increased excess of bisulfite a t the lower chromium lerels is necessitated in part at least by increased air oxidation of the bisulfite. Several attempts to determine chromium at the micromole level resulted in only 80 to 90% recovery. Amounts larger than 0.1 mmole present a problem 1,ecau.e the intense color of C r ( I I I ) - E D T 1 obscures the end point of the back-titration. Attempts to eytend this technique to the analysiq of miytures of V(1V) and Cr(V1) by first titrating the V(IV), adding excess EDTA, and reducing the Cr(V1) t o Cr(II1)-EDTA were also unsucccs?ful liecause the intense color of Cr(T’1) prevented observation of the
V(1V)-EDTA end point. The use of either luminescent indicators or nonvisual methods of end point detection should be possible in both cases. Other variations such as the careful selection of reducing agent, buffer system, masking agents, and chelon should increase the scope of the method and permit simpler, more direct chelometric determinations of other anions with lower states amenable to chelation. The case of molydate has been cited. Still another possible extension is the determination of small quantities of reducing agents by the use of excess chromate and EDTA. Tinder appropriate conditions the reductant reacts quantitatively with the chromate, yielding Cr(II1)-
EDTA. The number of moles of E D T A consumed is simply one third the number of equivalents of reducing agent present. LITERATURE CITED
(1) Beck, RI. T., Bardi, I., Acta Chim. Budapest 29, 283 (1961). 12) Flaschka. H.. Ganchoff, J.. Talanta 8. 885 11061).
RECEIVED for review June 11, 1962. Accepted October 11, 1962. Research sponsored in part by the U.S. Bir Force, Office of Scientific Research and Development Command, Contract No. 49(638)-333.
Coprecipitation of Trace Quantities of Metal Chelates Using Organic Coprecipitu nts W. P. TAPPMEYER’ and E. E. PICKETT Department of Chemistry, University o f Missouri, Colombia, Mo.
b Preliminary investigation with four different chelate systems and six metal ions indicated that the factors influencing coprecipitation of these metal chelates with organic coprecipitants like P-naphthol and phenolphthalein were the same as those that influence solvent extraction. Further preliminary studies indicated that the metal chelates that were extracted well with solvents like CCI, were also coprecipitated to a high degree with organic coprecipitants. Thus, per cent coprecipitation vs. pH relationships were formulated and plotted employing the same type of equilibrium considerations as are used for solvent extraction. A comparison between these calculated plots and those determined experimentally for three metal chelates and at different levels of excess chelating agent showed good agreement.
S
x C T H o D s have been developed for the chemical concentration or enrichment of trace elements in agricultural and biological materials prior t o spectrographic determination. The most n-idely used of these probably has been that of Mitchell and Scott (9). Several modifications of this method have been proposed by Heggen and Strock (j),Chichilo, Specht, and Whittaker ( I ) , Pickett and Hankins ( I I ) , EVCRAL
1 Present address, Department of Chemical Engineering and Chemistry, Missouri School of Mines and Metal-
lurgy, Rolls, Mo.
and Dehm, Dunn, and Loder ( 2 ) . All have employed the coprecipitation of the trace metal oxinates on a carrier of aluminum or indium oxinates a t p H 5.2 or above, with or without additions of thionalide or tannic acid to improve recoveries of certain elements. Coprecipitation has been followed by ashing of the precipitate and spectrographic analysis of the resulting aluminum or indium oxides containing the trace elements. Mechanisms of coprecipitation of organic chelates on organic precipitates have not received extensive study. Kuznetsov (8) has investigated several types of organic coprecipitation processes and likened them to “extraction by solid extractants,” but has not presented a systematic study in support of the comparison. The present work n-as undertaken to test its validity in detail. The influence of pH, mode of addition of carrier precipitate, and level of excess of various chelating agents on the efficiency of coprecipitation of six metals in trace amounts upon phenolphthalein and @-naphthol carrier precipitates are reported. These carriers were chosen because they were used by Kuznetsov and met most of the characteristics required of a good coprecipitant. In the present work and in that of Kuznetsov, the main object is to bring down or carry substances (metal chelate complexes) which are present in such small amounts as to be in true solution before the carrier is added. The phenomenon thus falls in the area of coprecipitation and bears a formal resemblance to solvent extrac-
tion of metal chelates, but by use of a solid rather than a liquid phase. EXPERIMENTAL
Standard stock solutions of AgSOs, CoC12, IrTi(N03)2, Cd(N03)2, Pb(N03)2, A1Cl8, and (NH4)2M004 mere prepared by dissolving ACS Reagent Grade salts, oxides, or free metal in distilled water and/or the appropriate acids or ammonia. All acids and bases used were Reagent Grade chemicals and had been distilled in the laboratory, as had the ethyl alcohol, 95%. The chelating agents, 8-quinolinol (oxine), N-nitrosophenylhydroxylamine (cupferron), diphenylthiocarbazone (dithizone), and n’-(2-naphthy1)thioacetamide (thionalid, available from K and K Laboratories), were used without purification. These chelating agents were dissolved in either distilled ammonia or distilled acetic acid and dispensed from those media. ACS Reagent Grade phenolphthalein powder and technical p-naphthol (neither was recrystallized) were dissolved separately in distilled ethyl alcohol and added from that medium as the coprecipitant. PROCEDURE
To 200 ml. of water containing 10 pg. each of Pb, Xi, Mo, and Cd and 5 pg. each of 4 g and Co, one of the above chelating agents was added a t the desired level of excess, the solution was buffered with 15 mmoles of ammonium acetate, and the pH adjusted to the proper value by the addition of ammonia or acetic acid. At this point in the procedure there was never any visible precipitate; the concentrations VOL. 34, NO, 13, DECEMBER 1962
1709
8oli';p
/
k
/
&-
_ -_ -
-
I I
701
-6---
'A
I
A I-
:
40
-
io
V
a
w 300
,b / I i
IO;
l 01 2 WEIGHT
OF
-A-7-
-c L-
PHEN6LPHTHALEIN ( 6 . )
Co and tenfold oxine -AMo and tenfold oxine -W-
- Cd and tenfold oxine
- 4- -
Co and 200-fold oxine
Mo and 1000-fold oxine
- -0- - Cd and 200-fold oxine
1710
ANALYTICAL CHEMISTRY
6
7
Figure 2. Relative effectiveness of coprecipitation with phenolphthalein and extraction with chloroform for three metal oxinates at 1 000-fold excess of oxine -0-
-A-W-
- -0- - -A- - -5- -
Cd and 1000-fold oxine
of the complexes were well below their solubility limits as established by measurements similar to those of Pickett and Hankins (11). An alcohol solution of the organic coprecipitant, containing 0.1 gram of coprecipitant per milliliter of alcohol, was then added usually while stirring. (Four milliliters of this coprecipitant solution would result in a carrier precipitate of about 0.3 gram of phenolphthalein or 0.2 gram of 8-naphthol.) The carrier precipitate containing the coprecipitated trace metal chelate was generally allowed to settle overnight to facilitate the filtering operation. (This was particularly important when phenolphthalein was used as the carrier precipitate because i t forms a fine-grained precipitate which filters slowly. On the other hand, the precipitations made with 8-naphthol could be filtered immediately with no detectable change in degree of coprecipitation.) The mixture was filtered on Whatman No. 42 paper and washed several times with water. The filtrate was evaporated to approximately 10 ml. and the organic matter was wet-ashed if it was excessive. The wet-ashing was accomplished by the successive addition and digestion with nitric acid, mixed nitric, perchloric and sulfuric acids, and hydrochloric acid. The solution, free of organic matter, was transferred t o a crucible lined with polyethylene paper containing 100 mg. of pure aluminum oxinate. The crucible and contents were placed in an oven and the
5
4
PH
Figure 1. Percentage coprecipitation as a function of amount of excess of complexing agent
-
3
Table 1.
Copper and phenolphthalein Cobalt and phenolphtholein Molybdenum and phenolphthalein Copper and 5 ml. of CHC13 Cobalt and 5 ml. of CHCI3 Molybdenum and 5 ml. of CHCb
Per Cent Coprecipitation Using Various Complexing Agents
(Conditions: 1 gram of phenolphthalein used as carrier precipitate and data from emission spectrometry) Complexing agent and amount Element coprecipitated, yo (in 200 ml. sol.) Co Ni Ag Cd Pb MO pH 81 2 5,s Thionalid, 2 mg. 4 7 97 99+ 47 0 6.0 24 35 21 1 Cupferron, 10 mg. Dithizone (saturated), 1 mg. 99+ 92 96 99 74 .. 5.0 Oxine (tenfold excess), 2 mg. 93 90 50 5 25 18 5.5
+
Table
II.
Effectiveness of the Various Carrier Precipitates for Causing Coprecipita tion
(Conditions: using one thousand-fold excess of oxine as complexing agent at a pH and using emission spectrometry) Element coprecipitated, % Pb Carrier Weight, g. Co Ni Ag Cd 76 78 80 23 Benzil 0.303 25 1.701 75 84 34 0.614 14 77 90 80 8 Biphenyl 1.543 13 79 90 93 14 0.459 15 91 91 94 21 p-Nitrotoluene 91 92 90 21 1.10 14 Naphthalene (three trials) 0.45 56 96 50 87 13 Phenolphthalein 0.347 99+ 97 92 99f 75 0.487 96 95 96 99+ 74 @-Naphthol
of 5.5 Mo
16 14
14 55
52 12
78 72
solution was evaporated t o dryness, then transferred to a muffle furnace, and ashed a t 500' C. for several hours. The efficiency of this convenient means of collecting traces of metal salts from solution for spectrographic determination had been established by Koirtyohann ( 7 ) . The matrix of A1203 containing the small quantity of elements escaping coprecipitation was mixed with a n equal weight of spectrographic grade graphite powder containing 0.3% of PdO as internal standard. This was filled into a 3/16-in~hcavity and arced a t 8-amperes d.c. The following line pairs were read: P b 2833.1/Pd 3251.6; Xi 3101.6/Pd 3251.6; M o 3170.2/Pd 3251.6; Cd 3261.1/Pd 3251.6; Ag 3281.2/Pd 3251.6; and Co 3453.5/Pd 3489.8 A. Data for standardization curves were obtained by taking the appropriate aliquot of the prepared stock solution, adding directly to the crucible lined with polyethylene paper and aluminum oxinate, and proceeding a s described above. To compare the results of solvent extraction with those of coprecipitation, a single extraction with 5 ml. of CHCla, with 20 minutes on a mechanical shaker, rvas substituted for the coprecipitation operation. All subsequent operations were as described for the coprecipitation experiments. For the more accurate data desired for the quantitative formulation, radioisotopes were used. Data were obtained using hfogs, Cued, C060, or Cd115, taken singly, by using a n appropriate aliquot of the radioisotope and the desired amount of stable element and proceeding with the coprecipitation or extraction as described above. I n each experiment, the solution of radioactive and stable salt was evaporated t o a small volume with several milliliters of nitric acid and then diluted to 200 ml. with water before addition of the chelating agent to ensure uniformity of oxidation states. After the coprecipitation and the filtering operation, the solution was evaporated t o approximately 10 ml., transferred to a 25 ml. volumetric flask. and diluted to volume for counting. Activities were measured with a Tracerlab dip counter and scaler. The measured activities exceeded 3200 counts per minute while the background registered approximately 55 counts per minute. Statistical treatment as given by Friedlander and Kennedy (3) was then applied to the measured activities and the fraction of coprecipitation for a constant weight of carrier found by dividing the activity of the radioisotope coprecipitated by the activity of total radioisotope present. DISCUSSION OF RESULTS
Table I contains the results of coPrecipitation of the various metal chelates using a common carrier. The effect of various levels of excess of the chelating agent upon the extent of coprecipitation is shown for three metal oxinates in Figure 1. When chelating agent was omitted in the above procedure, the extent of coprecipitation was
always found to be negligible. The excess of the chelating agent was always calculated as the amount in excess of that needed for stoichiometrically combining with all the metal ions present. Table I1 contains the results obtained when various organic materials were used as the carrier precipitates. The method of adding the carrier precipitate to the aqueous solution of trace metal and chelating agent and its effect on the completeness of coprecipitation were studied extensively. Table I11 summarizes the results for three of the best carrier precipitates, namely, aluminum oxinate, P-naphthol, and phenolphthalein, and indicates that the extent of coprecipitation is not largely dependent upon the method of adding the carrier precipitate. For the case of Al(oxj3 precipitating in the presence of the trace quantities of other metal oxinates, a calculated quantity of aluminum ions was first added to the solution and then sufficient oxine was added to precipitate all the aluminum as oxinate and to provide a calculated stoichiometric excess for the other metal ions present. The extent vf coprecipitation is independent of the concentration of metal ion present throughout a wide range of concentration (Table IW. The foregoing results indicate that the extent of coprecipitation resembles that for solvent extraction. Some preliminary direct comparisons indicated
that the metal chelates that were coprecipitated well by certain organic carrier precipitates were also extracted to a large degree with organic solvents such as chloroform. Figure 2 shows that the coprecipitation properties of 0.25 gram of phenolphthalein and solvent extraction properties of 5 ml. of CHC& are practically identical for CU(OX)~,Cd(ox)2, and M002(0xj2. However, no exact significance can be assigned to the close similarity of the two sets of results. EXTRACTION OR COPREClPlTATlON EQUATIONS
To pursue the similarity between solvent extraction and coprecipitation with organic coprecipitants further, the same type of quantitative formulation t h a t is used for predicting results for solvent extraction was applied to the coprecipitation by organic carriers. For many solvent extraction systems the course of the chelate extraction may be followed by the equation given b y Morrison and Freiser (10) and others (12):
D
=
[
K/KDxK? (H%jn klKi
where ki and K J
=
Ki
=
f
(HRsq x)n-l] -' (1)
first and total formation constants of the given chelate ionization constant of the chelating agent
Table 111.
Effect of Different Methods of Adding the Carrier Precipitate Upon Coprecipitaiion (Conditions: using approximately one thousand fold excess of oxine at pH of 5.5 and data
taken from emission spectrometry) E1ements Carrier Weight, precipitate g. Co Xi Cd Mo Ag Phenolphthalein 0 121 99 96 99 fO 95 0.347 99 97 99 78 92 Phenolphthalein 0 156 0.402
99 99
P-Xnphthol
0 226 0.694
93 97
@-Naphthol
0 094 0 187
96 96
Aluminum oxinate
0.167
0.291
95 98
Aluminum
0.201
94
oxinate a These values were obtained in the case of cadmium.
Conditions of coprecipitation Phenol hthalein (in alcohofi stirred while being added 97 99 83 93 Phenolphthalein (in al97 99 93 93 cohol) added carefully t o top layer and allowed t o settle through overnight 92 93 61 91 &Naphthol precipitated 98 99 76 93 in separated container then added t o buffered solution containing oxine and trace metals 94 96 .. 95 ,%Naphthol added and 95 99 7% 96 precipitated in presence of trace metal ions and oxine 95 72a 99 80 Al(ox)l precipitated in 98 95 99 91 150 ml. of H20 and then 50 ml. of buffered solution containing trace metals and oxine added . . 80. 99 83 Al(ox), precipitated in presence of the trace elements using only an 800-fold excess of oxine-which is crucial
VOL. 34, NO. 13, DECEMBER 1962
1711
n 70 6
k
n
60 W K
n
50 I-
z w u
40
a W
a
30
,
01
3
4
5 PH
I
6
-@-
-a--A-G-
- -C- =
-.-
--A--@-
-a-
- -3- -
range under consideration and under the other conditions of this extraction, their contribution is negligible. Values as measured for k l , K,, K,, and KHZOXT by Johnston and Freiser (6) in 50% dioxane were used. These authors found hydrogen ion concentrations could be determined in 50% dioxane with an accuracy equivalent to that obtained in aqueous media. For the oxinate system, association of H + with Hox is quite appreciable in
Table IV.
Per Cent Coprecipitation as a Function of Quantity of Metal Ion Present (Conditions: radioisotope data for Cow and Cu64 a t pH of 5.5 with 1000-fold excess of oxine
Metal
1712
as complexing agent) Quantity of metal present, Carrier pg./200 ml. precipitate, g. Phenolphthalein, 0 . 3 500.0 Phenolphthalein, 0 . 3 50.0 5.0 Phenolphthalein, 0 . 3 5 Phenolphthalein, 0.26 0.5 500.0 6-Naphthol, 0 . 1 5 50.0 6-Saphthol, 0 . 1 5 5.0 @-Naphthol,0.16 0.5 6-Naphthol, 0 . 1 3 41.0 8-Kaphthol, 0 . 1 5 4.8 6-Naphthol, 0 . 1 5 0.8 ,%Naphthol, 0 . 1 5
ANALYTICAL CHEMISTRY
4
5
7
6
Figure 4. Comparisons between the experimental and calculated coprecipitation for cobalt oxinate using pnaphthol as carrier and three different levels of excess oxine using CO-60
Experimental ( 1 mg./ml. oxine) Experimental (0.1 mg./ml. oxine) Experimental ( 1 mg./ml. oxine) Calculated ( 1 mg./ml. oxine) Calculated (0.1 mg./ml. oxine) Calculated (0.01 mg./ml. oxine)
concentration of chlelating agent in the aqueous phase K D X = distribution coefficient of the chelate heh e e n the organic and aqueous phases D = distribution ratio of the metal betm-een the two phases n = charge on the metal ion Other terms appear in the denominator of the complete expression but in the p H [HR,,]
3
PH
Figure 3. Comparisons between the experimental and calculated coprecipitation for cobalt oxinate using phenolphthalein as carrier and three different levels of excess oxine using CO-60
--A-
,
01
7
Coprecipitation,
70
Experimental ( 1 mg./ml. oxine) Experimental (0.1 mg./ml. oxine) Experimental (0.01 mg./ml. oxine) Calculated ( 1 ma./ml. axinel Calculated (0.1 &/ml. oxine) Calculated (0.01 mg./ml. axine)
the range of pH employed and is formulated in the usual way KHo.T
=
[H+][Hos] [ H ~ o+s]
The total oxine content ( T o y )of the aqueous phase was measured spectrophotometrically at the p H of 2 in order to convert practically all of the oxine into the H20x+ species. Under the conditions of the extraction the total oxine (Tax) is:
+
[Tm] = [ H ~ o x + ] [Hox] [ox-]
+
+ [M(ox)+l
. . [M(OX),l
Simple calculations indicate that only the first two terms need be considered, so that, by combining with the expression for the ionization of Hzox+ above, the following expression results
As the volume of the aqueous phase
(V,) for the coprecipitation is quite different from that of the organic phase (V,,), the percentage extracted or coprecipitated ( E ) is related to the dis-
n 70 w I-
I4
-
b 60 V W
K
50 U
I-
t 40 W U
a
w 30 n
3
2
4
5
6
0~ 3
5
4
DH
Figure 5. Comparisons between the experimental and calculated coprecipitation for copper oxinate using phenolphthalein as carrier and two different levels of excess oxine using Cu-64 -A-0-
-A-
- -0- -
6
7
8
PH
Experimental ( 1 mg./ml. oxine) Experimental (0.1 mg./ml. oxine) Calculated (1 mg./ml. oxine) Calculated (0.1 mg./ml. oxine)
Figure 6. Comparisons between the experimental and calculated coprecipitation for cadmium oxinate using phenolphthalein as carrier and three different levels of excess oxine using Cd- 1 1 2
-.-A-
-0--
- -A- --O-0-
Experimental ( 1 mg./ml. oxine) Experimental (0.1 mg./ml. oxine) Experimental (0.01 mg./ml. oxine) Calculated ( 1 mg./ml. oxine) Calculated (0.1 mg./ml. oxine) Calculated (0.01 mg./ml. oxine)
tribution coefficient (D)by the expression :
-E
lated. The most characteristic point on increased association of hydrogen ions these plots is the p H at 50% extraction, with oxine at the higher acid concentraBy combining Equations 1, 2, and 3 henceforth known as tions. The metals copper, cobalt, and the percentage extraction, E, can be Figures 3 to 6 present comparisons becadmium were used because the stabilcalculated in terms of the constants tween the calculated and experimental ities of their oxinate complexes extend above and the three measured quantities extraction (coprecipitation) curves of over a considerable range of values. All ~ pH, (Tax), and K D X . The value of K D ~ three metal oxinates a t three different the plots show a good agreement between is given by the highest value experilevels of excess of oxine, using radioisothe calculated and experimental plots mental obtainable for a particular topes. These curves are not spaced with the exception of copper oxinate. metal chelate as D (IZ)-i.e., a t the linearly on the p H scale because of the This anomalous behavior of copper optimum value of pH and at saturation with respect to chelating agent. I t is also imperative that the same weight of carrier precipitate be used for Table V. Summary of Experimental and Calculated Values of pH,/, ~ as for the establishing the K D value other coprecipitation results in that the Metal PH1/2 oxinate Carrier Absolute KDX quantity ( V J V J is difficult t o measure coprecipitated precipitate (:VO/VW) TOX Exp. Calcd. deviation but would appear on both sides of EquaCU(0X)l Phenolphthalein 1.73 x 10-3 2.23 2.0 18 0.23 tion l and would cancel in calculating CU(0X)Y 5.25 x 10-4 Phenolphthalein 2.90 18 0.32 2.58 the final percentages of coprecipitation. CO(OX)? Phenolphthalein 7.13 x 10-3 3.52 89 0.03 3.55 [KDXfor C O ( O X )with ~ 0.25 gram of 6.67 x 10-4 0.05 CO(0X)l 4.21 4.26 Phenolphthalein 89 phenolphthalein as the coprecipitant was 0.00 2 . 0 x 10-5 Phenolphthalein 5.52 5.52 89 Co(ox)z Co( ox j z 7 . 0 x 10-3 3 . 3 6 0 .14 3 . 5 0 &naphthol 49 established at 89 (V,/V,) from an 7 . 1 x 10-4 4.07 0.06 4.13 P-naphthol 49 CO(0X)l average maximum per cent coprecipiCo( ox j2 0.00 p-naphthol 7 . 0 x 10-5 49 5.07 5.07 tation of 98.89.1 The ( T O x )values Phenolphthalein 7 . 0 5 x 10-3 0.27 4.33 4.60 48 Cd(OX )z change slightly with p H but were Cd( 0x)p 7.25 x 10-4 Phenolphthalein 0.18 5.30 5.48 48 2 . 4 1 x 10-5 G.75 Phenolphthalein 0.23 6.98 48 Cd( 0x12 measured only on a sample showing approximately 50% extraction. Thus, .4v. absolute dev. 0.14 plots of E/100 - E us. pH can be calcuE/100
=
D(Vu/V0j-'
(3)
VOL. 34, NO. 13, DECEMBER 1 9 6 2
1713
oxinate was not unexpected in the light of the inefficiency which previous workers found for coprecipitation of traces of copper oxinate with aluminum oxinate (4). Table V presents a summary of the experimental and calculated values for p H ~ / z as taken from the plotted data of Figures 3 t o 6. CONCLUSION
The results of this study indicate that the factors that influence the extent of coprecipitation of metal chelates with organic carriers can be combined into one unified representation, which is identical in form t o the quantitative expression used for the various equilibria involved in extraction of metal chelates by organic solvents. Furthermore, in
this type of coprecipitation, as in solvent extraction, only the metal ions that are chelated can be coprecipitated. LITERATURE CITED
(1) Chichilo, P., Specht, A. W., Whittaker, C. W., J . Assoc. O j i c . Agr. Chemists 38, 903 (1955). ( 2 ) Dehm, R. L., Dunn, W. G., Loder, E. R., A N A L . CHEM.33, 607 (1961). (3) Friedlander, G., Kennedy, J. W.,
“Introduction to Radiochemistry,” pp.
203-10, Wiley, New York, 1949. (4) Hankins, B. E., Ph.D. thesis, Universitv of Missouri. 1957. (5) Heggen, G . E., itrock, L. W.,ANAL. CHEM.25,859-63 (1953). (6) Johnston, W. D., Freiser, Henry, J . Am. Chem. Soc. 74,5239-42 (1952). ( 7 ) Koirtyohann, R . S., M.S. thesis, University of Missouri, 1958. (8) Kuznetsov, V. I., “Organic Co-
precipitants,” Session of the U.S.S.R. Academy of Science on Peaceful Uses of Atomic Energy, June 1-5, 1955, Chemical Sciences section; Zh. Analit.
Khim. 9 , 199 (1954). (9) Mitchell, R. L., Scott, R. O., J . Soc. Chem. lnd. (London) 66, 330-6 (1947). (10) Morrison, G. H., Freiser, Henry,
“Solvent Extraction in Analytical Chemistry,” pp. 51-2, Wiley, Xew York, 1957. (11) Pickett, E. E., Hankins, B. E., ANAL.CHEM.30, 47 (1958). (12) Steinback, J. F., Freiser, Henry, lbid., 25,881 (1953).
RECEIVED for review January 31, 1962. Accepted September 20, 1962. The authors express their ap reciation to the National Science Founfation for the fi. nancial assistance in the form of a Faculty Fellow to the first author while doing the above research.
Teflon Dropping Mercury Electrode for Polarography in Hydrofluoric Acid and Other Glass-Corroding Media HELEN P. RAAEN Analytical Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn.
b A Teflon D.M.E. satisfactory for polarography in glass-corroding media was developed for the first time, It is dimensionally stable and has a round smooth-walled orifice that can be made to within f10 microns of a specified diameter, a lapped face, and a bore of shape that can be varied. Teflon D.M.E.s of orifice diameter in the range from 15 to 1 10 microns were made. Satisfactory geometric and performance characteristics were proved by means of photomicrography and high-speed motion-picture photography and b y determining: the nature of drop formation; values of m, t, and w; electrocapillary curves; current-voltage recordings; polarographic reduction waves; and a diffusion current v5. concentration curve. Data were taken for glass-corroding solutions of hydrofluoric acid and sodium hydroxide. Reference data were taken for a glass D.M.E. in several media that do not corrode glass. The Teflon D.M.E. behaves essentially the same as a glass D.M.E. The few differences in behavior favor use of the Teflon D.M.E. It appears that certain data taken with glass and Teflon D.M.E.s can be intercompared,
A
TEFLONdropping mercury elec-
trode (D.M.E.), satisfactory for polarography in hydrofluoric acid and strong caustic media, has been developed
1714
0
ANALYTICAL CHEMISTRY
for the first time. Its design, fabrication, and evaluation are discussed herein. It performs essentially the same as a glass-capillary D.M.E. The few differences in behavior have been delineated; they favor the use of the Teflon D.M.E., even for polarography in media that are not glass-corroding. The development of this electrode is significant because the lack of a D.M.E. that performs satisfactorily in glasscorroding media has prevented polarographic studies in such media. Clifford and coworkers (5-8) fabricated a capillary (-S/i6-inch long) for the D.M.E. by squeezing (or molding) a short length of drilled Kel-F rod, heated to 210’ C. in glycerol, around a 0.03-mm. tungsten wire. They state that the fabrication mas decidedly an art. I n some 100 attempts, they made only one completely satisfactory capillary. I n liquid HF, the others gave such difficulties as irreproducible drop rate, spraying, and cessation of dropping. Nevertheless, by means of the one capillary they carried out the only polarographic studies of the hydrogen fluoride solvent system that have been made heretofore with a D.M.E. (6-8). Their experiments showed that practical polarography in anhydrous HF is feasible. They proposed t o “exhaust all possibilities not yet considered for obtaining a dropping mercury electrode which will give reproducible and definitive performance in HF” (8). Apparently, they did not succeed in so doing, for in
a later publication, Sargent, Clifford, and Lemmon (SI) describe the use of a rotating nickel wire electrode, instead of a D.M.E., for “polarographic” studies of the hydrogen fluoride solvent system. Griffiths and Parker (IS) attempted to fabricate capillaries for the D.M.E. from polyethylene, but the persistence of erratic behavior led them t o abandon the work and to fabricate instead a polyethylene flowing mercury electrode. They stated that polarographic studies in fluoride media with their flowing mercury electrode were proceeding and would be reported. Hume et al. (14) were able to make limited studies of metal fluoride complexes in aqueous acid solutions with a capillary covered on the outside with Tygon, but they found that, in a n acidic solution, rapid attack and leaching of lead from the lead-glass capillary occurred. Their efforts to develop plastic capillaries and to treat glass-capillary interiors m-ith plastic or silicone coatings were unsuccessful. Others have also attempted and failed t o make a D.M.E. from Teflon @8). DESIGN
Criteria. The major design criteria were defined by t h e intent t o develop a Teflon D.M.E. of geometric properties and performance characteristics like (or, if possible, superior to) those of a glass-capillary D.M.E. This objective demands t h a t , both in t h e absence and presence of applied po-
