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Polarographic determination of zinc in hydroxide media
JCWRNAL
OF
ELECTROANALYTIGAL
CHEMISTRY
407
During an. investigation of the zinc gluconate complexes, wo had occasion to analyze for zinc ion in the presence of sodium hydrorride, l!!b%YROVSK%? ANI) I~~ovrC-2 first -1.45 V “JS. S.C.E. for zinc ion in hydroxide solureported a half-wave. potential of tions, but did not give any details of solution composition, or the diffusion cnrrent constant. More recently, LINGANE$ discussed the rednction of zinc ion in I F sodium hydroxide with 0_or0/~ gelatin as a maximum suppressor. IIe reported2 half-wave potenti~ of ---r-53 V ZIS,S.C.E. and a dif~~ion current constant, J”*of 3.r4 (average current) _ MEITES~ has tabulated poIarographic data for zinc under a variety of solution conditions, Because we could not reproduce the data of LINGANE”, a brief study has been made of the best conditions for determining zinc in sodium hydroxide solutions. The stack solution of zinc perchlorate was standardized gravimetricallyd and by titration with EDT.A5_ All materials were reagent grade.
-1.3
-1.5
E Yf. S.C.EW
““-1.7
Fig- x.. Pols~ogfams of zinc ion in the presence of I F sodium hydroxide. F in zinc perchlorate. Curve A: no geIa&; curve l3: o.ooz~~~~ gefatia;
All solutions were r * x0-3 curve c: ~-oxo~$~ gelatin_
* This \s-ork‘cftassuppol-ted bqr the United States Atomic Energy Commisston under Contract No. AT(rx--I)-34, Project No, 45 Appreciation is expressed to tie National Science Foundation for an Undergraduate Summer Fellowship which was akzrded to one of the aq#xors (R.J.K.) _
SHORT CO1IBIUNICATIONS
408
In the absence of gelatin, zinc ion in I F NaOH gives a broad maximum as shown by curve A in Fig. I, with a half-wave potential of --I.49 V ‘us. S.C.E. The height of the maximum is proportional to zinc ion concentration and obeys the relation : where id is the diffusion current at the maximum, DLZ/~ and tl/s are the capillary constants (2x0 at -x.5 V vs. S.C.E.), C is the concentration in mmoles per liter, and Imax is the diffusion current constant (maximum rather than average currentj. The equation indicates that the current-concn. curve does not go through the origin. The diffusion current constant, Ins,, has a value of 4-89 for r-o-r.5 Fe NaOH and is applicable for zinc ion concentrations between 0.2 - 10-3 and 2 - 10-3 F. Large amounts of perchlorate ion interfere by causing surface effects and nitrate ions interfere by giving a nitrate wave. If gelatin is used at the concentration suggested by LINGANE~, curve C of Fig. I results, which is totally useless for analytical purposes. The discrepancy between our data and LINGANE’S may be due to the source of gelatin (we used either Baker and Adamson powdered gelatin, Code r7g7 or Knox U.S.P. gelatin)..Curve B in Fig. r is for a solution containing r F NaOH and o_oo25°/o gelatin; the half-wave_ potential is --I -49 V vs- SC-E. For o_oo20°/o gelatin and I F NaOH a linear current-concn. relation is obtained with the curve going through the origin (the half-wave potential is --I_49 V vs. S.C.E.). The diffusion current constant, Imsx, is 3-54 (maximum rather than average current) ; this value is applicable for zinc ion concentrations from 0-05 - 10-a to 8 - ro-a F_
ODOO
oooz
Gelorin
0006
0004
con
c.
(wt-
%I
ofgelatinconcentraFig- z. Diffusioncurrentforzincionin I Fsodiumhydrosideasafunction tion..AIisolutions were I - 10-3 Fin zinc perchlorate-
The effect of gelatin- concentration upon the diffusion current of zinc ion in the presence of r F NaOH is shown in-Fig_ z ; a useful reduction wave is not obtziined for gela& concentrations above o_o05~/~:- A gelatin concentration of o,oo2d~/o appears to be the most ~&tisf&tory~ from an _~analytical.standpoint. J.
Electroanal.
CF+em.,
3
(1962) 407~409
SHORT COMMUNICATIONS
409
CANNAN AND KIBRICE~ indicated that gluconate ion forms a weak complex with zinc ion. To confirm this, a series of polarograms has been run with various solution conditions. Table I summarizes the half-wave potentials for these studies. TABLE HALF-XvAVE
POIEh'TIAB
FOR
ZIh'C ION
IX
I
THE PRESEXCE GLUCONATE
Sodiumglucomztcconctr. A.
0.01
FNaOH
B. 0.03 F NaOH C.
1.0
F
NaOH
0.00 F 0.05 F O-15
F
0.00 0.02
F F
0.00 F 0.~5 F x.00
F
OF
SODIU%I
Em
HYDRO2iIDE
AXD
SODIUM
vs. S.C.E.
-1.31 --r-35 -r.qr -1-35 -1-35
--r-49 --I_50 -1.5"
The results in this Table indicate that the complex with gluconate is weak and that hydroxide ion competes in the complexing of the zinc ion. -In fact, the gluconate concentration must be equal to or greater than the hydroxide ion concentration, if significant gluconate complexing is to be observed. All of the data in Table I are for solutions without gelatin ; in the presence of gluconate, maxima are not observed for zinc ion. Because the waves are irreversible, quantitative conclusions concerning the stability constant of the zinc-gluconate complex are not possible. Qualitatively, the data indicate a complex with a constant comparable to that given by CANNAN AND KIBRICK~ (pK equal-to 1_7)_ However, the complex under our conditions is formed from the Zn(OH)3 species?, which has a stability constant of 1014. Thus, if equal molar concentrations of gluconate ion and hydroxide ion show an effect for a gluconate complex (and if it is a I : I complex), then the stability constant for this gluconate complex, when formed from the free zinc ion, might be approximately ION*. Department
DONALD T. SAWYER RICHARD J. KULA
of Chemistry,
U9aiversity of California, Riverside,
1 2
CaZif_ (U.S.A.)
J_ HEYROVSKT~
ILKOVIC, CoUeectionCzechosEov. Chem. Conmmns-. 15 (1913) 583. PoZarogvaphic Techniques, Interscience Publishers, Inc., New AND
D.
J_ J_ L~~~_~~,;Zlzab.Chenz..
7 (1935) 19s.
L. MEITES, York, N.Y.. Ig5s.p~. 294.2954 W. F. HILLEBRAND ASD G. E-F. LuNDuL,;~&%~~~ Inorganic AnaZysis, 2ndedn.. John Wiley andSons. Inc_.Ne\vYork,N.Y.. I953,pp_428-431~ 5 H. A.FLASCHKA. EDTA Titrations. Pergamon Press,Nq+York,N.Y., 1959,pp75. 76. 6 R. K.CANtiAxAND -4. KIBRICK, J_ Am.Chenz_Sdc.-,60(193S) 23X4~7 J_ BJERRUM, G. SCHW_~RGENBACH A~'D L. G. SILLI%, Stability Co?zsta?zts. II.Inor~a&c&ands. SpecialPublicationNo.7,TheChemicalSociety.London.I9583
Received J_ EkctroandZ.
November Chenr..
r8th,. 1961
3 (rg62)
407-409
OF
ELECTROANALYTIGAL
CHEMISTRY
407
During an. investigation of the zinc gluconate complexes, wo had occasion to analyze for zinc ion in the presence of sodium hydrorride, l!!b%YROVSK%? ANI) I~~ovrC-2 first -1.45 V “JS. S.C.E. for zinc ion in hydroxide solureported a half-wave. potential of tions, but did not give any details of solution composition, or the diffusion cnrrent constant. More recently, LINGANE$ discussed the rednction of zinc ion in I F sodium hydroxide with 0_or0/~ gelatin as a maximum suppressor. IIe reported2 half-wave potenti~ of ---r-53 V ZIS,S.C.E. and a dif~~ion current constant, J”*of 3.r4 (average current) _ MEITES~ has tabulated poIarographic data for zinc under a variety of solution conditions, Because we could not reproduce the data of LINGANE”, a brief study has been made of the best conditions for determining zinc in sodium hydroxide solutions. The stack solution of zinc perchlorate was standardized gravimetricallyd and by titration with EDT.A5_ All materials were reagent grade.
-1.3
-1.5
E Yf. S.C.EW
““-1.7
Fig- x.. Pols~ogfams of zinc ion in the presence of I F sodium hydroxide. F in zinc perchlorate. Curve A: no geIa&; curve l3: o.ooz~~~~ gefatia;
All solutions were r * x0-3 curve c: ~-oxo~$~ gelatin_
* This \s-ork‘cftassuppol-ted bqr the United States Atomic Energy Commisston under Contract No. AT(rx--I)-34, Project No, 45 Appreciation is expressed to tie National Science Foundation for an Undergraduate Summer Fellowship which was akzrded to one of the aq#xors (R.J.K.) _
SHORT CO1IBIUNICATIONS
408
In the absence of gelatin, zinc ion in I F NaOH gives a broad maximum as shown by curve A in Fig. I, with a half-wave potential of --I.49 V ‘us. S.C.E. The height of the maximum is proportional to zinc ion concentration and obeys the relation : where id is the diffusion current at the maximum, DLZ/~ and tl/s are the capillary constants (2x0 at -x.5 V vs. S.C.E.), C is the concentration in mmoles per liter, and Imax is the diffusion current constant (maximum rather than average currentj. The equation indicates that the current-concn. curve does not go through the origin. The diffusion current constant, Ins,, has a value of 4-89 for r-o-r.5 Fe NaOH and is applicable for zinc ion concentrations between 0.2 - 10-3 and 2 - 10-3 F. Large amounts of perchlorate ion interfere by causing surface effects and nitrate ions interfere by giving a nitrate wave. If gelatin is used at the concentration suggested by LINGANE~, curve C of Fig. I results, which is totally useless for analytical purposes. The discrepancy between our data and LINGANE’S may be due to the source of gelatin (we used either Baker and Adamson powdered gelatin, Code r7g7 or Knox U.S.P. gelatin)..Curve B in Fig. r is for a solution containing r F NaOH and o_oo25°/o gelatin; the half-wave_ potential is --I -49 V vs- SC-E. For o_oo20°/o gelatin and I F NaOH a linear current-concn. relation is obtained with the curve going through the origin (the half-wave potential is --I_49 V vs. S.C.E.). The diffusion current constant, Imsx, is 3-54 (maximum rather than average current) ; this value is applicable for zinc ion concentrations from 0-05 - 10-a to 8 - ro-a F_
ODOO
oooz
Gelorin
0006
0004
con
c.
(wt-
%I
ofgelatinconcentraFig- z. Diffusioncurrentforzincionin I Fsodiumhydrosideasafunction tion..AIisolutions were I - 10-3 Fin zinc perchlorate-
The effect of gelatin- concentration upon the diffusion current of zinc ion in the presence of r F NaOH is shown in-Fig_ z ; a useful reduction wave is not obtziined for gela& concentrations above o_o05~/~:- A gelatin concentration of o,oo2d~/o appears to be the most ~&tisf&tory~ from an _~analytical.standpoint. J.
Electroanal.
CF+em.,
3
(1962) 407~409
SHORT COMMUNICATIONS
409
CANNAN AND KIBRICE~ indicated that gluconate ion forms a weak complex with zinc ion. To confirm this, a series of polarograms has been run with various solution conditions. Table I summarizes the half-wave potentials for these studies. TABLE HALF-XvAVE
POIEh'TIAB
FOR
ZIh'C ION
IX
I
THE PRESEXCE GLUCONATE
Sodiumglucomztcconctr. A.
0.01
FNaOH
B. 0.03 F NaOH C.
1.0
F
NaOH
0.00 F 0.05 F O-15
F
0.00 0.02
F F
0.00 F 0.~5 F x.00
F
OF
SODIU%I
Em
HYDRO2iIDE
AXD
SODIUM
vs. S.C.E.
-1.31 --r-35 -r.qr -1-35 -1-35
--r-49 --I_50 -1.5"
The results in this Table indicate that the complex with gluconate is weak and that hydroxide ion competes in the complexing of the zinc ion. -In fact, the gluconate concentration must be equal to or greater than the hydroxide ion concentration, if significant gluconate complexing is to be observed. All of the data in Table I are for solutions without gelatin ; in the presence of gluconate, maxima are not observed for zinc ion. Because the waves are irreversible, quantitative conclusions concerning the stability constant of the zinc-gluconate complex are not possible. Qualitatively, the data indicate a complex with a constant comparable to that given by CANNAN AND KIBRICK~ (pK equal-to 1_7)_ However, the complex under our conditions is formed from the Zn(OH)3 species?, which has a stability constant of 1014. Thus, if equal molar concentrations of gluconate ion and hydroxide ion show an effect for a gluconate complex (and if it is a I : I complex), then the stability constant for this gluconate complex, when formed from the free zinc ion, might be approximately ION*. Department
DONALD T. SAWYER RICHARD J. KULA
of Chemistry,
U9aiversity of California, Riverside,
1 2
CaZif_ (U.S.A.)
J_ HEYROVSKT~
ILKOVIC, CoUeectionCzechosEov. Chem. Conmmns-. 15 (1913) 583. PoZarogvaphic Techniques, Interscience Publishers, Inc., New AND
D.
J_ J_ L~~~_~~,;Zlzab.Chenz..
7 (1935) 19s.
L. MEITES, York, N.Y.. Ig5s.p~. 294.2954 W. F. HILLEBRAND ASD G. E-F. LuNDuL,;~&%~~~ Inorganic AnaZysis, 2ndedn.. John Wiley andSons. Inc_.Ne\vYork,N.Y.. I953,pp_428-431~ 5 H. A.FLASCHKA. EDTA Titrations. Pergamon Press,Nq+York,N.Y., 1959,pp75. 76. 6 R. K.CANtiAxAND -4. KIBRICK, J_ Am.Chenz_Sdc.-,60(193S) 23X4~7 J_ BJERRUM, G. SCHW_~RGENBACH A~'D L. G. SILLI%, Stability Co?zsta?zts. II.Inor~a&c&ands. SpecialPublicationNo.7,TheChemicalSociety.London.I9583
Received J_ EkctroandZ.
November Chenr..
r8th,. 1961
3 (rg62)
407-409