.
(14) (15) (le) (17)
ton. N.J.. 1087. BB 252-3 rr W. ‘Huber, “Tltratlons In Nonaqueous Solvents,” Aosdemlc Press, New York. N.Y.. 1067. BO 151-4. J. Frltr and 5. Ya&mura, Anal. Chem., 29, 1070 (1967). T. Jaalnskl and H. Smagowskl, Chem. Anal. (Warsaw), 10, 1321 (1965). F. Crltchfleld and T. Johnson, Anal. Chem., 28, 1803 (1054). D.Bruss end G. Wyld, AM/. Ctwm., 29,232 (1957). D. Morman and G. Harlow, Anal. Chem., 39, 1850 (1067). R. Cundlff and P. Markunas, Anal. Chem., 28, 702 (1066). Q. Harlow and 6. WyM, Anal. Ctwm., 30,SQ (1066). I. M. Kolthoff and T. Reddy, horg. Chem., 1, 189 (1062). I, M. Kolthoff and M. K. Chantoonl, Jr., J. Am. Ctwm. Soc.,97, 1376 (1975). M. K. Chantoonl, Jr., and I. M. Kolthoff, J. Phys. Chem., 79, 1176 (1976). F. Wrathelmar end 0. Benfy, J. Am. Chem. Soc., 78,6309 (1056). See revlew, I. M. Kolthoff, Anal. Chem., 48, 1902 (1074). I. M. Kolthoff and M. K. Chantoonl, Jr., Anal. Chem., 39, 1080 (1967). I. M. Koithoff, S. Bruckensteln, and M. K. Chantoonl, Jr., J. Am. Ctmm. ~
Soc.,83, 3927 (1081). I. M. Kolthoff and M. K. Chantoonl, Jr., J. Fhys. Chem., 76, 2024 (1972). I. M. Kokhoff, M. K. Chantoonl, Jr., and H. Smagowskl, Anal. Chem., 42, 1622 (1070). I. M. Kolthoff and M. K. Chantoonl, Jr., J. Am. Ctwm. Soc., 87, 4426 (1065). I. M. Kolthoff, M. K. Chantoaril, Jr., and S. Bhowmlk, Anal. Chem,, 39, 315 (1967). R. Benolt and 5. Lam, J. Am. Chem. SOC.,98,7386 (1974). See e.9. D. Mclnnes, “The Prlnclples of Electrochemlshy,” Dover Editlon, New York, N.Y., 1061, p 400. Q. Plmental and A. MoCklbn, “The Hydrogen Bond,” W. Freeman 8 Co., San Franolsoo, Callf., 1960, pp 218-19,
~
RECEIVEDfor review May 9, 1975. Accepted July 3, 1975. This work has been supported by NSF Grants GP-20605 and MPS70-01756 A02.
Rapid Precipitation of Trace Metals from High Saline Matrices by Poly-5-vin y l-8-hydroxyquinoline John A. Buono Analytlcal Instrument Dlvlslon, Flsher Sclentlflc Company, 590 Llncoln Street, Waltham, Mass. 02 154
Jane C . Buono Personal Care Dlvlslon, QliletteCompany, Boston, Mass. 02 199
James L. Faschlng” DepatTment or‘Chemistry, Unlverslty of Rhode Island, Kingston, RJ 0288 1
Preclpltatlon has long been used for the concentratlon and separation of trace metals In solutlon. Recent emphasls has been placed on the use of polymerlc preclpltatlng reagents as a means of reduclng the tlme needed for the preclpltate to form completely. Thls work reports on the use of poly-5vlnyl-8-hydroxyqulnollne (PVO) as a trace metal preclpltatlon reagent. PVO quantltatlvely preclpltates alumlnum, cobalt, copper, Iron, lead, manganese, nlckel, vanadlum, and zlnc In less than two mlnutes. Slnce PVO doer not preclpltate alkall salts, It Is an excellent reagent for the fast, quantltatlve removal of trace metals from sallne matrlcw.
Precipitation has long been used as a technique for the concentration and separation of trace metals in solution. 8-Hydroxyquinoline (oxine) has been used as a trace metal precipitating reagent for more than sixty years. Fox ( I ) precipitated silver, lead, cadmium, zinc, cobalt, magnesium, calcium, and barium as oxinates as early as 1910. Haendler and Thompson (2) precipitated aluminum from sea water using oxine. Fleck and Ward (3) and Fleck (4) determined the composition of the oxinates of copper, manganese, nickel, cobalt, molybdenum, zinc, and magnesium. They also determined the effect of p H on the formation of the precipitate. Goto (5) determined the optimum p H range for the oxine precipitation of fifteen elements in 1937. Mitchell and Scott (6) precipitated copper, molybdenum, cobalt, zinc, and nickel from plant and soil digestates. The precipitation technique was also used to concentrate the above metals up to 500 times in the plant and soil digestates. Smith (7) also precipitated various metals from plant and soil digestates. Mitchell and Scott (8, 9) found that additional organic precipitating reagents were neces-
sary for the precipitation of lead, tin, and vanadium. Subsequent work by Heggen and Strock (10) supported this observation. Heggen and Strock used oxine to precipitate ten metals from biological and brine matrices. One of the largest drawbacks of using oxine as a precipitating reagent is the need to boil solutions for periods of three to twenty-four hours to quantitatively precipitate most metals. Riley and Topping (11) used 5,7-dibromooxine as a precipitation reagent. While this reagent exhibited a significantly lower solubility than oxine, solutions still had to be boiled for an hour to facilitate the precipitate formation. Muzzarelli ( 1 2 ) used a naturally occurring chelating polymer (chitosan) as a precipitating reagent in 1971. The precipitate formation required one half to one hour for completion. Chitosan would appear to be a very promising reagent for the preconcentration of trace metals but it is not useful as a fast separation reagent. Poly-5-vinyl-8-hydroxyquinoline(PVO) is a linear methylenic polymer which retains the chelating properties of oxine. The purpose of this paper is to demonstrate the utility of PVO as a precipitating reagent.
EXPERIMENTAL Reagents. All reagents were of analytical reagent grade. Standard solutions of trace metals were prepared from Fisher Atomic Absorption Stock Standards. PVO was prepared by the method of Vijayaraghavan (13). The structure of PVO is:
Author to whom correspondence should be addressed. 1926
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
~ % A ~ I
L
I
I
~H---cH,-.,
S I
1
".GI
5 0 0
' I O
5 0 0
8.00
3.00
I
P..
Figure 1. Percent retention of aluminum by 0.05, 0.02, and 0.01M PVO (PVO/oxine = 110)as a function of pH
Figure 2. Percent retention of copper by 0.05, 0.02, and 0.01 M PVO (PVO/oxine = 110) as a function of pH
PVO was dissolved in 75% acetic acid (35 mg PVO/ml). Oxine was also dissolved in 75% acetic acid (28 mg/ml). The effective chelating group concentration in these solutions was 0.20M. The concentrations of all metal salts used in this study were between one and ten parts per million. Analytical Procedures. Atomic absorption determinations of trace element concentrations were made on a Fisher Jarrell-Ash 810 Atomic Absorption Spectrophotometer equipped with a twopen recorder. All p H measurements were made with a Fisher Microprobe combination electrode coupled to an Accumet p H meter. One ml of the PVO in 75% acetic acid solution was placed in a polyethylene cell equipped with a magnetic stirrer with the selected metal salt. The acetic acid was neutralized with 30% ammonium hydroxide and the p H adjusted to the desired value by the addition of 5-10 ml of an acetate buffer. The precipitate formed immediately. The precipitation solution was stirred for 30 seconds and filtered through a 0.2-,u millipore filter (Millipore GSWP03700). The filtrate was eluted with 5 ml of buffer and transferred to a volumetric flask. The PVO-chelate was destroyed by the addition of 5 ml of concentrated nitric acid. The resultant solution was diluted to 25 ml and analyzed by atomic absorption. The filtrate was also brought up to 25 ml and analyzed by atomic absorption. The percent retention (extraction) was calculated for each cation as a function of pH, PVO concentration, and the ratio of PVO to oxine. The effective chelating group concentration [Q-] was determined from the availability of N in either PVO or oxine, since an N atom is necessary for the formation of the chelate. [Q-] varied from 0.01 to 0.05M (e.g., a 0.20M [Q-] solution contained either 35 mg PVO/ ml or 28 mg oxine/ml). Equimolar solutions of PVO and oxine were mixed to yield 1/0, 1/1, and 1/2 PVO/oxine ratios. Each separation was performed in triplicate.
num extraction can be achieved only a t a pH greater than 7. The percent extraction of aluminum by an equimolar mixture of PVO and oxine (PVO/oxine = 1/11 as a function of p H a t a [Q-] of 0.05, 0.02, and 0.01M, respectively, was also determined. The oxine was added to see if the presence of any mobile chelating groups would increase the extraction efficiency at low pH and/or at a low [&-I. The opposite effect was observed on the addition of oxine to the precipitation solution. The extraction efficiency was slightly reduced a t low pH even a t high [&-]. At the low total [Q-] of 0.01M, the presence of the oxine did not significantly change the extraction. The same experiments were repeated with a 1/2 PVO/ oxine solution as a function of pH a t 0.05, 0.02, and 0.01M total [Q-1. These data are comparable to the data for the equimolar mixture of PVO and oxine. The addition of twice as much oxine as PVO has caused the efficiency to decrease as the pH is lowered from 9 to 4. This may be explained by the increase in solubility of oxine and its chelates with a decrease in pH. Quantitative precipitation of A1 may be carried out between pH 8 and 9 a t all three &- concentrations even when the PVO/oxine equals 1/2. The percent precipitation of copper by PVO is shown as a function of pH and [&-I in Figure 2. Solvent extraction studies (14) have shown that the efficiency of the extraction of copper by oxine is independent of oxine concentration between 0.01 and 0.1M. The same phenomenon is seen in the precipitation of copper by PVO. At 0.05 and 0.02M PVO, the extraction is quantitative from pH 4-9. When 0.01M PVO is used, the extraction is still quantitative over a pH range of 4.0-7.0. The addition of an equimolar amount of oxine does not detract from the high copper precipitation efficiency. Even when twice as much oxine is present as is PVO, copper is quantitatively extracted over the pH range 4.0-9.0 for a 0.05 and a 0.02M mixture. When the [&-I is reduced to 0.01M, the pH range of quantitative extraction decreases to pH 4.0-7.0. Manganese is quantitatively precipitated from neutral and basic solutions. Figure 3 shows the percent precipitation of manganese by PVO alone. Manganese is quantitatively precipitated by 0.05M PVO from pH 7.0-9.0. When the PVO concentration is reduced from 0.05 to 0.02M or 0.01M, this p H range is reduced to pH 8.0-9.0. The same series of experiments were reported with equimolar amounts of PVO and oxine. The presence of the oxine has little effect at a pH greater than 6.0; however, the solubility
RESULTS AND DISCUSSION As would be expected from its structure, PVO retains the chelating properties of oxine. The relationship of the precipitation of trace metals by PVO to the pH of the solution is analogous to the relationship of the precipitation of metals by oxine to pH. The relationship of the extent of precipitation of metals by PVO to the PVO concentration is similar to the relationship between the extent of extraction of trace metals by oxine to the oxine concentration in solvent extraction. Figure 1 shows the percent extraction of aluminum by PVO (PVO/oxine = 1/0) as a function of pH a t 0.05, 0.02, and 0.01M [&-I, where [&-I is the relative concentration of the chelating group. At 0.05M PVO, greater than 95% of the aluminum is extracted a t a pH of 6 to 9. A precipitation which removes more than 95% of the cation of interest is assumed to be quantitative in this work. When the PVO concentration is reduced to 0.02M, only 91% of the aluminum is extracted by the PVO. At 0.01M PVO, 95% alumi-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
1927
Y
..:g
5.70
'.cc
c.ja
B.CC
9.9:
P.i
Flgure 3. Percent retention of manganese by 0.05, 0.02,and 0.01M
PVO (PVO/oxlne = 1 /0) as a function of pH
0 1
' il
'I,. I Y
Flgure 5. Percent retention of iron by 0.05, 0.02,and 0.01M PVO
(PVO/oxine = 1/0) as a function of pH
I
I
P'I I 0
u
s !
-
'
4 0 C
5.00
6.00
1.00
8.00
I 9.00
1
DH
Flgure 4. Percent retention of vanadium by 0.05, 0.02,and 0.01M
p v o (PVO/oxine = 110)as a function of pH
Flgure 6. Percent retention of nickel by 0.05, 0.02,and 0.01M PVO (PVO/oxine = 1/0) as a function of pH
of oxine lowers the overall precipitation efficiency a t a pH less than 6.0. The addition of twice as much oxine to the PVO does not appear to further affect the efficiency of the manganese precipitation over equimolar mixture. Figure 4 presents the results of the study of the efficiency of the precipitation of vanadium by PVO as a function of pH for 0.05, 0.02, and 0.01M PVO. Quantitative vanadium precipitation by PVO occurs only a t p H less than 5.0 regardless of PVO concentration. When an equimolar amount of oxine is added, vanadium is quantitatively precipitated below pH 4.5 at 0.05 and 0.02M [Q-] and below pH 4.0 a t 0.01M [Q-1. The same effect is found when a one to two (1/2) ratio of PVO to oxine is used as the precipitating reagent. The total [Q-] concentration must be greater than 0.02M and the pH must be less than 4.5 to achieve quantitative vanadium precipitation. The precipitation of iron by PVO is presented in Figure 5. PVO quantitatively precipitates iron from p H 4.0-9.0 at PVO concentrations from 0.01 to 0.05M just as 0.01M oxine extracts iron from pH 2.0-12.O.The analogous study of the percent precipitation of iron by an equimolar mixture of PVO oxine shows a slight decrease in the percent precipitation of iron by 0.01M PVO/oxine (1/1)at pH 4.0. This decrease is probably due to the losses of iron as ferric oxinate. If this is the cause of the loss or iron; then an in-
crease in the oxine concentration should decrease the percent of iron precipitated by PVO even more, and a t a PVO/ oxine of 1/2, the percent iron precipitation decreases slightly a t 0.05M [Q-] and a t pH 4.0. The decrease is more evident a t a PVO/oxine = 1/2 and a 0.02M [Q-] and pH 4.0. However, iron is still quantitatively precipitated over a pH range of 5.0-9.0 when the [Q-] is maintained at 0.02M. When the [Q-]is reduced to 0.01M, this range narrows to pH 5.5-6.5. Figure 6 represents the precipitation of nickel by 0.050.01M PVO as a function of pH. Nickel may be quantitatively precipitated by 0.05 and 0.02M PVO from pH 5.09.0. The addition of an equimolar amount of oxine slightly increases the precipitation as compared to PVO alone (Figure 6). However, when the PVO/oxine ratio is reduced to 1/2, the pH range over which quantitative nickel precipitation occurs narrows to pH 5.0-8.0 for 0.02M [Q-] and to pH 6.0-7.0 for 0.01M [Q-1. Lead is quantitatively precipitated by 0.05M PVO at pH 6.0-9.0 as shown in Figure 7. The extent of precipitation decreases as the concentration of PVO is decreased. The precipitation is decreased a t low pH and increased a t high pH, if an equimolar solution of PVO and oxine is used as the precipitating reagent. This is observed even a t the low [Q-] of 0.01M in the equimolar mixture of PVO/oxine. The
1928
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
---I
4.::
---b P L
P i
Figure 7. Percent retention of lead by 0.05, 0.02, and 0.01 M PVO (PVO/oxine = 110)as a function of pH
Flgure Q. Percent retention of zinc by 0.05, 0.02, and 0.01M PVO (PVO/oxine = 110) as a function of pH
k ' z
00 c m. LL Z
N
,/-+e
--,--- _ _ _ + _ - . -
c cc d
S
'P
Figure 8. Percent retention of cobalt by 0.05, 0.02, and 0.01 M PVO (PVOIoxine = 110) as a function of pH
increase in lead oxinate's solubility accounts for the decrease in lead precipitation as the p H decreases from 7.0 to 4.0. If the formation of the lead-PVO chelate is sterically hindered, the addition of a small amount of oxine would overcome this problem. A lead-PVO-oxine chelate would still be insoluble because of the bulk of the PVO molecule. This effect explains the increase in the precipitation efficiency at p H 7.0-9.0 at 0.02 and 0.01M total [Q-]in the PVO/oxine = 1/1 case. The precipitation efficiency is reduced even more a t pH 7.0-9.0 with the addition of twice as much oxine as PVO. This is due to the loss of some of the lead as soluble lead-oxinate. The percent precipitation of cobalt by PVO as a function of p H is shown in Figure 8. The p H range of quantitative precipitation narrows considerably as the PVO concentration is reduced from 0.05 to 0.01M. The addition of an equimolar amount of oxine further decreases the precipitation a t low p H but it does not significantly affect the cobalt precipitation above 7.0. Figure 9 presents the percent precipitation of zinc as a function of pH by 0.05-0.01M PVO. The p H range of quantitative precipitation is reduced from p H 6.0-9.0 for 0.05M PVO to pH 7.0-8.0 for 0.01M PVO. The addition of an equimolar amount of oxine slightly increases the precipitaANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
1929
Table I. Optimal pH Ranges for Greater t h a n 95% Precipitation
w
Precipitation reagent, 0.05M
m
:, :x.0
Element
a
Aluminum Cobalt Copper Iron Lead Manganese Nickel Vanadium Zinc
..
0
,-
&E. U 0 U
PVO
6-9 6-9 4-9 4-9 7-9 7-9 4-9 4-5 5-9
111
112
PV0;oxine
PVOIoxine
6-9 7-9 4-9 4-9 7-9 7-9 4-9 4-5 6-9
8-9 7-9 4-9 4-9 7-9 8-9 4-9 4-5 6-9
Nuclear Science Center and the University of Rhode Island Computer Laboratory for their assistance in obtaining and analyzing the experimental data. Figure 11. Percent retention of sodium by 0.05, 0.02, and 0.01M
LITERATURE CITED
PVO after the sample was washed with a potassium solution
tion of total [&-I concentration, PVO/oxine ratio, pH, and time of contact are needed to better understand the mechanism of the precipitate formation. SUMMARY PVO and mixtures of PVO and oxine have been used to precipitate copper, aluminum, vanadium, manganese, lead, cobalt, iron, and nickel from solution. By varying the pH and the reagent concentration, these trace metals may be rapidly removed from salt solutions. ACKNOWLEDGMENT The authors acknowledge Dr. B. M. Vittimberga and Dr. Vijayaraghavan for their helpful comments and suggestions concerning our production of PVO using their original organic synthesis procedure. We also thank the Rhode Island
(1) J. J.Fox, J. Chern. Soc.,97, lll9(1910). (2)H. M. Haendler and T. G. Thompson, J. Mar. Res., 2, 12 (1939). (3)H. R. Fleck and A. M. Ward, Analyst (London), 58, 388 (1933). (4)H. R. Fleck, Ana/yst(London),62, 378 (1937). (5) H. Goto, SOC.Rep. Tohoku,26, 391 (1937). (6)R. L. Mitchell and R. 0.Scott, J. SOC.Chem. lnd., London, 62, 4 (1943). (7)D. M. Smith, Ana/yst(London), 71, 368 (1946). (8)R. L. Mitchell and R. 0.Scott, Analyst(London),66,330(1947). (9)R. L. Mitchell and R. 0.Scott, Spectrochim. Acta, 3, 368 (1947). (10)G.E. Heggen and L. W. Strock, Anal. Chem., 25, 859 (1953). (1 1) J. P. Riley and G. Topping, Anal. Chim. Acta, 44,234 (1969). (12)R. A. A. Muzzareili, Anal. Chim. Acta, 54, 133 (1971). (13)Vijayaraghavan, Ph.D. Thesis, University of Rhode Island, Kingston, R.I., 1968. (14)J. D. Stary, "Solvent Extraction of Metal Chelates", Macmillan, New York, N.Y., 1964.
RECEIVEDfor review April 21, 1975. Accepted July 14, 1975. Work supported in part by the Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health, Grant No. 1 R01 HD 06675.
Kinetics of the Formation of 12-Molybdophosphate in Perchloric, Sulfuric, and Nitric Acid Solutions P. M. Beckwith,' Alexander Scheeline,* and S. R. Crouch3 Department of Chemistry,'MichiganState University, East Lansing, Mich. 48824
The klnetlcs of formation of the 12-molybdophosphate anion (12-MPA) from Mo(V1) and phosphate have been Investlgated In strong acid solutions by stopped-flow methods. Mechanisms are proposed for the reaction in HC104, " 0 3 , and H2S04 solutions whlch involve an lnltlai reaction between a Mo(VI) species and phosphate followed by several polymerlzatlon steps. The rate laws obtalned from the proposed mechanisms are In agreement wlth experimentally determined rate laws. The proposed mechanisms along wlth rate constants and activatlon energles give insight about the 1
Present address, BASF Wyandotte Corp., Wyandotte, Mich.
48192.
Present address, Department of Chemistry, University of Wisconsin, Madison, Wis. 53706. Author to whom requests for reprints should be sent. 1930
influence of the ac.- anion on the formation L. 2. !PA. The results obtalned can be used to choose reagent concentratlons for phosphate analyses uslng 12-MPA procedures.
The reaction of Mo(V1) and phosphate in strong acid solutions (pH
