Amperometric Titration of Zirconium Application to Fluoride Solution EDWARD C. OLSON and PHILIP J. ELVING University
of M i c h i g a n , A n n Arbor, M i c h .
ilthough zirconium and its alloys are readily dissolved in hydrofluoric acid, the resulting solutions offer difficulties in the subsequent determination of zirconium. iccordingly, a method was sought for the direct determination of zirconium in the presence of excess fluoride ion. The amperometric titration of zirconiuni with cupferron in 2 M sulfuric acid solution was found to provide a satisfactory solution. The anal: tical precision is 0.4% or better. The method is subject to few interferences and may be applied without preliminary treatment to solutions containing zirconium in the presence of a 30- to 35-fold excess of fluoride ion without loss of accuracy; if aluminum is added, a much higher molar excess of fluoride can be tolerated. A graFimetric modification of the method may be used in solutions containing nearly a thousandfold excess of fluoride.
volume with distilled water. Zirconium solutions were prepared from the pure metal. Commercial nitrogen (Linde) was used for deoxygenating without further purification. All other chemicals were of reagent or C.P. quality grade, and were used without further purification. Reagent-grade cupferron (G. F. Smith Chemical Co.) was recrystallized twice from ethyl alcohol after treatment with Norite and, finally, once from ethyl alcohol. The dried crystals were stored in the dark over ammonium carbonate. A standard solution of approximately 0.05Y cupferron was prepared daily from this material by dissolving a weighed portion in air-free water. This purified reagent was essentially 100% pure. Solutions. The following test solutions n-ere prepai ed for use in testing the applicabilitj- of the method: Solution 1, 1.i214 grams of zirconium weie dissolved in 100 nil. of concentrated sulfuric acid containing sufficient hydrofluoric acid ( 3 to 5 ml.) to ensure complete solution without heating; this solution was then diluted to exactly 1000 ml. with water. Solution 2, 2.0253 grams of zirconium weie dissolved in 100 ml. of boiling concentrated sulfuric acid and diluted to 1000 ml. with water to give a fluoride-free solution.
T
H E determination of zirconium in fluoride-containing solutions cannot be readily accomplished by existing methods. The application to fluoride-containing solut'ions is indicated by the ease with which zirconium and its alloys can be dissolved in hydrofluoric acid, or in mixtures of other acids with hydrofluoric acid. However, the resultant presence of an appreciable fluoride to zirconium ratio results in serious interference in the usual methods for the determination of zirconium. Consequently, it is usually necessary to remove fluoride ion before determination of the zirconium, a time-consuming and, in some cases, an impossible procedure. I n addition, there are few, if any, rapid methods available for zirconium, such as titration procedures, which will yield satisfactorily precise results in the presence of fluoride and phosphate ions. The need for such a zirconium method, which would be both rapid and precise, prompted the present study. The determination of zirconium has largely been accomplished by gravimetric methods. Cupferron (7, 8),mandelic acid and its derivatives ( 2 , S, 6, 9), selenious acid (12, I S ) , phosphate (6, 1 4 ) , and phenylarsonic acid (10) are among the more common reagent,s used in the gravimetric determination of zirconium. All these methods are time-consuming and therefore unsuitable when a large number of analyses must be made in a limited time. The proposed colorimetric procedures (11) are subject to interference by fluoride ion in particular, and are therefore not applicable to fluoride solutions. Kolthoff and Libreti ( 4 ) studied the polarography of cupferron i n solutions ranging in p H from 1.1 to 12.5, and found in the acidic region a single well-defined wave whose height, is directly proportional to the concentration of cupferron. This suggested the possibility of using cupferron as a reagent for the amperometric t,itration of zirconium under conditions such that the zirconium is quantitatively precipitated (1) in a sufficiently rapid manner. The titration of zirconium with cupferron in 2-11 sulfuric acid solution, employing amperometric detection of the equivalence point, was found to be satisfactory even in the presence of large excess of fluoride ion. A gravimetric modification permits the determination of zirconium in the presence of a thousandfold excess of fluoride.
n
/ 0
EXPERIMENTAL
Chemicals and Reagents. Reagent-grade (48%) hydrofluoric acid was diluted to give a solution 8.13M in hydrofluoric acid, as determined from specific gravity measurement. Reagent grade sulfuric acid (specific gravity of 1.84) was diluted 1 to 10 by 1747
I
I
I
I
ANALYTICAL CHEMISTRY
1748 Procedure for Gravimetric Determination of zirconium. The sample, 5 to 10 ml., containing 0.1 to 1.0 millimole of zirconium is pipetted into a 250-ml. beaker, and diluted to about 100 ml. with 10% sulfuric acid. A 5% solution of cupferron is added slowly from a buret with vigorous stirring. The addition of cupferron is continued until the precipitate is coagulated, and then a few more milliliters are added. The precipitate is washed by decantation, and filtered through an ashless paper. After being transferred to the filter, the precipitate is washed with a minimum of 250 ml. of 0.1% cupferron solution, cautiously charred, ignited to constant weight, and weighed as zirconium oxide. Determinationof Optimum Conditions. Preliminary investigation showed that in 1 to 10 sulfuric acid solution, cupferron gives a single well defined wave with a pronounced maximum which is suppressed by gelatin but not by methyl red. The potential of -0.84 volt vs. S.C.E. was chosen because it is well out on the diffusion current plateau, and is also well in advance of the hydrogen discharge wave. Using the information obtained from these studies, a series of samples of varying size (obtained from Solution 1) was run and the results were calculated, assuming that the cupferron was 100% pure and that the reaction Zr(SO4)3-4 Cup- = ZrCup, 3S04-(1) is stoichiometric. These results are summarized in Table I.
+
Table I. Taken 0.0189 0.0189 0,0189 0.0378 0,0378 0.0378 0,0378 0.0566 0.0566 0.0566
Ion
a++Al-'" cu--
Sn++LT Fe++Sn++ Fe+T Ti-+'+ \r + + + + t Oxalate Citrate Tartrate Phosphate Phosphate Phosphate
Amperometric Titration of Zirconium in Pure Solution
Zr, Millimole Found Deviation 0.0183 -0.0006 0.0195 0,0198 0.0383 0.0387 0.0385 0.0385 0.0559 0.0566 0.0564
Table 11.
+
+0.0006 +0.0009 +0.0005 +0.0009
tO.0007 +0.0007 -0.0007 0.0000 -0.0002
Taken 0.0566 0.0566 0.0566 0.0765
0,0755
0.0755 0.0943 0,0943 0.0943
0.204 0.042 0.420 0.091 0.042 0.092 0.081
0.100 1.0 1.0 1.0 1.0 1.0
0.0564 0.0566 0.0586 0.0745 0.0753 0.0755 0.0930 0,0935 0,0930
-0.0002 0.0000 0.0000 -0.0010 -0.0002 0.0000 -0,0013 -0.0008 -0.0013
-4v. dev.
f0.00056
Survey of Interferences in Amperometric Titration of Zirconium
Added Species Millimole Salt AddedD 0.096 CrCla 6Hz0 KzAlz(S0a)n. 24Hz0 0.185 CUSO4 0.079 MnClp. 4Hz0 0.091
0.1
Zr, Millimole Found Deviation
MgSOa
SnClr .5HzO SnC14.5Hz0 FeCls. 6Hz0 SnCle. 2H20 Fe(NH4)z(SOi)z.6Hz0 Ti( SO41z (HzSO4 solution) NHiVOs ("4)2C204. HzO (NHa)zHCsH& KzC4Ha06. '/rH20 (NHdzHP04 (NHa)zHPOn (NH4)zHPOa
Zr, Millimole Taken
Found
Deviation
0.0943 0.0943 0.0943 0.0943 0.0943 0.0943 0.0548 0.0943 0.0943 0.0943
0.0964 0.0942 0.0950 0.0931 0.0953 0,0954 0.0769 0.1198 0.0945 0.0945
+o.
0.0943 0.0943 0.1108 0.1108 0.1108 0.1108 0.1108 0.1108
0.1750 0.1651 0.1100 0.1103 0.1110 0.1108 0.1100 0.1110
AY d e r
b
Table 111. Effect of Fluoride on Amperometric Titration of Zirconium Zra
Molar Ratio, F:Zr
Millimoles KF
0 1108 0 1108 0 1108 0.1108 0 1108 0 1108 0 1108 0 1108
0 0 0 2.0 3.0 4.0 5.0 6.0
Ziroonium, Mg Taken Found 10.13 10.13 10.13 10.13 10.13 10.13 10.13 10.13
0 0 0 18 27 36 45 54
10.12 10.18 10.18 10.15 10.10 10.15 9.51 8.28
a This amount of zirconium required about 5 ml. of 0.087M cupferron solution.
Table IV. Elimination of Fluoride Interference in Amperometric Titration of Zirconium by Aluminum Addition Millimoles Present F 0.1108 0 0.1108 0
A1
ZY
0.1108 0.1108 0,1108 0.1108 0,1108
0
2.0
0 2.0 4.0 8.0 10.0
6.0 6.0 6.0 6.0 6.0
Molar Ratios F:AI A1:Zr 0 0 0 0 0 18
Zirconium, Mg.
F:Zr
Taken
Found
54 54 54 54 54
10.13 10.13 10.13 10.13 10.13 10.13 10.13
10.15 10.18 8.29 10.42 10.20 10.22 10.18
3 1.3 0.75 0.60
0 18 36 72 90
Table V.
Gravimetric Determination of Zirconium in Presence of Fluoride Alillimoles Molar ZrOr, Mg. Present
Zr 1,108 1.108 1.108 1.108 1.108 1.108 1.108 1.108 1.108 1.108 0 1108 . 0.1108 0.1108
F 0
Ratio, F : Zr
2 4 6
0 0 0 1.8 3.6 5.4
8 10 50 100 0 10 100
9.1 45 91 0 91 910
0 0
7.2
Error, Taken
Found
%
136.5 136.5 136.5 136.5 136.5 136.5 136.5 136.5 136.5 136.5 13.7 13.7 13.7
136.8 137.1 136.8 136.9 136.1 137.8 137.9 137.1 137.3 136.8 13.9 14.1 13.6
+0.22 t0.44 +0.22 +0.29 -0.29 +0.95 +1.02 +0.44 +0.58 +0.22 $1.46 +2.90 -0.73 ztO.75
Av.
0021 -0.0001
+ O . 0007
-0.0012 +0.0010 +0.0011 +0.0178 + O . 0255 +o, 0002 +0.0002
+ O . 0807 f0.0608 -0.0008
-0.0005 0002 0,0000 -0.0008 +0.0002 *0.00065
+o.
Weighed amount of salt dissolved in 10% sulfuric acid and appropriate volume of this solution added. b Excludes F e + + - , S n + + " + , T i - + ' + , and V + + + ' + . a
The effect of various possible impurities on the method was checked using 5-ml. samples of Solutions 1 and 2; these results are given in Table 11. The effect of fluoride concentration upon the amperometric titration results is shown by the data in Table 111. The results in Table IV show that it is possible to eliminate the interference caused by very large relative amounts of fluoride by the addition of aluminum ion to the sample. Data for the gravimetric determination of zirconium in the presence of varying fluoride concentrations are given in Table T.'. DISCUSSION OF RESULTS
If Reaction 1 is stoichiometrically obeyed and if the cupferron is pure, each millimole of zirconium should require 4 millimoles of cupferron. The data in Table I and Figure 2 show that this condition is satisfied. Thus, it is unnecessary to standardize cupferron solutions, prepared as previously described, against solutions of known zirconium content.
Several attempts were made to modify the method described by adding the zirconium to a known excess of cupferron, and measuring the diffusion current before and after the addition of zirconium. In several trials equilibrium had not been reached in times ranging from 10 to 15 minutes, so no further investigations were carried out along these lines. It is possible that addition of a known excess of cupferron to the zirconium solution and determination of the decrease in the cupferron wave would be successful. However, the results would be expected to be poor, because they would be based on the difference of two polarographic measurements, each of which is precise to only 2 or 3%. The titration, as previously outlined, requires between 10 and 15 minutes after degassing. Thus, by degassing one sample while titrating another, it is a simple matter for a single analyst to analyze one sample in triplicate or three individual samples in an hour or less. Possible Interference. In Table IT, ferric iron, titanium, pentavalent vanadium, and large amounts of stannic tin are the only interfering cations; hafnium shows behavior similar to that of zirconium. Of the anions tested only large amounts of fluoride (Table 111)were found to interfere. The failure of phosphate to interfere is particularly interesting inasmuch as a precipitate of zirconium phosphate appeared almost immediately after adding the phosphate solution, However, titration of the resulting suspension gave good results if sufficient time ( 5 to 10 minutes) was allowed for equilibrium to be established after each addition of cupferron. Table I11 shows the effect of varying the mole ratio of fluoride to zirconium. From these data, plotted in Figure 3, it is apparent that interference occurs only a t greater than a 30- to 35-fold ey-
1749
V O L U M E 26, NO. 11, N O V E M B E R 1 9 5 4 cess of fluoride, providing the points immediately following the equivalence point are chosen for estrapolation. The varying shapes of these titration curves appear to bc the result of the formation of different zirconium-fluoride complexes depending upon the fluoride to zirconium ratio. The reasons for the shapes of the curves and the complexes formed are now being investigated. The data in Table IV show that it is possible to eliminate the interference caused by large MILLIMOLES OF ZIRCONIUM amounts of fluoride by Figure 2. Relation of Millithe addition of alumimoles Cupferron Used to 1Millinum to the solution. moles Zirconium Present Figure 4 shows the effect 0 - Theoretical--1 e , calriilated ralrirs of the addition of alumiExDrrirnentallv observed Iahic. num upon the titration curve; the latter is seen to approach the curve for a fluoride-free solution as the aluminum concentration is increased. However, even a 90-fold excess of aluminum over zirconium, amounting to ten times as much aluminum as nTould be required for the formation of AlFs---, was not sufficient to restore the curve to its original shape, indicating the stability of the zirconium fluoride complex formed. Analytical Precision. The data in Table I were obtained from curves such as those in Figure 1, which were uncorrected for dilution due to the addition of reagent; correction of several curves revealed a change of from 0.01 to 0.02 ml. in the volume of the
cupferron solution required. This error was found to be of the same order of magnitude as that inherent in the graphical method for the determination of the equivalence point. Therefore, except for the most exacting work, it is unnecessary to correct the observed diffusion current for dilution due to the addition of i-cagent, thuc saving considerable time. These data show an average deviation of 0.00056 millimole over the range from 0.02 to 0.10 millimole of zirconium. This deviation amounts to approximately 0.01 ml. of cupferron solution. The precision-i.e., the average of the precision figures for each samplr size-over this same samplc rangc n-as zk0.00015 millimole or f-0.3797,.
~
O
H I
0
1
I
2 3 4 5 CUPFERRON SOLUTION ADDED, ml
6
I
1 1
7
Figure 1. Removal of Fluoride Interference by Addition of Aluminum
f
1
2
3
CUPFERRON
4
5
6
SOLUTION ADDED,d
Figure 3. Effect of Fluoride OII Galvanometer Deflector hlolar ratios of fluoride to zirconium 0. 36 018 A. 54
2.
7
Molar ratios of fluoride to zirconium, fluoride t o aluniinurn. and aluminum t o zirconium X . 54, 1.5, RC, 54, infinity, 0
2, 54,
3, 18
0. 0 , 0. I 8
Gravimetric Determination. Completeness of precipitation a t the amperometric end point is indicated by the agreement between the titrimetric results (Tables 111 and IV) and those obtained 'by gravimetric means (Table V). In the gravimetric method a large excess of cupferron was added and, after filtration, washing, and careful charring, the precipitate was ignited to constant weight and weighed as zirconium oxide. From the data in Table V i t is apparent that even a very large excess of fluoride is without effect upon the accuracy of the gravimetric determination of zirconium by precipitation with cupferron. &o a much larger excess of cupferron is required for complete precipitation when the fluoride to zirconium ratio is large and precipitation does not commence in some cases until 5 to 10 ml. of 5% solution of cupferron have been added. There is also a very marked difference in the nature and volume of the precipitate when a large excess of fluoride is present. Under the latter condition, the precipitate formed has a much larger volume and appears more flocculent. This flocculent precipitate is coagulated by excess cupferron, rendering i t easy to filter but difficult to wash. Prolonged washing was found necessary to remove all of the potassium and sulfate ions from the precipitate before ignition. The analytical insolubility of the precipitate formed is indicated by the solubility found for the reverse of Reaction 1 in 10% sulfuric acid a t 25' C. h value of 5 X lo-' mole per liter
1750
ANALYTICAL CHEMISTRY
obtained by a polarographic determination of the cupferron in a 10% sulfuric acid solution saturated with zirconium cupferrate.
(5) Kumins, C. A.. ANAL.CHEM..19. 376 (1947). (6) Lundell, G. E. F., and Knowles, ’H. B.: J . Am. Chem. Soc., 41,
ACKNOWLEDGMENT
(7) Ibid., 42, 1439 (1920). (8) Lundell, G. E. F., and Knowles, H. B., J . Ind. Eng. Chem., 12,
The authors wish to thank the Atomic Energy Commission, which helped support the work described.
1801 (1919).
562 (1920). (9)
Furman, K.H., Mason. W.B., and Pekula, J. S., . 4 s . 4 ~ CHEM., .
1509
(1949).
(12)
Rice, A. C., and James, C., J . Am. C h a . Soc., 48, 895 (1926). Sandell, E. B., “Colorimetric Determination of Traces of Metals,’’New York, Interscience Publishers, 1950. Simpson, S. G., and Schumb, W.C., J . Am. Chem. Soc., 53, 921
(13) (14)
Smith, 11.&and I., James, C., Ibid.,42, 1764 (1920). Willard. H. H., and Hahn, R. B., A x . 4 ~ CHEM., . 21, 293
(10) (11)
LITERATURE CITED
Oesper, R. E., and Klingenberg, J. J., ANAL.CHEM.,21,
21, 1325 (1949).
(1931).
Hahn, R. B., Ihid., 21, 1579 (1949). Klingenberg, J . J., and Papucci, R. 4..Ihid., 24, 1861 (1952). Kolthoff, I. hI., and Libreti. A , , J . A m . Cheni. Soc.. 70, 1885
RECEIVED for review Map 28, 1954.
(1948).
(1949).
Accepted July 22, 1954
Coulometric Titrations with Photometric End Point Titration of Arsenic with Electrically Generated Iodine GROVER W. EVERETT
and
CHARLES
University o f North Carolina, Chapel Hill,
N. REILLEY
N. C.
This investigation was undertaken to test the feasibility of using a spectrophotometer to detect end points in coulometric analyses for the microgram range. A cell and associated apparatus were so constructed that the entire unit fitted into a Beckman B spectrophotometer. Prior to the end point the absorbance remained at zero. Beyond the end point the absorbance due to excess iodine changed quicldy. The end point was obtained by plotting time versus absorbance and extrapolating the linear lines to a point of intersection. End points could be determined easily in the microgram region and results compared favorably with other published results.
C
titrations using a constant current is one of the most recent and intriguing phases of research in the field of analytical chemistry. .4s has been pointed out ( l d ) , coulometric titrations can be applied to accurate determinations of macrogram samples, but its greatest advantage lies in the microgram region. I n this range of very small concentrations its distinctive characteristics-Le., ease of addition of reagent, elimination of reagent impurities, nondilution of solution, and the possibility of adding extremely small quantities of reagent-can be used to advantage OULOMETRIC
(9).
Since 1950, numerous papers have been published describing coulometric titrations of ovidimetric and acidimetric systems in both the macro- and microgram ranges of concentration. Szebelledy and Somogyi (26) used a visual method for determining the end point of a titration. However, most recent workers in the field have been concerned with electrical svstems for endpoint detection-i.e., amperometric (3, 4,12, 17, 19, 24, 68), potentiometric ( 5 . 7 , 8, 10, 11, IS, 14, 18, 20, de), and derivative polarographic (6, 22). Wise, Gilles, and Reynolds ( 2 7 ) stressed the desirability of eliminating the indicator electrodes and attendant troubles by using a photometric system of end-point detection. They constructed a device for the automatic coulometric titration of solutions using a photoelectric cell and circuit for detecting changes in the light transmittancy of a solution. Macrogram concentrations were used in their experiments. Bricker and Sweetser ( 2 ) have described experiments in which a specially constructed beaker was placed in the cell compartment of a
Beckman DU spectrophotometer and various solutions were titrated by adding reagents from a microburet. The end point was determined by observing the sudden change in absorbance. The possibility of using a spectrophotometer for detecting end points in coulometric analysis in the microgram range seemed to offer several advantages. Such a procedure would combine a method for controlling very accurately the amount of reagent added, with a sensitive device for detecting small changes in absorbance. Handelman (16) described coulometric titrations of resorcinol with electrically generated bromine in which a photometric end point was used. Samples of solution were taken from the titration cell a t intervals, placed in an absorption cell of quartz, and their absorbances determined by using a Beckman DU spectrophotometer set at a vave length of 270 mp. The excess bromine could be detected easily bv the greatly increased absorbance when the end point had been passed. A4titration cell and accessories were constructed which could be completely housed inside of a spectrophotometer so that a complete titration could be made without lifting the cover. The course of the titration was followed by determining the absorbance of the solution a t suitable time intervals. After the end point, a considerable change in the absorbance occurred. The titration of arsenious acid with electrically generatediodine was selected for the initial oxidation-reduction reaction. Several investigators (10, df, 27, 28) have used generated iodine as an oxidizing agent against arsenious acid, thus comparisons of different methods ceuld be made easily. Iodine can be generated o (28) and it will react stoichiometrically with with 1 0 0 ~efficiency arsenious acid. Also, an excess of free iodine will immediately cause a large change in the absorbance in the lower wave lengths of the visible spectrum. AI’PAR4TUS
Titration Cell. Two transparent plastic cells were constructed to fit into the spectrophotometer. The large cell, 37 X 70 X 58 mm. with a capacity of approximately 70 ml., was used u hen additions of unknown solution were made by means of a volumetric pipet. The small cell. 43 X 37 X 4 i mm. with a capacity of approximatelv 25 ml., mas used when additions were made from a small weight buret. Each cell contained a platinum electrode, 1 X 2 cm., a glass stirrer fitted with a vanelike attachment a t the top so that it could be rotated by a current of conipressed air or nitrogen, and another electrode. This second electrode consisted of a platinum wire enclosed in a filter stick which contained a fine sintered glass plate (10 em.) a t one end. This
