Analytical Applications of Zirconium Electrode STEPHEN MEGREGIAN Division of Dental Public Health, Public Health Service, Departmenf of Health, Education, and Welfare, Washington 25, D. C.
,The current generated by the spontaneous electrolysis of the zirconiumplatinum cell is proportional to fluoride concentration between 2.5 and 20.0 mg. per liter. The determination may b e performed in from 5 to 30 minutes, and the apparatus required is simple to assemble. Zirconium metal can also be used as an indicator electrode in titrations of the zirconyl ion with fluoride. This electrode exhibits a sharp rise in potential when as little as 0.25 mg. of fluoride in 100 ml. is present in solution. The titer of the fluoride solution for zirconium depends upon the acidity of the zirconyl sobtion. At constant acidity zirconium can b e titrated with fluoride. The reverse titration is not feasible.
T
PROCEDURE of Baker and Morrison (1) for determining fluoride ion concentration by means of the current generated by a n aluminum-platinum cell was investigated to determine if it could be used in the control of fluoridation of public water supplies. Because the chloride ion which interferes with this procedure HE
could not be removed by simple means short of the Willard-Winter (2) distillation, the method mas not considered adaptable to the routine analysis and control of fluoridated waters. However, it was believed that other electrode systems might be found which would be more specific in their response to the presence of the fluoride ion. The metals to be considered would be among those high in the electromotive series and whose ions would unite with fluoride to produce a complex ion in acid solution. The following metals were tested for sensitivity to a fluoride ion concentration of 20 mg. per liter in approximately 0.1M perchloric acid solution : aluminum, beryllium, iron, germanium, niobium, lead, tantalum, titanium, and zirconium. None showed a sensitivity to fluoride ion at this concentration except zirconium. Therefore, only the zirconium-platinum electrode system was studied further. During the course of this investigation it was discovered that the zirconium electrode exhibited a sharp rise in potential when as little as 2.5 mg. per liter of fluoride was present in solution. This observation indicated that the
IO00
I
,900-
,300I
.o
I
I
1.0
3.0
2.0 ML.
Figure 1.
I
I
4.0
5.0
F- SOLUTION (1.0 ML.= I O MG.
1
6.0
electrode might be a useful indicator in reactions involving the fluoride ion as titrant, and the conditions under which this titration might be carried out were also studied. DETERMINATION O F FLUORIDE BY SPONTANEOUS ELECTROLYSIS
Materials. The zirconium electrode was a ‘/cinch diameter wire of the metal obtained from the Carborundum Metals Co., Inc., of Akron, N. Y. T h e platinum electrode was a strip of commercially available metal. The electrodes were coated with wax except for about l/z inch of the lower portions. Fluoride solutions were prepared by diluting a stock solution of 0.526M sodium fluoride (1 ml. equals 10 mg. of fluoride). T h e acids used were the standard reagent grade. The current was measured by the galvanometer circuit of a Sargent Model I11 Polarograph. Procedure. A 100-ml. sample of the fluoride solution was acidified with the appropriate acid to a predetermined concentration in a 250ml. beaker. T h e beaker was placed over a magnetic stirrer set a t a predetermined speed, and t h e electrodes were immersed in t h e solution. The galvanometer circuit was closed and after a time interval of 5 to 30 minutes, during which the current reached stability, the reading was taken. Experimental. Various concentrations of several acids were tested to discover the optimum conditions for the reaction. Either perchloric or hydrochloric acid could be used, although perchloric was preferred because the current reached stability somewhat sooner than with the hydrochloric acid. Sulfuric acid or phosphoric acid could
I
7.0
8.0
F)
Titration curves a t various acidities
1 16 mg. of zirconium ion in 100 ml. Molarity of nitric acid
0
3.20
X
1.60
0 A
0.80 0.16
ACIDITY, MOLES
“03
Figure 2. Effect of solution acidity on mole ratio of zirconium to fluoride end point VOL. 29, NO. 7, JULY 1957
1063
produced by the previous increment. However, when the end point was exceeded, a rapid rise in potential occurred which oscillated only slightly between its maximum and minimum readings. Further addition of titrant tended to decrease the potential of the electrode gradually. The end point of the titration was taken a t the occurrence of the sharp rise in potential. RESULTS
0
I
I
I
5
IO
15
20
ML NAF ( I M L . = l O O MG F )
Figure 3. Titration of zirconium ion with sodium fluoride at constant acidity of 0.8M nitric acid
Curve 1
2 3 4
Mg. Zr 11 6
lleq. Zr
58 0 116 0 290 0
0 635
n
127
1 270
3 180
not be used because these rendered the zirconium electrode insensitive to fluoride a t the concentration levels tested, It appears that the concentration of acid is not critical and that any strength between 0.1 and 1.OM can be used without appreciable change in sensitivity to fluoride. A linear relationship exists between fluoride concentration and current in fluoride solutions containing 0.1M perchloric acid. The maximum concentration of fluoride which may be determined by this procedure would depend on the range of the galvanometer and the surface area of the electrodes. The lowest concentration which could be measured with precision was about 2.5 mg. of fluoride per liter. Below this value the cell became insensitive to small changes in fluoride. Attempts made to sensitize the zirconium electrode to the concentrations of fluoride of interest in water fluoridation were fruitless. The use of this technique with relatively pure fluoride solutions will probably produce satisfactory results. The following precautions must be exercised throughout a qeries of determinations: Electrode area, electrode spacing, stirring rate, and temperature of the test solution must be held constant. When the calibration curve is prepared, the highest concentration of fluoride should be run first, followed by the other standards in the order of decreasing concentration. When an unknown is run, the cell should first be sensitized by immersion in the highest concentration of the fluoride standards until the current output agrees with 1064
ANALYTICAL CHEMISTRY
111. NaF 0 65
2 90 5 70 13 90
lIeq. F 0 31 1 53 3 00 7 32
Mole Ratio, Zr to F 1 to 2 70 1 to 2 40 1 to 2 35 1 t o 2 30
that of the standard curve; then the electrodes may be transferred to the unknown solution. KO further sensitization is necessary during a series of determinations so long as the fluoride concentration of the unknown does not fall below 2.5 mg. per liter. The elapsed time between the closing of the galvanometer circuit and the stabilization of the current produced will vary from 5 to 30 minutes, with the lower fluoride concentrations requiring the longer time interval. POTENTIOMETRIC TITRATION OF ZIRCONIUM WITH FLUORIDE
Materials. Zii conium oxychloride. .prepared b y dissolving 35.4 grams of t h e salt in 1 liter of water. T h e solution contained 11.6 mg. of ziiconium per liter. Sodium fluoride, 0.52631. Sitric acid. concentrated, reagent grade. Saturated calomel reference electrode. Zirconium nire electrode, same as before. Beckman Model G p H meter. Procedure. A predetermined quantity of t h e standardized zirconium solution was transferred t o a beakel. A predetermined volume of nitric acid was added and the solution diluted t o 100 ml. T h e beaker was placed on a magnetic stirier, t h e electrodes were immersed in the solution, and it n-as titrated with thc standard sodium fluoride solution. A s the end point was approached, each small addition of the fluoride solution tended to produce a sudden rise in potential followed by rapid oscillations up and down until a stable potential was reached slightly higher than that
Sitric acid was preferred to the other mineral acids for this titration because the potential readings were more stable. Figures 1 and 2 illustrate the effect of acid concentration on the stoichiometric relationship betn-een zirconium and fluoride ions. It can be seen that acidity strongly influences the stoichiometry of this reaction. Consequently, all titrations should be carried out under conditions of constant acidity. Figure 3 is a n illustration of a series of titrations a t constant acidity with various quantities of zirconium in solution. Except for the sample containing the least quantity of zirconium, the ratio of zirconium to fluoride is nearly constant under these conditions, indicating that a quantitative relationship can be developed for most ranges of zirconium concentration. Fluoride cannot be determined directly by using a zirconium solution as titrant because a sharp potential drop is not produced when small increments of zirconium are added near the end point. DISCUSSION
It is obvious that the potential curyes do not have the same shape as the typical S curves obtained in the usual potentiometric titration. This is probably due to the fact that this titration does not follow the Sernst equation, because the rise in potential occurs when excess fluoride ion is present rather than in the presence of zirconium ions, as would be necessary for the Yernst equation to hold. What probably occurb is that under the conditions existing in solution the zirconium electrode is normally covered with a film of oxide and exhibits a potential clue to this film. When fluoride ion is present in the solution it attacks the oxide layer, dissolving it and exposing the true metal surface to the solution, thus exhibiting the true potential of the zirconium metal. Possibly other titrations could be carried out using the fluoride ion as titrant in acid solutions in which the Zirconium electrode can function as the indicator. It should be possible to titrate thorium, lanthanum, beryllium, aluminum, and other ions capable of producing a complex fluoride anion. Also, lead could be titrated as lead
chlorofluoride. The aluniiiium electrode might also be used as an indicator in those titrations in which no mineral acid is present and the chloride ion is absent. Although this research is incomplete, it is reported now because this laboratory has no further interest in its derelopment. The data presented nlay be of sufficient interest to stimulate further work with these electrodes.
sheet, all of which were used in this study.
ACKNOWLEDGMENT
The author is indebted to the Aluminum Co. of America for its donation of various alloys, the Brush cO. for its donation Of liuni foil, the Carborunduni Metals C O . for its donation of zirconium n-ire, and the Fansteel 1\Ietallurgical Corp. for its donations of niobium and tantalum
LITERATURE CITED
(1) Baker, B. B., Morrison, J . D., h . ~ . CHEM, 27, 1306 (1955). ( 2 ) Willard, H. H., \\-inter, 0. B., Isu; E S G . CHEM., .kS.kL. E D . 5 , 1 (1933). R~~~~~~~ for review september 12 1956. Accepted February 19, 1987.
Testing of a Rotary Concentric-Tube Distilling Column BEVERIDGE J. MAIR, NE0 C. KROUSKOP, and FREDERICK D. ROSSlNl Petroleum Research laboratory, Carnegie lnstifute o f Technology, Pittsburgh I 3, Pa.
b A large laboratory concentric-tube distilling column, with rotor 4.871 inches in diameter and 60 inches in length, and with an annular space of 0.0465 inch, has been tested for throughput and separating power a t speeds up to 4000 r.p.m. For speeds up to 2400 r.p.m. the results confirm data previously obtained on a smaller column. Above 2 4 0 0 r.p.m. the separating power is much lower than was expected. This is attributed to the generation of heat by friction in the vapor phase.
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AMERICANPetroleum Institute Research Project 6 published ( 3 ) in 1 9 4 i a description of the assembly and testing of a small model of a rotary concentric-tube distilling column. The rectifying section was the empty annular spsce formed by the outside surface of a rotating, closed, inner cylinder and the inside surface of a stationary outer cylinder. The annular fractionating section was 0.043 inch in width, 23.0 inches (58.4 em.) in length, and 2.93 inches (i.44 cm.) in smaller diameter. For high values of throughHE
put, 2000 to 4000 ml. per hour, the column when operated a t 4000 r.p.m. had a n efficiency factor (the throughput divided by the holdup per theoretical plate. or the number of equivalent theoretical plates through which the material being fractionated passes in unit time) about 10 times t h a t of the best values previously reported for other rectifying columns. The efficiency factor increased markedly with the speed of rotation of the inner cylinder. Extrapolation of the results indicated that a column with a rotor 5
Figure 1 . Assembly of rotary concentric-tube distilling column
Motor B. Transmission Baffles D. Shaft of rotor Guide bearings, graphital Wall of rectifying section, 4.964-inch inside diameter G . Cylindrical rotor, dynamically balanced, 4.871-inch outside diameter H . A4nnularspace, 0.0465 inch in width, 5 feet in length Transite shell, 2 inches thick I. J . Heating jacket for rectifying section K . Corrugated sheet asbestos covered Tvith aluminiim foil (Alfol) L. Guide bearings, graphital M. Pot, stainless steel, 3-gallon capacity *Y. Tube for introducing charge and withdran.iiig samples 0. Thermocouple, copper-eonstantan P . Insulation, magnesia asbestos, 2.5 inches thick (2. Heaters for pot R. Tube connecting to manometer, detcrniines pressure difference between pot and head S . Thermocouple, copper-constantan, determines teniperature of vapor-liquid equilibrium in pot 1'' to 114. Thermocouples. single and differential, to determine temperatures of vall of stationary cylinder and heating jacket, and difference between these temperatures a t four positions along rectifying section C . Condenser on distillate line I-. Connection from head of column to condenser, insulated with magnesia asbestos tf-. Thermocouple, copper-constantan, 10-junction, to determine temperature of vapor-liquid equilihrium a t head X. Ball and socket joint, borosilicate glass Y. Reflux regulator Z. Condenser A. C. E. F.
i
VOL. 29, NO. 7, JULY 1957
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