Use of polarographic methods for the ... - ACS Publications


Mar 16, 1970 - Calibration curves were linear over the tin range studied except near the lowest limit of detection where curvature was observed. Tin(I...
1 downloads 0 Views 576KB Size


anodic wave is being observed, as is expected, but with ac stripping, it is not possible to distinguish between cathodic and anodic electrode reactions. EDvalues in both ac and dc stripping were found to be independent of Sn(IV) concentration. Peak heights of both dc and ac scans were investigated as a function of Sn(1V) concentration. Calibration curves were linear over the tin range studied except near the lowest limit of detection where curvature was observed. Tin(1V) could be determined from 10-3-10-7M by dc stripping and from 10-8M by ac stripping. The higher sensitivity of the ac method was due to the much lower background current observed compared with the dc method. Alternating current stripping analysis, because of the lower limit of detection and higher sensitivity would appear to be more suitable than dc stripping analysis of tin(1V). Reproducibility studies of both methods were similar and no advantage in this direction was found for either method. Although ac stripping analysis of tin(1V) can be carried out in the range 10-3-10-8M, only the range 10-6-10-8 should be analyzed by this method. The range 10-3-10-6M would best

be done by conventional ac polarography as this method is much faster, more accurate, convenient, and reproducible than stripping analysis. CONCLUSIONS

Tin(1V) solutions can be analyzed polarographically down to 10-8M in hydrochloric acid supporting electrolytes. Inverse ac (anodic stripping) polarography with a hanging drop mercury electrode is recommended for analysis for tin in the range (10-6-10-8)M. For higher concentrations ( l o + 1OP6)M,conventional ac polarography with a dropping mercury electrode of natural drop frequency is recommended. Other polarographic methods of ac and dc rapid ,polarography, dc conventional polarography, and inverse dc polarography have also been investigated but for analysis for tin(1V) were not as useful as the above-mentioned methods. RECEIVED for review September 8, 1969. Accepted March 16, 1970.

Use of Polarographic Methods for the Determination of Tin in Geological Samples A. M. Bond, T. A. O’Donnell, and A. B. Waugh Department of Inorganic Chemistry, University of Melbourne, Parkville, Victoria, 30.52, Australia R. J. W. McLaughlin Department of Geology, University of Melbourne, Parkville, Victoria, 30.52, Australia Quantitative methods for determination of tin in geological samples, by polarographic methods, have been established. Conventional alternating current polarography was found to be highly selective and specific for tin in rock samples examined, enabling quantitative determinations to be achieved rapidly and directly without any prior separation of the tin. The method has been used for the geochemical survey of tin on a large number of samples. A comparison was made of several methods of fusion used to prepare tin solutions prior to polarographic analysis. Results obtained by inverse ac and dc polarographic methods using a hanging drop mercury electrode are also described for determination of tin in trace amounts. Use of hydrofluoric acid as a solvent, and rapid ac or dc polarographic techniques provided rapid, simple, and reliable estimations of tin. MOSTEXISTING methods for determination of tin in geological samples are tedious and time consuming as they require preliminary extraction and concentration of tin ( I ) , and are subject t o considerable error because they are nonspecific and because of the multistage processes necessary t o complete the analysis. Tin exists in rock samples almost exclusively in oxidation state (IV). Fusion techniques, such as sodium carbonatesodium peroxide fusions, however, ensure that tin present in (1) M. Farnsworth and J. Pekola in “Treatise on Analytical Chemistry,’’ Part 11, Vol. 3, I. M. Kolthoff and P. J. Elving, Ed., Interscience, New York/London, 1961. 1168

solution is in oxidation state (IV). In conventional chemical analysis, tin(1V) is reduced t o tin(I1) and determined volumetrically against a standard solution of iodate (1-3). The volumetric method is not specific for tin, being subject t o interference from a considerable number of species ( I ) unless specific preliminary procedures are used to prepare fairly pure tin solutions. Gravimetric methods ( I ) and colorimetric methods ( I ) are in use for determination of relatively small amounts of tin; but these methods lack selectivity and sensitivity, and again preliminary separation is generally required. Methods based on atomic absorption ( 2 ) , emission ( I ) , and X-ray spectroscopy ( I ) have been used. Bowman ( 2 ) has applied the atomic absorption method to tin ores and concentrates and has used it for routine analysis. Heggen and Strock ( 4 ) and Mitchell (5) have used spectrographic methods of analysis of tin in geochemical and other samples; but again prior enrichment and separation were necessary ( I , 4 , 5). Recently, neutron activation analysis has been used (6) for determination of tin in geological samples.

(2) J. A. Bowman, Anal. Chim. Acfa, 42,285 (1968). (3) I. M. Kolthoff and E. B. Sandell, “Textbook of Quantitative Inorganic Analysis,” 3rd Ed., Macmillan, New York, 1952, p 604. (4) G . E. Heggen and L. W. Strock, ANAL.CHEW,25, 859 (1953). (5) R. L. Mitchell, Analysf,71,361 (1946). (6) 0. Johansen and J. Richardson, ibid.,94,976 (1969).

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970

In these laboratories a rapid simple method was required which could be applied without any concentration or separation of tin for the purposes of a geological survey. Rock samples for analysis ranged from tin ores to those in which traces of tin were present. Since none of the available methods met all of these requirements, several polarographic techniques for the determination of tin(1V) were investigated and compared(7). Conventional ac polarography was found to be the most accurate, reproducible, and selective technique. Inverse or anodic stripping ac polarography was found to be considerably more sensitive than conventional ac polarography, but it is less accurate and reproducible and was recommended only for very low tin(1V) concentrations. However these polarographic results previously reported were for pure tin(1V) solutions. Rock samples normally have high concentrations of other entities which may or may not interfere with the polarographic determination, so the feasibility of direct application of polarography to determination of tin in rock samples was investigated. Ferret and Milner (8) had previously directly determined tin over the range 0.0005 to 0 . 2 6 z in steel samples by square wave polarography technique similar in some ways to ac polarography, since both provide considerably higher specificity and sensitivity than conventional dc methods. Love and Sun (9) had used conventional dc polarography to determine directly the tin in ores and had reported relatively little interference. Other workers have found that dc polarography can be used after preliminary separation of tin [e.g. reference ( l o ) ] . Direct application of the reported ac methods (7)to rock samples which varied widely in tin concentration and nature of foreign ions was expected to be a substantial improvement on dc methods from the point of view of specificity, accuracy, and reproducibility. Apart from the actual analysis, a major problem in the determination of tin is that fusion of the rock sample is usually necessary for solution preparation. Most of the fusion methods used leave some undissolved material and it is usually assumed that the residue does not contain tin, but may be an insoluble compound such as barium or calcium sulfate, calcium fluoride, graphitic carbon, silicon, or intractable minerals such as tourmaline, etc. However this assumption is not necessarily valid and this aspect of analysis for tin in geological samples was also investigated by comparing various fusion methods. EXPERIMENTAL

Standard tin(1V) solutions were prepared from ammonium chlorostannate(1V). All chemicals used were of reagent grade purity. Polarographic measurements were made at (25.0 k 0.1) "C and solutions were de-aerated with oxygenfree nitrogen, unless otherwise stated. The instruments and procedures used were as previously reported (7). Atomic absorption measurements at the 2246.1-A line for tin were made in the nitrous oxide-acetylene flame with an AA-4 Techtron atomic absorption spectrophotometer. Geochemical samples of the following type were obtained for tin determination: (i) Tailings from State Battery, Irvinebank, Queensland, Australia, being sand, silt, and clay samples. (ii) Granodiorite (GSP-l), Andesite (AGV-l), and a Dibase (W-1) (American Rock standards) (11, 12). (7) A. M. Bond, ANAL.CHEM., 42, 1165 (1970). (8) D. J. Ferret and G. W.C. Milner, Analysf, 81, 193 (1956). 27, 1557 (1955). (9) D. L. Love and S. C. Sun, ANAL. CHEM., (10) S. Kallmann, R. Liu, and H. Oberthin, ibid., 30,485 (1958). (11) M. Fleischer, Geocliim. Cosmochim. Acm, 29, 1263(1965). (12) F. J. Flanagan, ihid., 31, 289 (1967).

Table I. Comparison of Results Obtained by AC Polarography and Atomic Absorption Spectrometry Sample no. 1 2

3

za

AC polarography, 0.876,0.879,0.876 0.748, 0.744,0.746 0.686,0.687,0.688

Atomic absorption spectrometry, z0 0.883,0.871,0.876 0.757, 0.743,0.755 0.690,0.678,0.685

Triplicate results refer to consecutive determinations on the solution.

(iii) A large number of rock samples mostly containing extremely low tin concentrations from a geochemical survey of wide areas of Australia. In all tin determinations 0.5-1.0 gram of rock samples were fused, using various methods described later, and solutions of known volume, usually 20 to 50 ml, were prepared. Aliquots were made 5M in hydrochloric acid.

RESULTS AND DISCUSSION Quantitative Determination of Tin by AC Polarography. Following the procedure described previously (7), linear ac calibration curves for tin over the range to lO+M were obtained from standard solutions prepared with 5M HC1 as 2e Sn (0) supporting electrolyte and using the Sn(I1) wave. Reference to these calibration curves provides the basis of the analytical method for unknown samples. Geochemical samples, however, contain many elements other than tin, and the possibility of interference from these other entities was checked. For all tin ores and concentrates tested in this work, in which the percentage tin was greater than 0.005%, no interference from overlapping waves of foreign species was found. The peak potential of the ac wave, us. Ag/AgCl as reported previously (7), was observed between -0.4 and -0.7 V, depending on the nature of the supporting electrolyte, To test the validity of the analytical figures for tin in these rock samples, known standard tin concentrations were added to the rock samples and the solutions re-analyzed. Complete additivity of the tin concentration was observed in all cases, indicating absence of interference and the probable reliability of the direct analytical figure. Comparison of polarographic results with those using atomic absorption spectrometry (Table I) showed polarographic results to be more reproducible and, therefore, more preferable. The absorption method is less sensitive, and the high noise level militates against reproducibility. The limit of detection by the atomic absorption method of 0.02% (2) restricts the use of this method to tin ores and concentrates. The polarographic method is by comparison directly applicable to samples down to 0.0005 in tin. For geochemical samples containing less than 0.005 tin, ac polarograms were sometimes complex and several ac peaks were normally observed in the vicinity of the Sn (11) waves. Figure 1 shows a typical ac polarogram for such a system. Identification of the tin wave can be made by accurate location of the summit potential or, better, by deliberate addition of pure tin which increases the height of the tin wave without altering the summit potential of this wave, waves due to other species remaining unchanged. Once the tin wave has been located definitely, determination of tin can be carried out on solutions with tin concentrations greater than 10-6M, Le. on rock samples containing more than about 0.0005 2 of tin.

+ +

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970

1169

i

otherSn

-0.8

-0.6 -0.4

VOLT

-0.2

L

-1.0

Vs. Ag/AgCI

-0.6 VOLT Vs. Ag/AgCI

-0.2

Figure 3a

-10

- 0.6

-1.0

VOLT

Figures 3a and b. Typical inverse dc polarograms of tin in rock samples when tin is present only in trace quantities

Vs. Aq/AgCI

-0.6

- 0.2‘ I

VOLT Vs. Ag/AgCI Figure 2b

Figure 2a and b. Typical inverse ac polarograms of tin in rock samples when tin is present only in trace quantities Reproducibility of results was found to be that of 1 t o 2xnormal for ac polarography (13). Calibration curves were found t o be extremely reproducible over long periods of time and frequent check calibrations were not necessary. In routine analysis in these laboratories samples could be analysed by ac polarography at the rate of about twelve per (13) B. Breyer and H. H. Bauer, “Chemical Analysis,” Vol. XIII,

Alternating Current Polarography and Tensammetry, Interscience, New York/London 1963. 1170

-0.2

V O L T Vs. A g / A g C l Figure 3b

-0.2

Figure 2a

a1.0

-0.6

hour. This rate includes time for degassing and thermostating, although a much faster analysis rate could be obtained if these latter steps were eliminated. If ambient temperatures d o not vary greatly, there is little loss in precision, and the rate of routine analyses can be increased t o between 25 and 30 per hour. The high selectivity and specificity of the ac method for determination of tin in rock samples, which enables direct analysis without separation procedures, may well be due t o the fact that the ac method is sensitive only to reversibly reduced or oxidized species. Presumably most elements present other than tin are either reduced or oxidized reversibly at potentials very different from that of the Sn(I1) wave or they are irreversibly reduced or oxidized and, consequently, are not detected by the ac method. Quantitative Determination of Tin by AC Inverse (Anodic Stripping) Polarography. As reported earlier (7), use of this technique enables determination of tin down t o 10-EM. Since the inverse method is not generally as accurate, reproducible, or rapid as conventional ac polarography, it should only be used in the tin concentration range low6t o 10-SM. Therefore geochemical samples were examined for tin by the inverse ac method only when the levels were thought to be in the range 0.0005-0.00005 %. The procedure followed was that previously described (7). Figures 2 and 3 show typical inverse ac and dc polarograms obtained from rock samples, in which tin is present only in

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970

trace quantities. The contribution to the polarogram by elements other than tin and present in greater concentrations is indicated by the presence of several waves and the tin wave is much smaller in height than many of these other waves. To establish reliability of tin analyses, known standard tin concentrations were added. However analytical figures for the tin in rock, calculated from comparison with calibration curves and taking the added tin into account, were always low. A possible explanation of this discrepancy is that different electrode reactions occur during the pre-electrolysis time for the tin in standard solutions and tin in rock samples. For pure tin standards, only tin amalgam is formed but in rock samples a very complex amalgam may be formed in which tin is only a minor component. In support of this view, it was found that results obtained using the inverse ac method on tin ores, in which tin is a major component, were quantitative, and results were in reasonable agreement with those obtained by conventional ac polarography. To use the inverse ac method, standard additions are necessary and quantitative additivity of peak height must be assumed. This method is tedious and time consuming as addition of a standard amount is required for each sample. As shown in Table 11, however, comparison of results with conventional ac polarography in the concentration range where both methods can be used to 10-6M) shows that this method of analysis is satisfactory. Simple Quantitative Polarographic Determination of Tin in Samples Dissolved in Hydrofluoric Acid. Many samples including rocks can be dissolved simply and directly in concentrated hydrofluoric acid itself or in mixtures of it with other acids, eliminating the necessity of fusions. While in some cases dissolution is not complete, usually a major portion of the sample dissolves. After addition of concd hydrochloric acid, an ac polarogram of the hydrofluoric acid solution can be recorded by scanning the potential range -0.3 to -0.8 V us. Ag/Ag C1, and an ac wave for Sn(I1) will be detected if any tin is present. To ensure that the wave observed is that for tin, a solution of tin should be added and the polarographic scan repeated to check that the height of the appropriate wave increases. This scan should be performed using rapid polarographic techniques so that the glass D M E is only in contact with the solution for as short a time as possible to prevent etching. The effect of concentrated hydrofluoric acid solutions on the glass DME has been discussed in detail by Bond and O’Donnell

Table 11. Comparison of Results Obtained Using Inverse AC Polarography with Standard Additions and Conventional AC Polarography Inverse Conventional Sample No. polarography, polarography, 1 0.0082 0.0068 2 0.0052 0.0056 3 0.0030 0.0041

z

z

(14), and they have stated the advantages of techniques and the procedures to be followed. Comparison of Fusion Techniques Used Prior to Polarographic Tin Determinations. The conventional method of rendering rocks and minerals soluble has been fusion with sodium carbonate, either alone, or with various admixtures designed to increase the ljuxing, complexing or oxidizing powers of the melt (15). This normal method of rock and mineral analysis is laborious and time consuming and much effort has been expended in devising rapid analytical techniques (16). The problem still exists in such analyses of rendering the sample soluble while avoiding addition of elements to be estimated. A further problem arising is the ease with which silica may precipitate under certain conditions. Wet digestion methods, using hydrofluoric acid mixed with other acids such as perchloric, sulfuric, nitric, will dissolve most silicate materials with the advantage that silicon is removed by volatilization (16). However this digestion technique frequently leaves a residue of unreacted refractory minerals (17, 18), unless a pressure digestion is employed (19). The problem is aggravated further when dealing with materials containing carbonaceous matter, such as indurated shales. Even evaporation with perchloric acid will not oxidize all of this organic material, and a black residue with considerable absorptive capacity remains. For materials

(14) A. M. Bond and T. A. O’Donnell, ANAL.CHEM., 41, 1801 (1969). (15) W. F. Hillebrand, G. E. F. Lundell, H. A. Bright, and J. I. Hoffman, “Applied Inorganic Analysis,” Wiley, New York, 1959, p 860. (16) L. Shapiro and W. W. Brannock, U. S. Geof. Sum. Bull., 1144A, 1962, p 24. (17) G. K. Hoops, Geochim. Cosmochim. Acta, 28, 405 (1964). (18) V. S. Biskupsky, Anal. Chim. Acta, 33, 333 (1965). 40, I682 (1968). (19) B. Bemas, ANAL.CHEM.,

Table nI. Comparison of Tin Analyses after Different Preliminary Treatments of an Irvinebank Silt Tailing Sample Sample treatment

Description of solid residue

(i) 5 g of NazOzadded to 0.5- to 1.0-g sample. Mixture sintered,

fused, treated with water and acidified with concd HC1 (ii) 0.5- to 1.0-g sample fused in Pt crucible for 10 min with 2 g of boric acid and 3 g of sodium fluoride. Solid residue treated with 10 ml of concd H2S04and heated until no further evolution of B203. Residue dissolved in water and diluted to known volume (iii) As in (ii) but with NaF replaced by LiF (iv) 2 ml of HzSO~, 1 ml of “ 0 3 , and 10 ml HF added to 0.5- to 1.0-g sample in FTFE crucible. Digested overnight, evaporated to dryness. Residue dissolved in 5 ml of concd H2S04 and 10 ml of H,O. Evaporated to dryness, redigested, with HzSO4 and made up to volume (v) As in (iv) except that residue was heated to low red heat after first treatment with HzSO4 Results quoted are for separate samples in each case.

Tin by ac polarography,

Gelatinous residue

0.156,0.152,0;159

Trace of white solid

0.160,0.162,0.157

Small amount of white solid Carbonaceous solid

0.153, 0.156, 0.150 0.064,0.085,0.072

White solid

0.085,0;076,0.071

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970

z4

1171

containing organic matter, ignition must follow digestion. The problem of rendering the refractory materials soluble still remains, and certain minerals are very resistant, e.g., tourmaline, sillimanite, zircon. The use of sodium peroxide as a sinter and fusion mixture is frequently recommended, (15, 20, 21) but this reagent is frequently impure and not applicable to certain investigations. Recently, interest has centered on the use of lithium tetraborate or metaborate either with or without fluoride additions (18, 22). Fluoride addition allows removal of silicon by evaporation, and boron may be removed by a subsequent acid digestion (18). This fusion technique is very rapid and effective with most minerals. For the small number, e.g., chromite, which take a considerable time to react, the substitution of lithium fluoride by sodium fluoride gives a higher melting fusion mixture and reaction is extremely rapid. Table I11 gives the details of the preliminary treatment of a sample of Irvinebank tailings and allows a comparison of results. Results using sodium peroxide-sodium carbonate, lithium fluoride-boric acid, and sodium fluoride-boric acid fusions are equivalent, despite the fact that differing amounts and types of undissolved residue were left by each method. (20) R. J. W. McLaughlin and V. S. Biskupsky, Anal. Chim. Acta, 32, 165 (1965). (21) T. A. Rafter, Analyst, 75, 485 (1950). (22) N. H. Suhr and C. 0. Ingamells, ANAL.CHEM., 38,730 (1966).

This suggests that any of these three methods can be used satisfactorily. The lithium or sodium fluoride-boric acid is to be preferred because generally there was considerably less residue than with peroxide methods. The wet digestion methods, either on ignited or nonignited samples, invariably gave considerably lower tin values compared with the fusion methods and were not used subsequently for quantitative studies. Presumably this method does not provide complete dissolution of tin. Geochemical Survey for Tin. Conventional ac polarography has been used without the necessity to control temperature or to degas the solution to provide a cheap, simple, and reliable method of determination of tin in rock samples from a geochemical survey conducted over a wide area of Australia. Samples with tin contents down to 10 ppm were analyzed. Those in which tin could not be detected were simply classified as containing less than 10 ppm. For surveys of this type, currently being made in several laboratories, the attainable sensitivity of 5 ppm, or lower using stripping techniques, was not required, RECEIVED for review September 8, 1969. Accepted May 22, 1970. Acknowledgment is made of a grant from the Australian Research Grants Committee for the instruments used in this and the preceding paper (7).

Computer Approach to Ion-Selective Electrode Potentiometry by Standard Addition Methods M. J. D. Brandl and G . A. Rechnitz Department of Chemistry, State University of New York, Bufalo, N. Y. 14214 Methods are described for determining an unknown concentration using an ion-selective electrode without prior calibration of the electrode. The methods are based on standard addition procedures. In the simplest case, only two standard additions are required and a simple calculation i s described which can be performed by a computing calculator. To obtain high accuracy in the determination of unknown concentrations, multiple additions are made and a least squares curve fitting method is used to evaluate the unknown concentration, electrode slope, and standard potential. A computer program to accomplish this calculation, ADDFIT, is given in Fortran IV. The effectiveness of these methods is demonstrated by experiments on lead and chloride samples.

ANALYTICAL METHODS using ion selective membrane electrodes may be largely classified as direct determinations of ionic activity or indirect titrimetric procedures (1). Direct potentiometry is based on the relation of the measured electrode potential to the logarithm of ionic activity through the equation

E

=

E,

+ S log a

(1 )

I Present address, Research Department, Imperial Chemical Industries Ltd., Agricultural Division, Billingham, Teesside, U. K.

(1) R. A. Durst, in “Ion Selective Electrodes,” R. A. Durst, Ed., National Bureau of Standards Special Publication 314, Washington, D. C., 1969, p 375. 1172

where E, is the standard potential and S is the slope, usually near to the Nernstian (RT/nF). It may readily be shown (2) that an error of A 0.1 mV in measurement of the electrode potential leads to an error of 0.39 Z in the value of a, under Nerstian conditions with n = 1. However this accuracy is rarely achieved for a single determination except under ideal laboratory conditions with very stable electrodes. If the error is random, in principle it can be made as small as is desired by making a sufficient number of replicate determinations. Thus an accuracy of +0,2% has been obtained for the determination of silver at the 10-’M level (3). The differential technique of null point potentiometry (4, 5) has been applied to direct potentiometry with ion selective electrodes. This method can in theory achieve high accuracy but it has been shown ( 5 ) that with nonideal electrodes having different values of S, the accuracy is limited by the measurement of the cell null potential. Standard addition methods provide an alternative approach to obtaining high accuracy by direct potentiometry. Methods

*

( 2 ) T. S. Light, ibid., p 356. (3) D. C. Muller, P. W. West, and R. H. Muller, ANAL.CHEM., 41, 2038 (1969). (4) R. A. Durst, ibid., 40,931 (1968). (5) M. J. D. Brand and G . A. Rechnitz, ibid., 42, 616 (1970).

ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970