NOTES
I
Known Addition Ion Selective Electrode Technique for Simultaneously Determining Fluoride and Chloride in Calcium Halophosphate Lowell G . Bruton Syluania Electric Products, Inc., Towanda, Pa.
CALCIUM HALOPHOSPHATE [Ca10(F,C1)2(P04)61 is a phosphor used in the production of fluorescent lamps. The color of these lamps can be partly controlled by the fluoride and chloride content, hence the need for precise analysis of these elements. The chloride was analyzed previously by X-ray fluorescence. The fluoride has been analyzed previously by a microdistillation, colorimetric method and later by ionselective electrodes using the two-standard calibrated electrode technique which brackets the sample fluoride concentration. This paper describes a method for simultaneously determining fluoride and chloride in calcium halophosphate by the ion selective electrode known addition technique. Simultaneous analysis is achieved by using a n electrode switch in conjunction with a direct readout electrometer, and by preparing the standard addition t o contain all the ions to be measured, and last, by using the known addition technique which simplifies the operation by eliminating calibration. All calculations are reduced to a tabulated chart which converts the change in potential directly to total concentration. The tabulated chart is prepared initially by a computer. The result is a method which is both fast and simple and a relatively inexperienced operator can obtain good results. The theoretical limitations of the known addition, such as theoretical electrode slope, ionic strength, and complexation are discussed in relation t o the example and the precision and advantages of the technique are compared to conventional activity measurements. I n reviewing the literature o n this subject, it was found that relatively few papers have been published. Orion’s Newsletter (1-3) has done as much as any t o publicize and discuss the theory of this approach. Durst ( 4 ) has discussed the method in his book, and Brand and Rechnitz (5) recently reported o n a multiple addition-computer approach. However, very little was found on the practical applications of this technique t o real samples, and if the literature is any criteria, this method has been slow t o gain acceptance. Technical publications have concentrated on activity measurements and potentiometric titrations, and instrument manufacturers have responded with readout activity scales, and the advantages of known addition have either been ignored or overlooked. EXPERIMENTAL
Equipment. Orion Models 94-09 and 94-17 solid state fluoride and chloride ion electrodes and a Model 90-02 double (1) Newsletter, Specific Ion Electrode Technology, July 1969, Orion Research Inc., Cambridge, Mass. (2) Ibid.,February 1970. (3) Ibid.,July/August 1970. (4) R. A. Durst in “Ion-Selective Electrodes,” R. A. Durst, Ed., National Bureau of Standards Special Publication 314, Washington, D.C., 1969, p 381. (5) M . J. D. Brand and G. A. Rechnitz, ANAL.CHEM.,42, 1172 (1970).
junction reference electrode using 10% K N O j as a n external filling solution were used. An Orion Model 605 manual electrode switch allowed rapid switching of electrodes and the incorporation of a calibration potential. A Corning Model 101 digital electrometer was used (all that is necessary is a millivolt display accurate to 0.1 mV). Nalgene 100-ml beakers and volumetric flask and a 100-ml automatic measuring pipet were employed. Reagents. A known addition solution of 100.00 grams F-, 10.00 grams of Cl-/liter was prepared from anhydrous potassium fluoride (Allied Chemical Corp.) and potassium chloride and stored in 100-ml Nalgene volumetric flasks. A second solution of 10% H N O , for sample dissolution was stored in a large reservoir connected to a 100-ml automatic pipet. Procedure. A 1 .OO-gram sample of calcium halophosphate is weighed into 100-ml Nalgene beakers and dissolved with 100 ml of 10% HNO,(. The fluoride and chloride electrodes and a double junction reference electrode are connected to the electrometer cia the electrode switch. These electrodes are equilibrated in a sample solution until the electrode potential varies no more than 0.1 mV in 2 minutes (approximately 10-15 minutes). The electrodes are then placed in the unknown sample and equilibrated until a stable potential is obtained(30-40 seconds). The millivolt display for the F- and CI- electrodes is adjusted to zero with the calibration controls o n the electrode switch. [Normally, this would be recorded as the initial potential ( E l ) ; however, with the electrode switch the initial potential can be adjusted t o zero so the final potential (E2) will now be AE.] A 1.00-ml addition of the 100.00-gram F-, 10.00gram Cl-/liter solution is made and equilibrated 1 minute. The total concentration of the unknown F- and C1- is determined from a tabulated chart. Table I is a n abbreviated form of this chart, the actual form covers a range of 1-120 mV in 0.1-mV increments. The change in potential (AE) for the sample F- and CI- is located under the first column (AE) o n the chart, and the corresponding value in the next column (CtjCA) will be the total concentration (ZF, ZCI). I n this analysis, all calculations were eliminated by weighing a 1.00-gram sample and making the known addition a factor of ten (1 .OO ml of 100.00 grams of F-, 10.00 grams of CI-/liter standard addition). The final result is obtained by moving the decimal point of the Ct/CA value the appropriate distance. The tabulated chart (abbreviated in Table I) was prepared from a n expression which has elsewhere been derived ( I ) from the Nernst equation: CtjCA
=
1 (Antilog AEjS)
-3
where Ct is the total concentration, CA is the change in concentration, A E is the change in potential, and S is the Nernst factor or a theoretical electrode slope of 59.16 mV a t 25 “C. F o r a given change in potential (AE), the value of the expression l/[(Antilog AEjS) - 11 will be constant. Therefore, the calculation was performed by a computer to produce ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
579
1 (Antilog AEIS) - 1 or Ct/CA as a Function of Change in Potential CAE) for'Monovalent Electrodes and Theoretical Electrode Slope of 59.16 mV at 25 "C AE Ct/CA AE CtICA 1 .OO 25.1961 3.00 8 ,0740 22.8607 3.09 7.7980 1.10 7.5393 20.9146 3.20 1.20 7.2964 19.2679 3.30 1.29 17.8566 3.40 7.0677 1.40 6.8521 16,6334 3,50 1.50 15.5632 3.59 6.6485 1.60 6,4560 14.6189 3.69 1.70 13.7796 3.80 6.2735 1.79 1.90 13.0287 3.90 6.1005 12.3529 4.00 5.9361 2.00 11.7415 4.10 5.7798 2.09 11,1857 4.19 5.6309 2.20 10.6782 4.30 5,4890 2.30 5,3535 10.2131 4.40 2.40 5.2241 9.7852 4.50 2.50 9.3902 4.60 5,1003 2.59 2.70 9.0246 4.69 4.9817 2.80 4,8682 8.6851 4.80 2.90 8.3690 4.90 4.7593
Table I. Values for the Expression
~
Table 11. Comparison of Precision Obtained for F- Analysis in Calcium Techniques, Halophosphate by Known Addition and Calibrated Electrode F- by known additions F- by calibrated electrodes Sample 1 1 = Av RSD IZ= Av RSD A 4 3.05 0.5 4 3.09 0.7 2.99 B 4 0.7 4 2.99 1.3 3.16 0.5 C 4 4 3.15 1.0 a t i 1 = number of determinations.
a tabulated chart over the range of 1 to 120 mV in 0.1-mV increments showing CtIC4 as a function of 4 E . RESULTS AND DISCUSSION
Although the method of known addition solves some of the basic problems of conventional activity measurements, there are still problems of interference, complexation, and ionic strength which have to be understood in successfully designing a method for a particular sample. I n order to obtain precise values for total concentration (Ct), it is necessary to obtain precise values for all the components of Equation 1. The change in potential ( 4 E ) was found to be very important. Precise values for 4 E were obtained by digital readout equipment accurate to 1 0 . 1 mV; by expanding the tabulated chart (Table I) to a 0.1-mV accuracy; and finally, by optimizing the known addition so the change in potential ( 4 E ) will be as large as possible without altering the total ionic strength. The volume change accompanying the known addition (1.0 ml in 100 ml) was assumed negligible. The slope factor (S) was assumed to be the theoretical Nernst slope of 59.16 mV at 25 "C. A theoretical electrode slope was assumed because the limit of detection of the electrode had not been approached, and there were no appreciable electrode interferences. Therefore, the electrode response should approximate the theoretical Nernst slope. Recently, there has been some work (5) on a multiple addition-computer approach t o the problem of slope correction in the known addition. It needs t o be recognized however, that if you can't assume a theoretical slope and have to resort 580
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
to multiple additions, the advantages of this technique over activity measurements will rapidly diminish with increasing operations. The method of standard additions also requires a n understanding of the individual factors which contribute to the activity of an ion in solution ( 2 ): Ion activity
=
yfCt
(2)
where y is the activity coefficient of the ion, f is the fraction of the total concentration of the ion that is free, and Ct is the total concentration of the ion (free and complexed). It can be seen from Equation 2 that in order t o determine Ct, the standard addition must not alter the y and f factors, or y1E 7 2 andfi E h Maintaining a Constant Activity Coefficient ( 7 ) . The y is a function principally of the ionic strength of the solution. If the known addition does not significantly alter the total ionic strength, the y factor will remain essentially constant. The relative concentration of the ion to be measured as compared with the total ionic strength has to be considered. Where this ratio is unfavorable (high concentration, low ionic strength), the addition of a noninterfering salt has been suggested and also a method has been devised to test if the known addition has altered the y factor (2). I n the analysis of F- and CI- in calcium halophosphate, the relative concentration of these ions (3x, 0 . 5 x ) is low in comparison to the sample ionic strength. (Ca2+ and P043- ions.) Because the ratio is favorable, a more concentrated known addition can be made, which increases the precision of the measurement due t o a larger AE. Maintaining a Constant f Factor. The method works because the degree of complexation of the added species and the test species will be the same if there is a considerable excess of free complexing agent. The total concentration of the test species will be increased with the known addition, but the fraction of the test species complexed will remain the same. For example, in the mass action equation for F- in equilibrium with H+:
(3) Thef'factor in Equation 3 will depend upon the free complexing agent [H+] and not upon the total [F-] concentration. If the [H+] is in considerable excess, the addition of F- t o this system would not alter the equilibrium o r the fraction of the free fluoride ion. If there are several complexing agents present, it is necessary to determine which forms the strongest complex and that it is in excess. Any shift in the degree of complexation will produce error. In the analysis of calcium halophosphate, the phosphate competed with the F- and CIfor protons. It was determined empirically that if the [H+] was approximately 100 times greater than the fluoride ion, theffactor remained constant. The method of known addition offers several advantages over activity measurements. An improved precision (see Table 11) was obtained by controlled experiments. The improvement was attributed to running a standard with each sample. By the calibrated electrode technique, two standards are used to determine the slope and bracket the test ion concentration. A series of samples is then run and the instrument is rechecked for drift. Electrode calibration is not necessary in the addition technique. The method is faster than activity measurements. A technician can analyze 10-15 samples per hour for both fluoride and chloride, and by the previous method less than half that number could be analyzed
for fluoride alone. By combining the electrode switch and the addition technique, several ions may be analyzed by a single addition which contains all the ions to be analyzed and with the speed and ease of a single ion measurement. Previously, multiple ion analysis meant preparing a calibration curve for each ion by plotting activity us. millivolt readings, o r by calibrating a logarithmic scale with two standards to bracket the unknown. The known addition obviates these shortcomings and prepares the way for multiple ion analysis with millivolt readings. Ion selective electrodes measure ion activity, but the method of known addition gives total ion concentration. The problems of p H adjustment, ionic strength differences between sample and standard, and complex formation which are critical in activity measurements can be used to advantage by the method of known addition.
Table 111. Comparison of Data Obtained by Known Addition for F- and CI- to an X-Ray Fluorescence for C1- and a Microdistillation, Colorimetric Method for F- in Calcium Halophosphate % F- by F- by C1- by C1- by std add. microdist std add. X-ray Sample 1 2.95 2.95 0.93 0.90 2 2.88 2.87 0.77 0.75 3 2.90 2.83 0.78 0.77 4 2.89 2.85 0.80 0.80 5 2.92 2.85 0.84 0.82 6 3.03 3.03 0.59 0.58 0.59 0.58 7 3.03 3.03 8 3.04 2.95 0.51 0.48 9 3.07 3.10 0.50 0.49 10 3.01 3.01 0.55 0.54
PRECISION AND ACCURACY
for fluoride and 0.8 for chloride. In Table 111, the accuracy of the method is checked against X-ray fluorescence and a microdistillation colorimetric method. In conclusion, although the theory of the method is somewhat complicated, the actual practice is rapid, simple, and precise and well-adapted for multiple ion analysis.
The following data were obtained both by controlled experiments and by including standard samples with routine samples for analysis. In Table 11, the precision of the addition method is compared to activity measurements. On replicate analysis of 10 samples by the known addition, a relative standard deviation of 0.7% for fluoride and 0.4% for chloride was obtained. On a production basis, a standard sample submitted six times with routine samples over a 2month period, gave a relative standard deviation of 1.1
RECEIVED for review May 4, 1970. Accepted October 22, 1970.
Polarographic Determination of Perbromate in the Presence of Bromate Bruno Jaselskis and J o h n L. Huston Department of Chemistry, Loyola Unicersity, Chicago, Ill. 60626 SINCETHE DISCOVERY of perbromates, several titrimetric methods for the determination of perbromate have been reported (/, 2). These methods are based on the reduction of bromate either by bromide in 12M HBr or by stannous chloride. In addition, a spectrophotometric method (3) for perbromate analogous to the perchlorate determination ( 4 ) has been described. All of these methods are accurate; however, they are tedious and long. A faster polarographic method has been developed and is described in this note. EXPERIMENTAL
Materials and Apparatus A stock solution of approximately 0.2M potassium perbromate was obtained from E. H. Appelman of the Argonne National Laboratory. Potassium perbromate was recrystallized and the crystals were dissolved to yield approximately 0.1M stock solution. The resulting solution was standardized by the iodometric method as described by Appelman. All supporting electrolytes were prepared from reagent grade chemicals. Polarographic determinations were carried out using a Sargent Model XXI polarograph and thermostated H-cell with a saturated calomel electrode(SCE). Appelman, J. Amev. Cl7ern. SOC.,90, 1900 (1968). Appelman, Zmrg. C/7em., 8, 223 (1969). (3) L. C. Brown and G. E. Boyd, ANAL.CHEM., 42,291 (1970). (4) S. Uchikawa, Bid/. Chem. Sac. Jup., 40,798 (1967). (1) E. H. (2) E. H.
Procedure Typical polarographic solutions of perbromate in the concentration range 5 X 10-5 to 5 X 10-4M were prepared by taking suitably sized aliquots, adding them to 10 ml of the supporting electrolyte stock solution, and diluting the contents to the 25-ml mark. Stock solutions of electrolytes were prepared by using weak acid and its conjugate, and sodium perchlorate to yield an ionic strength of approximately 0.1 upon dilution. The determination of perbromate alone or in the presence of bromate in amounts equal to or larger than perbromate was carried out in supporting electrolytes having pH > 3, such as phosphoric acid-phosphate, acetic acid-acetate, or sodium perchlorate-bicarbonate. The height of the perbromate reduction wave was measured by subtracting the current of the supporting electrolyte alone from the current in the presence of perbromate. The concentration of perbromate was then determined from the calibration curve. Both perbromate and bromate, if present in amounts less than perbromate, were determined by first recording the polarogram in 0.05M perchloric acid and then recording the polarogram after the neutralization of the same solution with solid sodium bicarbonate. In acid medium, only one reduction wave was observed, corresponding to the reduction of perbromate and bromate to bromide : In neutral or alkaline media, two reduction waves resulted, the first corresponding to the reduction of perbromate to bromate and the second due to the reduction of bromate to bromide. The perbromate concentration was determined from the calibration curve obtained in perchlorate-bicarbonate medium, while the bromate conANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
*
581
