procedure was investigated. The preferred order of addition of the reagents to the volumetric flasks was: Calcein Blue or zirconium, pH 5.5 solution, distilled water to volume. Under these conditions maximum fluorescence due to the zirconium complex is obtained immediately. When the Calcein Blue was diluted with pH 5.5 solution before the addition of the zirconium solution, very little fluorescence was observed initially for the complex. The fluorescence was then found to increase with time and reached a maximum more than one hour after mixing. Effect of Temperature. In all experiments, the temperature of the solutions was 21 =t3 “C. No significant variation of fluorescence intensity at 405 nm with temperature was noted, but no specific study of temperature effects was made. Precision. The recommended procedure was applied to the repetitive determination of 100 ng of zirconium over several days. The eight determinations produced a coefficient of variation of *1.8%. Effect of Foreign Ions. The effect of 30 cations and 12 anions on the determination of 50 ng of zirconium by the recommended procedure was investigated. A 500-fold molar excess of the foreign ion was employed. An ion was considered to interfere at this level when it produced an error in the fluorescence intensity compared to that of 50 ng of zirconium alone of greater than 3 times the standard deviation (5%). The effect of those ions which were found to interfere was subsequently reinvestigated at lower concentrations, uiz. 50-fold and 5-fold molar excess over 50 ng of zirconium. The results are shown in Table I. Thorium, aluminum, and fluoride cause positive interference even at the 5-fold molar excess level, while iron(II1) and tungstate completely quench the fluorescence of the zirconium complex at this level. Phosphate also causes a serious negative interference at the 5-fold molar excess level. The most unexpected interference was that of fluoride. In many other methods for the determination of zirconium, fluoride and phosphate cause serious interference by the formation of stable anionic complexes with the zirconium. In the procedure reported here, phosphate does bleach the reaction in the expected manner, but fluoride gives a positive interference.
The separation of interfering ions in the determination of trace amounts of zirconium has received considerable attention (7). A highly selective extractant for micro amounts of zirconium is thenoyltrifluoroacetone (TTA). Zirconium has been separated from aluminum, iron, the rare earths, thorium and uranium in 6M HCl by extraction with n A in xylene (8) and back-extraction of the zirconium from the organic phase with 0-1M HC1. Any small amount of residual TTA in the O.lMHC1 may then be removed by extraction with a single aliquot of xylene. Freund and Miner (9) have also separated zirconium from iron and aluminum by ion-exchange, Structure of the Complex. The nature of the complex formed between zirconium and Calcein Blue was investigated by the mole-ratio (10) method. The results, after correction for the reagent fluorescence, indicate the formation of a 1:1 complex between zirconium and Calcein Blue. Similar results were obtained by the method of continuous variations (11). CONCLUSIONS
The determination of trace amounts of zirconium by the spectrofluorimetric method with Calcein Blue reported here is very sensitive and moderate amounts of many ions may be tolerated without interference. The sensitivity compares favorably with those of other procedures reported for the spectrofluorimetric determination of zirconium with similar instrumentation. RECEIVED for review December 5, 1969. Accepted February 16, 1970. We thank the Ministry of Technology for financial support of this work, and the Science Research Council for an equipment grant for the purchase of the spectrofluorimeter used in the work. (7) R. B. Hahn, “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Ed., Part 11, Vol. 5, Interscience, New York, N. Y., 1961. (8) F. L. Moore, ANAL.CHEM., 28,997 (1956). (9) H. Freund and F. J. Miner, ibid., 25,564 (1953). ANAL.ED., 16, (10) J. H. Yoe, and A. L. Jones, IND.ENG.CHEM., 111 (1944). (11) P. Job, Ann. Chirn. Frame, 9, 113 (1928).
Coulometric and Titrimetric Reduction of Iridium Fumed in Perchloric Acid Edward J. Zinser and John A. Page Departmetit of Chemistr.y, Queen’s UriiGersity, Kingston, Otitario, C‘ariadn
MOSTOF THE TITRIMETRIC methods for iridium have utilized the reduction of Ir(1V) to Ir(III), with iron(I1) the recommended titrant ( I ) . These methods have been mostly applied to the assay of pure salts, and little attention has been paid to obtaining the Ir(1V) required for the titration. Reagents that have been used to give Ir(1V) for titration include Cl., with the excess removed by boiling ( 2 ) , and aqua (1) F. E. Beamish, “The Analytical Chemistry of The Noble Metals,” Pergamon Press, Oxford. 1966. (2) D. I. Ryabchikov, Zhur. Anal. Khim., 1, 47 (1946); Chern. Abstr., 43, 2542 (1949).
regia, where the excess was removed by evaporation (3). Heating with an excess of Ce(1V) after an H2S04 fuming has also been used with conflicting claims of an Ir(1V) product ( 4 ) and a species that could be titrated through the states +4.5 to +3.5 to +3.0(5). A particularly useful oxidizing agent is fuming perchloric (3) G. Milazzo and L. Paolini, Rend. Zst. Super. Sariita (Rome), 12,693 (1949); Chem. Abstr., 44,6337 (1950). (4) N. K. Pshenitsyn, S. I. Ginzburg, and L. G. Sal’skaya, Russ. J . Inorg. Chem., English transl., 4, 130 (1959). (5) W. A. E. McBryde and M. L. Cluett, Can. J . Res., 28B, 788 (1950). ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
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