Table I. Comparison of X-ray, Spectrophotometric and Spectrographic Results Mo uum
Sample No 1 2 3 4 5
6 7 8
9
X-ray with inter. Spectrostandard photometric 473 480 1250 1200 540 450 140 127 1200 1100 825 900 740 700 230 250 ... 690
Spectrographic 400 1040 360 120 970 645 492 180
...
X-ray direct
... 1230 470 90 1290
...
720 ... 670
molybdenum peak to the scattered radiation as the ordinate us. the concentration of the molybdenum as the abscissa. From the counting rate of the unknown samples we read the molybdenum concentration from the graph. COMPARISON WITH OTHER ANALYTICAL METHODS
molybdenum in lubricating oils, the procedure described by Meeker and Pomatti (10) was adopted in our laboratory. Agreement is in the range of the precision of the X-ray spectroscopy. The comparison does not indicate that any method is less accurate than the other three. The four methods are all reasonably accurate, and the choice among them will depend on convenience, cost, and required sensitivity. CONCLUSION
X-Ray fluorescence analysis greatly reduces the time required for the analysis of molybdenum in lubricating oils. It is possible to do molybdenum analysis directly on lubricating oils samples. If reliable and more accurate results are required, the addition of an internal standard such as yttrium is necessary. The simplicity of the method is an important feature. Sample preparation and operation of‘the X-ray equipment are both simple procedures that require a minimum of technical skill. The chief limitation of the direct method is that a calibration curve must be prepared for each type of oil.
The molybdenum contents of lubricating oils obtained by the internal standard and direct methods are compared with those obtained by spectrophotometric (9) and spectrographic methods in Table I. For the spectrographic determination of
RECEIVED for review October 17, 1969. Accepted February 10, 1970. This report was presented at XV Colloquium Spectroscopicum Internationale, Madrid, May 1969.
(9) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, p 644, 1959.
(10) R. F. Meeker and R. C. Pomatti, ANAL. CHEM.,25, 151 (1953).
Mass Spectrometric Determination of Zirconium Hafnium and Titanium in Mixtures Using the Integrated Ion Current Method M. G . Allcock,l Ronald Belcher, J. R. Majer, and Roger PerryZ Chemistry Department, University of Birmingham, P.O.Box 363, Edgbaston Birmingham, 15, England BECAUSE of the closely similar size of the ionic and atomic radii of zirconium and hafnium, the properties of the compounds of these two metals are very similar. As a result the separation of the two metals is difficult and analytical procedures for the determination of one metal in the presence of the other have been difficult to devise. Methods involving differences in molecular weight have been suggested in which a mixture of the pure metal salts is weighed and converted quantitatively to a mixture of a second type of compound [e.g. oxide (1-3)]. The second mixture is then weighed and the relative amounts of the two metals calculated knowing their atomic weights and the appropriate conversion factors. A variation Present address, Dudley College of Education, Dudley, Worcestershire. Present address, Department of Civil Engineering, Imperial College of Science and Technology, London. (1) R. B. Hahn, ANAL.CHEM., 23, 1259 (1931). (2) S.Fujiwara, Chem. Absfr.,45, 4171 (1951). (3) Ibid.,46, 10047 (1952). 776
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7 , JUNE 1970
of this technique uses the molar extinction coefficients of complexes of the metals with dyes such as Alizarin Red S and Xylenol Orange ( 4 , 5) rather than their molecular weights. An alternative approach is to utilize differences in stability of metal complexes in solution by measuring absorbances at different pH values (6, 7). Among the physical methods which have been recommended are X-ray emission and fluorescence (8,9), emission spectroscopy (IO),and neutron activation (11).
(4) H. Freund and W. F. Holbrook, ANAL.CHEM.,30, 462 (1958). ( 5 ) R. Z. Bachmann, ibid.,37, 103R (1965). (6) A. Mayer and G. Bradshaw, Analyst, 77, 476 (1952). (7) L. J. Kononenko, R. S. Lauer, and N. S. Pluektov, Chem. Abstr., 54, 9603 (1960). (8) G. deHevesey, “Chemical Analysis by X-rays and its Applications,” McGraw-Hill, New York, 1932. 22,1017 (1950). (9) L. Birks and E. Brooks, ANAL.CHEM., (10) G. W. Kingsbury and R. B. Temple, J. Appl. Chem., 25, 296
(1953). (11) W. D. Mackintosh and R. E. Jervis, ANAL.CHEM.,30, 1180 (1958).
Ti L, I
u 46 47
D
a
-
4
II
HI LzF
Hf L 4 Hf L,F,
I 176177178179 1
L3
.I
I
u i 844
1040 mie
Figure 1. SimpliRed mass spectra of titanium, zirconium, and hafnium derivatives of benzoyl trifluoroacetone
EXPERIMENTAL
Preparation of Metal Chelates. Five grams of the metal halide was dissolved in 200 ml of dry carbon tetrachloride in an atmosphere of dry nitrogen. The stoichiometric amount of benzoyl trifluoroacetone in 25 ml of dry carbon tetrachloride was added slowly with stirring. After the evolution of hydrogen chloride had ceased, the mixture was boiled under reflux for 30 min. The solvent was removed at reduced pressure in a rotary evaporator and the residue was purified and collected by vacuum sublimation at 0.05 Torr. Determination of Metal Chelate. The mass spectrometric method used for the determination of zirconium, hafnium, and titanium was that described originally by Majer and Jenkins (12) and subsequently used in the examination of the volatile chelates of the alkali metals and the rare earths (13, 14). In the construction of the calibration graphs, standard solutions of the three pure metal chelates were prepared in carbon tetrachloride (1 pg/pl). These solutions were diluted as required and volumes in the microliter range were injected into the direct insertion probe of an A.E.I. MS9 mass spectrometer using a Hamilton microsyringe. The probe was
Materials. Benzoyl trifluoroacetone was obtained from Kochlight Laboratories Ltd. Titanium tetrachloride was obtained from Hopkin and Williams Ltd. Zirconium tetrachloride and hafnium tetrachloride were supplied by Kingsley and Keith Laboratories, Inc.
(12) A. E. Jenkins and J. R. Majer, Talunfu,14, 777 (1967). (13) R. Belcher, J. R. Majer, R. Perry, and W. I. Stephen, Anal. Chim. Acfa., 43, 451 (1968). (14) Zbid., 45, 305 (1969).
The X-ray methods are complicated by the difficulties of preparing suitable targets and the overlap of the X-ray fluorescence spectra. The determination of the two metals by emission spectroscopy is complicated by the difficulty of preparing suitable electrodes and the refractory nature of the metals leading to incomplete or irreproducible evaporation. The neutron activation technique is the most successful of those proposed but involves elaborate apparatus and lengthy counting procedures. There is a need for a simple rapid method of determining hafnium in the presence of zirconium because of the use of the latter metal in the construction of fuel rods for nuclear reactors and the very great difference in the thermal neutron cross section of zirconium (0.18 barn) and hafnium (105 barns). The present paper describes such a procedure and has the further advantage that the determination may be made in the presence of other metals (such as titanium) yielding volatile chelates.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, J U N E 1970
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Table I. Precise Masses of Characteristic Ions Metal chelate Ion used Exact mass Ti(Btfa)? Ti(Btfa)zF+ 497.010318 Zr(Btfa)4 Zr(Btfa)*F+ 538.966629 Hf(Btfa)4 Hf(Btfa)*F+ 629.003167 Table 11. Limits of Detection of Metal Chelates Metal chelate Limit of detection, gram Ti(Btfa)s 1 x IO-" 5 x 10-14 Zr(Btfa)a Hf(Btfa)4 5 x 10-12 Table 111. Analysis of Mixture Amount Amount Chelate introduced, pg detected, pg Ti(Btfa)a 2.0 1.7 Zr(Btfa)4 2.0 1.82 Hf(Btfa)4 3.0 3.1
then inserted into the vacuum system of the spectrometer and the solvent pumped away. The mass scale of the instrument was calibrated using a standard substance in one of the alternative inlet systems (heptacosafluoro-tri-n-butylamine). The probe and sample were then lowered into the heated ion source and the ion current at a selected m/e value was recorded during the evaporation of the sample (1-2 min). The area under the peak of the recorder trace is proportional to the integrated ion current and to the amount of metal chelate evaporated. Similar aliquots of the sample mixture were evaporated and the records at three characteristic m/e values were measured independently. The amounts of the three chelates in the mixture were calculated by comparison of the peak areas with those of the calibration graphs. RESULTS AND DISCUSSION
When the integrated ion current technique is used, a recording is made of the rise and fall of the ion current at a selected m / e value during the course of the evaporation of the sample. The operator must be able to initiate the evaporation at will by lowering the sample into the heated region of the source. If the sample is too volatile, it will evaporate during the introduction sequence; if it is too involatile, it will evaporate at a rate too slow for the measurement of a peak in the integrated ion current record. The metal chelates of zirconium, hafnium, and titanium examined in this work fulfilled these requirements, being evaporated completely at a temperature of 220' C. The sensitivity of the technique depends upon a number of variables which include, apart from the instrumental sensitivity, the m/e value selected for measurement and the isotopic composition of the elements in the metals being examined. The intensity of the ion current observed at any m/e value for a given pressure of the sample in the ion source depends only on the voltage of the ionizing electrons and is normally constant. The m/e value which is selected for measurement should be one which is sufficiently characteristic of the sample to make misidentification of the sample unlikely and at the same time the ion current observed at this value should be a high proportion of the total ion current produced. That is to say, when a normal mass spectrum is recorded, there should be an intense peak at the selected m/e value. It is not always possible to reconcile these two requirements. The most characteristic m/e value would be that of the molecule ion itself, but frequently the molecule ion peak in a spectrum is not sufficiently 778
ANALYTICAL CHEMISTRY, VOL. 42, NO, 7, JUNE 1970
large. It is fortunate that the mass spectra of the three metal chelates examined are quite separate and no two spectra contain a peak at the same precise mass value, provided that the ion responsible contains a metal atom. It is thus possible to integrate the ion current for any such peaks without any interference. Figure 1 illustrates simplified mass spectra of the three metal chelates. The molecule ion in each case dissociates by the loss of one of the ligands to give an ion ML3+ which rearranges with the transfer of a fluorine atom to the metal accompanied by loss of the appropriate even electron fragment. This process is repeated with the transfer of a second fluorine atom to the metal and the appearance of an ion MLF2+. The transfer of atoms in metal chelates upon ionization has been observed previously by McDonald and Shannon (15) who demonstrated the formation of such ions as AlL(OH)+ upon ionization of aluminium acetylacetonate. The importance of these dissociative processes in the present work was that they provided a large proportion of the total ion current at distinctive and accessible m/e values in the mass spectrum. Table I gives the constitution and precise masses of the ion responsible for the mass peaks used for measurement in the present work. The calibration curves obtained by plotting the weight of pure metal chelate evaporated against the peak area on the integrated ion current record are linear over the range 0.510.0 pg. This record was obtained using the least sensitive ranges of the mass spectrometer and provides calibration in the microgram range. The use of other ranges of the instrument provides similar graphs in the nanogram range. The differences in sensitivity as shown by the slopes of the three graphs are due to differences in isotopic constitution of the metal and to differences in the contributions of the ion current at ML2F+ to the total ion current. Thus, for example, the mass peak chosen for zirconium with an integral mass of 539 is due to an ion current which is only 51.4 % of the sum of all ion currents for ions of the general formula ML2Ff. The corresponding value for titanium is 74% and that for hafnium is 37.1 %. Despite this advantage in isotope ratio, the sensitivity obtained for the measurement of titanium was very much lower because of the low intensity of the mass peak corresponding to the ion TiL2F+; this is due to the lower probability of dissociation of the molecule ion TiL3+ to give the rearrangement ion with transfer of a fluorine atom. It is significant that the large peak in the mass spectrum of the titanium chelate is due to the ion TiLF+ formed after loss of the first ligand fragment. This difference in sensitivity is also reflected in the limits of detection achieved for the three metal chelates. The very high sensitivity obtained with this technique is partly due to the fact that there is a considerable discrimination against noise (random excursions of the base line) in that measurements are made only during evaporation. Table I1 lists the limits of detection and reveals that the sensitivity for titanium is almost two orders of magnitude less than that for zirconium. Table I11 gives the results of an analysis of a mixture of the three metal chelates. Measurement was made in the microgram range with an accuracy of the order of 10-15 %. The present work was based on the conversion of the metal chlorides to their diketonates in nonaqueous solution. The technique is, however, not restricted to this procedure; zirconium and hafnium may be separated from all other metals by precipitating from aqueous solution with reagents such as mandelic acid. The two metals may then be converted to (15) C. G. McDonald and J. S. Shannon, Aust. J. Chem., 19, 1545 (1966).
their oxides and, hence, to volatile chelates of the form ZrO& (where X is the residue of a fluorinated diketone). It is possible to determine such chelates using the same method and, hence, to estimate zirconium and hafnium in aqueous solution. Work is proceeding upon the determination of zirconium and hafnium in alloys and ores by this procedure.
ACKNOWLEDGMENT
The authors are indebted to M. J. A. Reade for assistance in spectrometry.
RECEIVED for review December 15, 1969. Accepted February 16,1970.
Ferrozine-A New Spectrophotometric Reagent for Iron Lawrence L. Stookey Hach Chemical Company, 713 South Duff Avenue, Ames, Iowa 50010
SINCETHE LATTER part of the nineteenth century, organic molecules containing a certain atomic configuration, namely I
-N=C&N-, have been known to react as bidentate ligands with certain metal ions such as ferrous, cuprous, and cobaltous, to give colored complex species (1-6). This effect was first noticed with the ferrous ion ( I ) , and since this complex of a given compound is generally of a more intense color than are those with other ions (7), the atomic configuration in question was given the trivial name of the ferroin group. During the last century, hundreds of compounds containing the ferroin group have been synthesized (7), the majority of which demonstrate the ability to form complexes with the ferrous ion. Most of these complexes are only weakly colored, are unstable under normal physical conditions, or are formed over a very narrow pH range. A few of these compounds, however, form stable, intensely colored species with the ferrous ion and are, therefore, suitable for the quantitative determination of iron (8). Examples of compounds which have found acceptance for such use are 1,lOphenanthroline (9), 4,7-diphenyl-l,lO-phenanthroline(IO), 2,2‘-bipyridine (I]), 2,6-bis(2-pyridyl)-pyridine (11). 2,4,6tris(2-pyridyl)-1,3,5-triazine (12), and phenyl 2-pyridyl ketoxime (13, 14). Many of these reagents are the product of difficult and tedious organic syntheses and, hence, are high in cost (14). Although for even the most expensive of these reagents, the cost of a single manual analysis is fairly low, when one considers automated continuous instrumental analysis, a lowcost yet highly sensitive reagent would be desirable, since the cost of maintaining such an analytical instrument with (1) F. Blau, Monatsh., 19, 666 (1898). (2) A. Smirnoff, Helu. Chim. Acta, 4, 802 (1921). (3) L. Tshugaeff, Ber., 39, 3382 (1906). (4) F. Burstall, J . Chem. Soc., 1938,1662. (5) F. Case, J . Amer. Chem. Soc., 68,2574 (1940). (6) A. Pinner, Ber., 27,984 (1894). (7) A. Schilt, Talanra, 13,895 (1967). (8) A. Schilt and W. Hoyle, ANAL.CHEM., 39, 114 (1967). (9) W. Fortune and M. Mellon, IND. ENG. CHEM.,ANAL.ED., 10, 60 (1938). (10) G. Smith, W. McCurdy, and H. Diehl, Analyst, 77,418 (1952). (11) M. Moss and M. Mellon, IND. ENG. CHEM.,ANAL.ED., 14, 862 (1942). (12) P. Collins, H. Diehl, and G. Smith, ANAL.CHEM.,31, 1862 (1959). (13) R . Chernin and E. Simonsen, ibid.,36, 1093 (1964). (14) H. Diehl and G. Smith, “The Iron Reagents,” The G. Frederick Smith Chemical Company, Columbus, Ohio, 1960.
Figure 1. Structure of ferrozine
presently available reagents becomes prohibitive. The purpose of this work, therefore, was the synthesis of a colorimetric reagent for iron which would have a sensitivity comparable to that of the 2,4,6-tris(2-pyridyl)-1,3,5-triazine (commonly known by the acronym TPTZ) now in limited use in automatic analysis (13, but which would also cost substantially less than other reagents commercially available. Such a reagent was found in the disodium salt of 3-(2pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine, hereafter referred to as ferrozine (Hach Chemical Co., Catalog No. 2304). This compound reacts with divalent iron to form a stable magenta complex species which is very soluble in water and may be used for the direct determination of iron in water (Figure 1). EXPERIMENTAL
Preparation of Ferrozine. The parent compound 3-(2pyridyl)-5,6-diphenyl-l,2,4-triazinewas prepared according to Case (16). This compound was subsequently sulfonated and isolated as the disodium salt. The product is a light yellow powder which melts with decomposition above 350 “C and may be recrystallized from water. Anal. Calcd for C20H12N4S20sNaz:C, 46.70; N, 10.89; and S, 12.46. Found: C, 46.78 and 46.64; N, 10.84 and 10.89; and S, 12.50 and 12.58. Physical Characteristics of the Ferrous Complex. The visible absorption spectrum of the ferrous complex of ferrozine exhibits a single sharp peak with maximum absorbance at 562 nm. At this wavelength, the molar absorptivity is (15) J. Dunbar, “Automated Continuous Determination of Total Copper and Iron in Boiler Feedwater,” Technicon Symposium on Automated Analytical Chemistry, Technicon Instruments Co. Ltd., Hansworth Lane, Chertsey, Surrey, England, 1965. (16) F. Case,J. Org. Chem., 30,931 (1965). ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
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