Determination of Diethylzinc by Thermometric Titration W. L. EVERSON and EVELYN M. RAMIREZ Shell Development Co., Emeryville Calif. Hydrocarbon solutions of diethylzinc can be titrated thermometrically with o-phenanthroline (PHEN) or 8quinolinol (oxine). PHEN apparently is specific for Et2Zn in the presence of its oxidation/hydrolysis products. 2,2'-Bipyridyl (BIPY) reacts similarly but is a less satisfactory titrant. Oxine reacts quantitatively with both EtzZn and its oxidation products, EtZnOEt and Zn(OEt),; reaction with the hydrolysis product, EtZnOH, is incomplete. The BIPY and PHEN complexes are deep orange-red and should b e suitable for the spectrophotometric determination of EtzZn.
T
indicates increasing interest in the use of diethylzinc in catalytic systems, but little has appeared to date concerning its determination. Inasmuch as EtpZn reacts rapidly with oxygen and fairly rapidly with water and alcohols, its determination requires a selective reaction in the presence of its oxidation and/ or hydrolysis products. Xovak (9) has described a volumetric method for Et& based on reaction with excess iodine and hack-titi,ation with thiosulfate; the oxidation products are ZnI, and E t I . As thermometric titration is convenient and selective for the determination of alkyllithium (6) and alkylaluminum (6) compounds, its applicability to the determination of diethylzinc was studied. HE CHEMICAL LITERATCRE
The apparatus ronsisted of a vacuumjacketed titration flask, stoppered to exclude air and moisture, a thermistor in a Wheatstone bridge circuit to det,ect temperature changes, a motor-driven syringe buret for addition of titrant, and a strip-chart recorder to display teniljerature change as a function of titrant added. Details were given in a previous paper ( 5 ) . The following compounds were t'ested as possible titrant's.
o-Phenanthroline
Concentration, M Solvent
(PHES)
0.8
Anisole
(BIPY)
1 0-1.3
Toluene
Isoquinoline
1 6-1.7 1 0 1 2
Anisole Toluene Toluene
2 0 2 2
Toluene Dioxane
2 , 2 '-Bipgridyl
8-Quinolinol (oxine) Pvridine
E; hano1
(absolute) Water 81 2
ANALYTICAL CHEMISTRY
Et'zZn
+ EtOH =
EtZnOEt
EXPERIMENTAL
Compound
P H E X (G. Frederick Smith Chemical Co.) is supplied as the monohydrate; it was converted to the anhydrous form by heating at 100-10" C. BIPY, Oxine, isoquinoline, and pyridine were Eastman Chemical White Label grade and were used as received. Calcium hydride was added to the amine t'itrants to prevent moisture pickup. For titration, a weighed amount of 0.2 to 2.5M Et2Zn solution in toluene was added, via a hypodermic syringe, t,o 40 to 50 ml. of toluene (dried over calcium hydride) in a dry, nitrogen purged titration flask. P H E S and oxine were used in most tests; oxine was used both alone and following titration with other titrants. Tests wit'h P H E K were also made with the "cleanup" procedure, previously described ( 6 ) , in which a small portion of sample is added t'o remove reactive impurities from the titration solvent prior to analysis. One test was made with tetracyanoethylene solution, and one titration was made by simply adding oxygen gas from a motor-driven syringe buret. The effects of oxidative and hydrolytic deterioration were simulated by adding known amounts of ethanol and water, respectively. In these tests, the solution was stirred for 10 to 150 minutes before titration. The available literature indicates that the end products of ethanol reaction are the same as those formed in vapor phase oxidation.
+ EtOH
=
The first step in hydrolysis is Et,Zn
+ H20 = EtZnOH 4- CZH6 (3)
The end product of hydrolysis is ZnO rather than Zn(OH)z ( I d ) . The intense color of the P H E S and BIPY complexes suggested their possible use for spectrophotometric determination of diethylzinc Spectral transmittance curteb of the cornidexes were obtained on a Cary Model 1111S recording spectrophotometer, absorbance of the P H E X complex was tested biiefly using a Beckman hlodel B syectio1)hotometer REACTIONS OF DIETHYLZINC
Coates (4) mentions the formation by Me,Zn of bright yellow complexes with P H E S and I3IPY. Diethylzinc reacted similarly ; the complexes are
deep orange-red. They contain 1 mole of amine per mole of Et,Zn; the structures are assumed to be
u PHEN complex
BIPY complex
Haurowitz ( 7 ) states t'hat EtnZn dissolves in pyridine, evolving heat and forming a yellow liquid which fumes only slightly in air. Pajaro, Biagini, and Fiumani (IO) report formation of a fairly stable complex of &Zn wit'h isoquinoline. Other complexing reactions are known ( 4 ) , but' in general the complex is too weak (tetrahydrofuran) or the complexing agent is too unstable (methyllithium) t'o be of analytical interest. Bock (3) states that hydrazine reacts with EtlZn to form EtZnSHNH,. Reactions of EtzZn with oxygen, ethanol, and water are of particular interest in connection with the work reported here as well as with possible deterioration mechanisms. The reaction with oxygen apparent'ly yields various products depending on reaction conditions. I n early vapor-phase studies Thompson and Kelland (11) considered the product, ZnEt,Oz, to be peroxidic. Later, l3amford and Seivitt (2) showed that. in vapor phase osidation EtZnOEt is the final product if Et2Zn is in large excess; excess oxygen yields Zn(OEt)z. Recently, Abraham ( I ) reported that in ether or anisole solution the diperoxide, Zn(OOEt)?, can be formed almost quantitatively. Both Zn(OEt), and Zn(OOEt)? might be expected to react with EtzZn to produce EtZnOEt and Et'ZnOOEt, respectively; or the latter might rearrange to Zn(OEt),. These possible reactions do not appear t'o have been studied. I t seems likely that slight advent'itious oxidation, such as might occur in handling and storage, leads to EtZnOEt and that other possible products can be ignored. Herold, hggarwal, and Keff (8) studied t'he reaction of Et2Zn with several alcohols (ethanol was not included) and with water. They found, somewhat surprisingly, that EtZn(OiPr) is stable in the presence of excess isopropyl alcohol. R e have assumed that EtzZn reacts with ethanol in accordance with Equations 1 and 2 to form the same products as those formed by vapor phase oxidation. Our results are consistent with this assumption.
I
0.1’C
I
I -
Figure 1
. Titration
4
of diethylzinc with o-phenanthroline and 2,2’-bipyridine
Vogel (12) found the end product of hydrolysis to be ZnO rather than Zn(OH)s. Herold ( 8 ) established that EtZnOH is formed as an intermediate and that it disappears through a firstorder reaction, possibly via HO-Zn-Et i
:
ET-Zn- OH which can decompose to ZnO and ethane. Our results confirm t h a t EtZnOH [rather than Zn(OH), or ZnO] is the initial product. Only a few of the above reactions are useful in thermometric titration. Ethanol, water, and iodine react too slowly with Et2Zn to permit direct titration. Hydrazine is insoluble in solvents (hydrocarbons, ethers) which do not themselves react with Et2Zn. Pyridine and isoquinoline react rapidly and exothermically, but the titration curves do not show any breaks. Oxygen gas is not a practical titrant because of the relatively large volume required, the necessity for slow addition, and the difficulty of accurately measuring the amount consumed. Tetracyanoethylene might be a useful titrant, but was not available in sufficient quantity for testing. Of the compounds considered, this leaves only P H E N , BIPY, and oxine for further consideration. RESULTS AND DISCUSSION
Figure 1 shows typical titration curves for P H E N and BIPY. Both compounds react exothermically, and the heats of reaction are about the same; they are 2 kcal./gram-mole. estimated at 10 However, B I P Y gives curves which are somewhat rounded, possibly indicating an unfavorable equilibrium; P H E N is a much better titrant giving sharp, well defined breaks and considerably better precision. Figure 2 shows typical titration curves for oxine. The reaction is more strongly exothermic than the PHEN-BIPY re-
*
I mrnole
actions, about 33 f 4 kcal./gram-mole of oxine. Curve I , the type obtained with relatively pure EtzZn samples, shows three breaks. The first one is fairly well defined and corresponds to reaction of one mole of oxine per mole of EtzZn. The second break is poorly defined and sometimes is not observed; it is thought to be related to reaction of the Et-Zn bond in compounds of the EtZnO- type, but agreement with this assumption is not good. In any event, this break is not useful analytically. The final break generally is sharply defined, usually with a slight characteristic peak a t the inflection point; it represents reaction of two moles of oxine per mole of Et2Zn, EtZnOEt, and Zn(OEt),. Reaction with EtZnOH
proceeds somewhat past the 1 : l stage but is not stoichiometric. If the amount of oxidation products is large, or if oxine is used to titrate a sample which was previously titrated with B I P Y or P H E N , the curve is similar to I1 of Figure 2. I n this case only the final break is significant, and it is sharply defined. If much EtZnOH is present, the final break is considerably less sharp and may take the form of a smoothly rounded dome where no definite end point can be located. The steep rise at the start of curve I1 is abnormal; it is observed only when titration is made shortly after addition of ethanol or water. Similar segments of abnormally high slope are observed when such samples are titrated with P H E N or BIPY. Most of the work discussed subsequently was done on a freshly distilled sample of EtzZn, approximately 1.M in toluene. Table I shows results obtained for this sample by titration with P H E X , BIPY, and oxine. The results indicate that both T H E N and oxine can give precise results. For comparison, the total zinc was determined by hydrolysis and EDTA titration; the values obtained were 1.124 and 1.131, in good agreement with the P H E N and oxine values. Rate of titrant addition causes no significant variation in results; within reasonable limits, reaction speed is not a significant variable. The first oxine break, less sharply defined than the final break, showed somewhat poorer precision and somewhat lower values. The small difference between the two values, in this case, is attributed to inability to define the first break precisely. For purposes of further discussion, the value 1.135 (with a possible variation of
1
1.OT
Figure 2.
Titration of diethylzinc with oxine VOL. 37, NO. 7, JUNE 1965
81 3
Table 1.
Titration of Diethylzinc with PHEN, BIPY, and Oxine
EtlZn, mmoles/gram, by titration with Oxine, to PHEN BIPY First break Final break 1,134 1,137 1.142 1.137b 1.115 1.138 1 ,140b 1,112b (1,009)d (1. 130)d 1.142 1.223 1.144 1.130 ( 1 , 143)d 1.135 1,136
Titration technique Simple titrationQ “Cleanup” titrationC “Cleanup” titration,c repeated
Av., excluding “cleanup” samples 1.138 1.176 1.121 1.140 Rate of titrant a 1.5-2.1 grams of sample added to 47 ml. of toluene and titrated. addition, 0.5 ml./min. except as noted. 6 Rate of titrant addition, 0.2 ml./min. Small “cleanup” sample added and allowed t o react with reactive impurities, if any, in titration system before main samples were added. Excess reagent from each titration counted as part of following titration. Variation of this result from those following is rough indication d “Cleanup” sample. of amount of reactive impurities, if any, present before starting the analysis. These results are not considered valid.
1.0.007) is considered to be the actual EtzZn content. The BIPY results lack precision and are included only to indicate the 1: 1 stoichiometry. There is little difference between results by simple titration and those where the “cleanup” technique was used. However, in the first set of P H E N titrations using the latter tech-
Table If. Titration of Diethylzinc with Oxine in Presence of a Complexing Agent
EtlZn, mmoles/gram, by titration with ComplexComplexing agent added PHEN BIPY Pyridine Isoquinoline
w
agent 1,136 1.176 b b
Oxinea 1,072 1.137 1,142 1,134
Curves showed no intermediate breaks; results calculated from final break. 6 K O result could be calculated; titration curve showed no break. 0
Table 111. Change in PHEN Titration of Et,Zn-EtOH Reaction Mixture with Time”
Zinc found by PHEN titration, mmole/gram, for Normal Time, Initial break min. steep slope 20 0.0118 0,1581 0,1558 40 0,0063 h 0.1561 75 b 0.1560 150 a 2.98 mmoles of EtOH added to 8.68 mmoles of Et2Zn in toluene. Calculated composition, mmolea/gram, total Zn 0.2358; EtZnOEt, 0.0812; unreacted Et& (by difference), 0.1546. None detected.
81 4
ANALYTICAL CHEMISTRY
nique, some reactive impurity evidently was removed by the “cleanup” sample. I n general, we consider this technique to be preferable (when it can be used; its usefulness with oxine was not tested) because it is less vulnerable to contamination errors and because comparison of results for the “cleanup” sample and subsequent samples gives some idea of the level of impurities in the titration system and the eflectiveness of the solvent purification and sample-vessel preparation techniques in use. Table I1 shows results in which titration was first attempted with an aminetype complexing agent and the resulting solution was then titrated with oxine. The presence of equimolar amounts of BIPY, pyridine, or isoquinoline does not have a significant effect on the oxine titration However, in all cases where oxine followed P H E N (see also Table IV, test 6), the oxine value was low. This might be attributed to the hygroscopic nature of P H E N and the difficulty of keeping the titrant absolutely dry; however, results with the “cleanup” technique (Table I) do not indicate that a significant amount of water was introduced with the P H E N titrant. If values for both P H E N and oxine are needed, it is better to obtain them on separate samples. A number of tests was made in which small, measured amounts of ethanol ( 2 M in toluene) were added to EtsZn solutions in the same manner as in making a titration. The resulting curves clearly showed the reaction to be slow. On addition of titrant, there was an appreciable time lag before the temperature started to rise and the initial portion of the curve was concave upward. When titrant addition was stopped, the curve became concave downward as the temperature continued
to rise for several minutes and slowly levelled off. Apparently the cessation of temperature rise did not mean that all reaction had ceased; titration with P H E S , BIPY, or oxine during the next half hour or more gave a titration curve with an initial segment of abnormally steep slope. Because EtZnOEt does not react with P H E N or BIPY, it seems that the situation is more complex than is indicated by Equation 1; possibly some transient intermediate is formed in the reaction of EtzZn with ethanol. Aifterreacting for varying lengths of time, these reaction mixtures were titrated with various titrants. Table I11 gives results for a test in which P H E N was used as titrant and the reaction time varied. The total value calculated for the normal P H E N break was approximately constant and in good agreement with the calculated value for unreacted Et2Zn. The initial segment of abnormal slope decreased rapidly and was not observed after the second test. Table IV presents additional results in which varying amounts of EtOH were added to EtzZn before titration. The initial segment of abnormally steep slope was observed in the three 15minute tests but not in the other tests where the reaction time was an hour or more. Test 2 indicates that xine reacts quantitatively with both EtZnOEt and Zn(OEt)2-i.e., with both of the likely oxidation products of EtzZn. An alternative explanation is that Equation 2 represents a slow reaction so that only EtZnOEt and excess alcohol, but no Zn(OEt)z, were present in this test. However, there is other evidence that Zn(OEt)2 reacts with oxine; in the case discussed below where a sample was titrated with oxygen gas and subsequently with oxine, the total Zn value by oxine titration corresponded to about 9674 of the EtpZn originally present. I n tests 3 and 6 the values found by P H E N titration for EtzZn in the presence of EtZnOH are 98% and l06%, respectively, of the calculated values. However, it is possible that they are actually more accurate then the latter. Test 3 involved an alcohol solution which may have contained a trace of water, while a new solution was prepared for test 6; the high result in the latter case may reflect slow reaction of ethanol in the absence of water. Test 5 , which involved the new alcohol solution and gave a result of 107y0 of the calculated EtPZn value, is a t least consistent with this view. Test 5 indicates that oxine can give fairly reliable values, in a single titration, for both EtpZn and total zinc. Earlier work, not reported here, supports this conclusion. The value for EtzZn obtained in test 4 is unreliable,
Table IV.
Titration of EtzZn-EtOH Reaction Mixtures"
Test NO.
EtOH Calculated composition* . Reaction added Et2Zn EtZnFEt Zn(0Et)z time, min. 1 1.296 0 0.974 0.161 15 2 1,905 0 0.365 0.770 120 3 0.484 0.651 0.484 ... 15 0,543 ... 0,592 15 4 0.543 0.558 ... 65 0.577 5 0.558 0.590 , . . 75 0.545 6 0,590 All values in mmoles/gram. Original EtZZn content taken as 1.135. b Based on original EtzZn content and Equations 1 and 2. c EtZnOEt determined by difference. d Titration with BIPY. Curve had abnormally high slope. Titration with PHEN. J Titration with oxine (following PHEN, in tests 2 and 6).
EtZZn 0 . 144d
Found by titration" EtZnOEt Total Zn ...
Oe
0.636e 1.181) 0.598J 0.579'
01499 ( neg ) 0.536 0.517
1.'135/ 1 ,'128/ 1,134f 1,096,
(I
but here the EtPZn break may have been shifted by the abnormally steep initial portion of the curve. P H E N , which gives only a single break, is considered a more reliable titrant for the EtnZn content. Test 6 is in line with the previous observation that the oxine value for total zinc is low when oxine titration follows P H E N titration. I n test 2 this effect probably was not noted because only a small amount of P H E X titrant was needed to establish the absence of Et2Zn, and oxine titration followed almost immediately. The reaction of water with EtzZn was studied in similar fashion. I t is also a slow reaction, although apparently somewhat faster than reaction with ethanol; and as with ethanol, titration curves obtained shortly after the addition of water show an initial segment of abnormally steep slope. Results are given in Table V. Results are consistent with the finding of Herold, Aggarwal, and Keff ( 8 ) that EtZnOH is the initial product of hydrolysis. Tests 3 and 4 indicate that this product slowly reacts to consume EtpZn; the amount of unreacted Etz% found is slightly less than the calculated value in test 3, while in test 4 it has dropped to 85% of the calculated value. Oxine does not give reliable values for total zinc in systems to which water has been added. Kot only are the results low but the shape of the final break changes, with increasing water content, from a sharply defined peak t o a broad, rounded dome where only a very ap-
Table VI.
Test XO.
1 2 3
Oxygen added as
Table V.
Titration of EtZnz-Water Reaction Mixtures"
Found by Titrationc Calculated composition * Reaction "Total" KO. EtzZn EtZnOH time, min. EtzZn Zn 1 0.989 0.146 5 0.976 0,983 2 0.760 0.375 5 0.757 1.08 3 0.577 0.578 5 0 560 1 02 120 0.654 4 0 769 0 366 0 813 0 322 5 15 1.02 1107 0 902 0 233 6 60 0 78 0 86 Original Et2Zn content taken as 1.135. 0 All values in mmoles/gram. b EtZnOH assumed equal to water added. Et2Zn by difference. "Total" c Et,Zn by PHEN titration on tests 1 to 4; by oxine titration on tests 5 and 6. Zn from final break of oxine curve. Test
-
proximate estimate of the end point is possible. Only the first test gave a sharp peak. I n most of the tests the oxine value was higher than the calculated EtsZn content, indicating that EtZnOH and oxine do react to some extent, An initial segment of abnormally steep slope was observed in all cases. For test 1 to 4 the values for this segment were 0.054, 0.104, 0.139, and 0.021 mmoles/gram; the steep segment in test 4 was very slight. I n tests 1 to 3 the values increase with increasing concentration of water added. During the exploratory phase of this work, a measured amount of oxygen was added from a motor-driven syringe buret, both to produce oxidation independent of the assumptions regarding the mechanism of the EtzZn-EtOH reaction and to see whether quantitative indication of a reaction could be obtained using oxygen gas as titrant.
Titration of Et,Zn with Oxine Following Air or Oxygen Addition"
Et2Zn
Calculated composition EtZnOEt
Zn(0Et)n ...
Air,' 6 :020 0:4i8 0 : 060 ... 0.139 0.099 0.339 4 02, 0.491' ... ... 0:47s 0 All results in millimoles per gram. b EtzZn from first oxine break, total Zn from final break, EtZnOEt by difference. c Sample titrated with oxygen. 02,
The solutions were subsequently titrated with oxine. Table VI summarizes the results of these experiments. These tests give some indication that direct oxidation and reaction with ethanol give equivalent products. Oxygen is not considered a practical titrant because of the large volume required, the need for slow addition, and the fact that the titration curve was not well defined. The initial portion was normal, but a break was obtained a t an O/Zn ratio of 0.65 rather than the expected 1.0. The temperature then remained approximately constant-i.e., heat production was balanced by heat loss-up to a second break a t an O/Zn ratio of 2.06, after which the temperature dropped. The end product was assumed to be Zn(OEt)n rather than the monoperoxide, EtZnOOEt, but the solution was not tested for peroxide. There was no evidence of formation of the diperoxide reported by dbraham
Et2Zn 0,438 0.436 0.329 ...
Found by oxine titrationb EtZnOEt Total Zn 0.040 0,478 0.044 0.480 0.147 0.476 ... 0.458
VOL. 37, NO. 7 ,JUNE 1965
81 5
(11, which would have required an O’Zn ratio of 4.0. The BIPY and P H E N complexes have rather similar spectral curves, showing a pronounced shoulder with maximum absorbance a t about 425 mp. In a single test, successive increments of EtnZn were added to 25 ml. of a toluene solution containing excess P H E N plus enough EtzZn to give a definite color. The plot of absorbance us. EtzZn added was linear over the range tested (0.4 to 3 mg. of Et2Zn) with an indicated absorptivity of about 14 liters/gram-cm. Spectrophotometric determination of EtrZn in this range should present no difficulty. Thermometric titration with P H E N provides a simple, rapid, and precise method for determining the net EtPZn content of a solution which may also contain its oxidation or hydrolysis
products. Titration with Oxine gives a precise measure of the total zinc content in Et2Zn solutions which contain its oxidation products, but is not reliable if hydrolysis products are present. Under favorable circumstances oxine gives, in the same titration, a measure of both total zinc and EtzZn. The use of oxine as titrant in a hydrocarbon system may be of interest as a possible means for the analysis of other organometallic compounds. ACKNOWLEDGMENT
The authors thank John Boor, Jr., for samples of purified diethylzinc and for helpful suggestions and encouragement. LITERATURE CITED
(1) Abraham, M. H., Chem. Ind. (London) 1959,p. 750.
(2) Bamford, C. H., Newitt, D. M,, J . Chem. SOC.1946, p. 688. (3) Bock, H., 2. “Vaturforsch 17b, 426 (1962). (4) Coate;; G. E., “Organometallic Compounds, 2nd ed., Wiley, New York, 1960. (5) Everson, W. L., ANAL. CHEM.36, 854 (1964). (6) Everson, W. L., Ramirez, E. >I., Ibzd., 37, 806 (1965). (7) Haurowitz, F., Mikrochemie 7, 88 (1929). (8) Herold, R. J., Aggarwal, S.L., Neff, W., Can. J . Chem. 41, 1368 (1963). (9) Novak, K., Chem. Prumysl 12, 551 (1962). (10) Pajaro, G., Biagini, S., Fiumani, D., Angew. Chem. (Int.. Ed.) 2, 94 (1963). (11) Thompson, H. W., Kelland, N . S., J . Chem. SOC.1933, p. 746. (12) S’ogel, C. H., Monatsber. Deut. A k a d . Wiss. Berlin 2, 115 (1960). RECEIVEDfor review January 8, 1965. Accepted ;\larch 29, 1965.
Polarographic Behavior of Molybdenum(V1) in Acidic Chloride Media J. J. WITTICK’ and G. A. RECHNITZ Department o f Chemistry, University of Pennsylvania, Philadelphia, Pa.
b The polarographic behavior of molybdenum VI has been investigated in the concentration range of 1 10-3 to 5 I O - 6 ~ M~(VII in supporting electrolytes of widely varying hydrogen ion and chloride ion concentrations. On the basis of polarographic, coulometric, and spectrophotometric evidence, the three waves present in dilute hydrochloric acid are ascribed to the reduction of two species of molybdenum(V1) in slow equilibrium, to produce molybdenum(V) and (111) on the first and third waves, on the one hand, and molybdenum(1V) at potentials corresponding to the second wave, on the other. A reversible, 2-electron wave for the reduction of molybdenum(V), formed by reoxidation of molybdenum(lll), is reported and discussed in critical detail along with other secondary reactions resulting from the disproportionation of molybdenum(lV) and the reoxidation of molybdenum(lll), respectively.
x
x
portion of our knowledge of the chemistry of molybdenum in hydrochloric acid rests upon conclusions drawn from polarographic data. Thus, the catalytic reduction of perchlorate ( I ?‘), nitrate (16), and oxalate (34) in the presence of molybdenum is attributed to certain oxidation states of molybdenum on the basis of SUBSTANTIAL
816
ANALYTICAL CHEMISTRY
polarographic investigations which demonstrate the enhancement of characteristic reduction waves. Polarography has also been employed as the basis of arguments concerning the nature of the isopoly acids which are characteristic of Mo(V1) in acidic media (12, 32) and to explain the complex and controversial color changes which accompany the quinquevalent and trivalent oxidation states (14, 21) in solution. I n all of these diagnostic attacks upon the structural or mechanistic problems of molybdenum chemistry, it is obvious that a reliable interpretation of the polarographic behavior of molybdenum(V1) is essential. Attempts to interpret the polarographic waves exhibited by Mo(V1) in hydrochloric acid began in 1941 with a rudimentary examination by Holtje and Geyer (20). Carritt (6) used controlled potential electrolysis to identify polarographic reduction products and postulated the existence of two species of Mo(V1) in sluggish equilibrium to account for the observed polarographic waves. Although his work was done a t relatively high concentrations of molybdenum where serious problems with maxima were encountered, he furnished the first clues to the difficulties inherent in molybdenum polarography. h comprehensive re-examination of the system was subsequently reported by Guibe and Souchay (14) and included polarographic data on the lower oxidation
states of molybdenum in partial contradiction to the results of Carritt. H o w ever, Guibe and Souchay gave no data regarding capillary characteristics or maximum suppressors, and their conclusions based on relative wave heights were unsupported by any of the usual tests for diffusion control. In a recent investigation, Haight ( I 7 ) concluded that one of the characteristic reduction waves of Mo(V1) involves the production of Mo(1V). Both Carritt and Guibe attributed this wave to the reduction of Mo(V) to Mo(IIIj, however. The present investigation represents an attempt to furnish a definitive explanation for the polarographic behavior of Jlo(V1) and thereby to resolve the above differences by the use of controlled potential coulometry and through the extension of measurements to new concentration ranges under conditions which permit the isolation of critical experimental variables. EXPERIMENTAL
Apparatus. A Sargent Model XV Polarograph, having a pen speed of 1 second for full-scale deflection and equipped with a micro range extender for high sensitivity work, was used to record all polarograms. Maximum suppressors, including agar, have been shown to distort the Present address, Merck, Sharp & Dohme Research Laboratories, Rahway, N. J.
