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Coulometric titrations with electrogenerated cyanide ion
JOURKAL. OF ELECTROANALYTICAL
COULOMETRIC FRED
CHEMISTRY
337
TITIt4TIONS WITH ELECTROGENERATED CYANIDE ION* C. ANSON.
CaZifomzia
Instiirtle
KARL
H. POOL
of Tec?rtzoZogy.
JOM
ABID
PasAGtza.
(Received September zznd,
RI. LVRIGHT
Calif.
(U.S.A.)
rg6o)
INTRODUCTION The numerous
titrations
of metal
cations
with cyanide
ion which
have been developed1
all suffer from the difficulty of maintaining standard solutions of potassium cyanide. One of the chief advantages of constant-current coulometric titrations is that unstable reagents from which it is difficult to prepare standard solutions can be generated electrolytically as needed*_ This led us to try to develop a procedure for generating cyanide ion electrolytically. The first method which suggests itself.for generating cyanide ion is one in which a metal ion in the form of a stable cyanide comples of fised stoichiometry is electrolytically reduced to the metallic state, thus releasing the cyanide ions which were in the comples ion. Analogous methods have been developed by REILLEY AND PORTERFIELDS for the electrogeneration of ethylenediamineof sulfhydryl tetraacetic acid and by MILLER AND HUME -2 for the electrogeneration compounds. The cyanide compleses of both mercury(I1) and silver were investigated but the silver complex gave more reproducible and accurate results, so that only the silver complex was employed for carrying out titrations_ It is possible to generate cyanide ion w-ith IOOO/~ current efficiency in a solution of potassium dicyanoargentate according to the electrode reaction: Ag(CN)s-
+ e-
+ Xg + 2 CN-
(I)
A fine-textured. adherent deposit of silx-er is formed on the platinum the generation, and the freed cyanide ion thus formed can be used cations. Titration of silver, nickel and gold will be described.
cathode during to titrate metal
EXPERIMENT_4L _4ppamt1is The constant current source was similar to that of LINGANE~. The generating electrode was a platinum gauze electrode having an area of approximately 50 cm2. The auxiliary electrode was a coil of platinum wire placed in a IO-mm sintered glass tube wkich was contained in a 30-rnm sintered glass tube. This double insulation of the auxiliary electrode was found to be necessary in order to insure that none of the osygen which is evolved at the anode gets into the cathode comp&tm&t. The solution in both the anode and cathode compartment& of the cell was tieed of o%gen and kept under an atmosphere of pre-purified-nitrogen which had been p&sed through two. * ConGbution
No_ ~627 from the Gabs
and
Crellk
L&d&tories
J-
of Ch&&try.
EZectroatid_
Cheqz_.
4
(1961)
237<3:41-
23s
F-C.
AKSOX:,
K.H.
POOL,J.
BI. WRIGHT
washing towers containin, Q a solution of -r-anadous chlorideG_ The a.Gode compartment was filled with a 0.2 F TiKOa solution which was also O.OI-_F-in NaOH. The titration ceh was a ~~o-ml beaker with the electrodes, nitrogen inlet tube, and sintered glass tubes mounted in a closely-fitting rubber stopper. About 50 ml of solution was used for a titration; the solution was stirred magnetically_ Potentials were measured ets. a saturated calomel electrode \;iith a Leeds and Northrup Model 7664 pH-meter. _ The indicating electrode was a coti of pure silver w-ire through which a constant cathodic current of 4-5 pA1 was continuously passed. Another silver electrode in the same cell compartment was used to complete the circuit. The passage of this small current through the indicating electrode greatly increased the rate of potential establishment, thus decreasing potential drift. Reagents
aizd
sokffio~zs
A3.I chemicals were reagent grade and were used without further purification. Standard solutions of silver nitrate were prepared by \\veight. Standard solutions of ~o~d(I~r) were prepared by d.ksofving pure gold wire in aqua regia and evaporating with hydrochloric acid to remove nitrate. Stock solutions of nickel nitrate were standardized with a solution of EDTA wh.ich had been standardized against pure zinc metal. The generating electrolyte was a 0.25 F solution of potassium dicyano argentate which was also 0.01 F in sodium hydro_xide. The generating electrolyte was prepared by dissolving o. 5 mole of potassium cyanide in ca. IOOO ml of 0.01 F KaOH. Approximately 0.~5 mole of silver nitrate was added, and the solution tvas stirred overnight or until all of the precipitate had dissolved. After equilibrium was attained small quantities of solutions of s&Fer nitrate and potassium cyanide were added to the solution until a shght turbidity due to solid Ag*Ag(CN)2 persisted in the solution. The presence of the precipitate of Ag-Xg(CN)_ Z)in the sofution insured that the sohrtion contained. no excess of either silver ion or uncompleted cyanide ion. To insure complete removal of oxygen both the generating electrolyte and the elecfrolyte in the anode compartment were made 0.1 F in NazSOa in most titrations.
Fifty ml of the generating electrolyte are placed in the cell, and the anode electrolyte is placed in the anode compartment. Both solutions are made 0.1 F in sodium sulfite (escept as noted below). Nitrogen is passed through the cell, and both compartments are maintained under a nitrogen atmosphere. The generating sohrtion is pretitrated to a potentiometric end-point by cathodic generation_ The pretitration serves to convert the small c&amity of solid Ag,-Ag(CN)z-present in_ the electrolyte into Xg(CN)a-. The sample is pipetted- into the cell, (If sulfite-has not been added -this results in the formation of a precipitate of Ag*Ag(CN)z.) The cathodic generation is then continued ro a second potentiornetric end-point_ Pot&ial measurements are mad~_=iit.h the generating current off. Although the presence pf-snlfite ion and the priss&eof a-few r&roampeGs through the indicating electrode &crease the rate of establi_shment .of _potential- equilibrium, some_ drifting of the -Pot&tials is obseived i;l -%e ’ ti&it$--of the ~~~c$oi+. LBy’ Feko&ng t‘he pdtential 1 dz a fixed _time ~_ (e.g. _ 15 sec)_foiloivingeach c&rrerit&c?ement I’ie-Arthe end-point, reproducible and accurate r*&u.hsare~o~&&ed. :-P&&s of potential & -time in -th-e-v&&y of -the end-p_oin& for-
COULOMETRS
\VlTH
ELECTROGENERATED
CYANIDE
239
ION
the pretitration and titration of-the sampIe were made, and the interval between the curves at a selected potential was taken as the titration time. In case of over-titration the current can be reversed and-the-excess cyanide back titrated lvith electrogenerated silver ion. RESULTS
AND
DISCUSSION
The results of titrations of silver, nickel, and gold(III) are summarized in Table I. Gold(II1) consumed esactly four cyanide ions per atom of gold when the titration was performed in the absence of sulfite. In the presence of sulfite, however. only TABLE
I
.X. TITRATION
5-435 5.435 40.43
4
120.75
3-050 15.29 30-57
I
2z.0;
5-433 5-4’5 40-36
51-33
h-43
;
99.96
80.98
+o-15
120.88
_tO.II
.o
f&t??&
106.16
3 -I 4
51.33
fO.12
3.005
IO.+.2
TITRXTION
--0_01
4.18 ---o-I7
15.2s
-1.5 *.07
30-5-l
--o-IO
IO.11
OF
GOLD
Per ce?zt *IDI
CUTW7d fm.-ll
Titrations
2
a-393
2
3
C.
rmgl
SILVER
1o.og
I
60.36 So.86
Au
OF
+1.18
IO-33
22.1X
6
20.36
22.11
2
31.16
22.11
I
2g.89
4
i-o.32
to-14
49-5s 3-l-33
4-o-27 +0.27
about two
+ Au(CN)i-
= Au(CN)a=
Au(C
f
(2) (3)
C&z
In the presence of sulfite, however, positive errors result if any of thk red&ion Au(III) proceeds. by reaction (3) rather than bji reaction (4.’ Au[CN)a-
+
& SOS=-
=_ Au(ChZ~-I.+
2 CN-
f
$ Sc)J’-
J. Elecivoianil.
Zhe-+.,:a
of (4)
(rg6r)_-2~3TT241
F. C. ANSON,
240
XC_ H.
POOL,
j.
M.
WRIGHT
Positive errors of I--2”/b were obtained in titrations- of gold(III) attempted in the presence of sulfite. These errors most likely result because both reactions (3) and (4) _ take place. For this reason all the titrations of gold reported in Table I were performed without sulfite. In order to avoid mechanikrl loss of some of the precipitate of Ag - Ag(CN) z w?&31 is formed when gold(I~I) is added to the generating solution, approximately 90% of the stoichiomeWc quantity of cyanide ion was generated before the gold(II1) was added to the generating solution_ The current efficiency for the generation of cyanide ion according to reaction (I) was established by titrating with standard silver nitrate a solution of KAg(CX)z had been passed, and by w-eighing the through which a known number o f coulombs deposit of silver on the cathode. The latter method wa.s more precise, but both pracedures indicated that cyanide ion could be generated with rooO;, current efficiency so long as the generating solution was maintained completely o_xygen-free_ Any oxygen in the solution was co-reduced at the electrode and rapidly o_s.idized the silver’ on the cathode in the presence of cyanide, thus reducing the current efficiency. The double insulation of the anode described in the expexime~tal section was found to be necessary to avoid contamination of the generating solution with some of the osygen evolved at the anode. No upper limit to the amount of metal which could be accurately determined was encountered_ There is, however, a lower limit to the quantity of metal which can be determined accurately. The change in potential of the silver-indicating electrode in the vicinity of the end-point becomes smaller and smaller as the concentration of metal taken and the generating current used decrease, until, with currents smaller than about 5 rnA, the end-point is too poorly-defined to be useable. Fig. I shows titration curves for the titration of silver at two different currents. Curve I resulted from a titration of silver at a current of 104 n-A. Cur-x-e 2 resulted from the titration of silver at a current of 26 mA. The curves are essentially independent of the quantity of silver titrated. Cu.rx~es3 and zgshow the effect of the addition of sulfite ion on the titration curves with currents of 26 and IO_+ mA, respectivel_v_ Sulfite ion complexes silver ion strongly enough to d.issol\rethe precipitate of Ag-Ag(CN)a which is otherwise present up to the end-point in the titration. As a result, the difference in the concen-
Fig_ 1. Poten_titimetic curve
3, 26 mA;
yithout
tztratipn cur& for t&e titration of silver: cur& r, 104 m-4. without sulfite; curve 3. a6 -4, &Zth sulfit&, 0. I f;; curve &ro_i ‘III& with 0-r F,
sulfite;
suffite.
cOUL&rETRY
WITH
ELECTROGENER-4TED
CYANIDE
ION
231
trations df_ free silver ion before
and after ~the equivalence-point is not so great as without sulfite and the titration curve is less steep i&the vicinity o-f the end-point. With currents of 35 II-LA or more the potential change in the vicinity of the endpoint is sufficiently sharp to give accurate titrations with sulfite present. In the absence of sulfite the potential inflection is considerably sharper but +ifts in the potential result which make the end-pdint more difficult to detect. Sodil.un suLfit.e was used in all thk titrations where the generatin g current being used was large enough to give a distiict end-point. For currents below 25 mA no sulfite was used. An attempt was made withput success to follow the titration amperometrically with two silver electrodes. Large currents resulted at all applied voltages presumably because the anode reaction prior to the end-point (5) _.g + Xg(CN)z- = _Ag-_Ag(CX)s + etakes place very close to the potential end-point (6) occurs. Xg f- 2 CKTzl~ation
of other
at which =
hg(CK)2-
(5)
the anode reaction f
following
e-
the (6)
iizetak
but potential drifts made end-PO&t The titration of copper(I1) was attempted, detection very difficult in the absence of sulfite. In the presence of sulfite the end-point was not sharp. Cobalt (II) rapidly reacts with Ag(CN)zto give silver metal. The resulting Co(II1) species appears to contain more than five but fewer than sis cyanide ions and the stoichiometry is not reproducible. This behavior corresponds to that reported by El-ANS~ in the \volumetric titration of cobalt(I1) with potassium cyanide. ACK?XOWLEDGEJIEST
Estensix-e undergraduate participation in this work was made possible from the California Corporation for Biochemical Research_
by a grant
Cyanide ion can be generated electrolytically with roo”h current efficiency according to the reaction Ag(CN)zj- e- = Xg +- a CN-. Constant current coulometric titrationsofsilver, nickelandgold(III)havebeenperformedwithanaccuracy ofafew parts per thousand_ Attempted titrations of copper(I1) and cobalt(I1) were unsuccessful. REFERENCES 1 I. RI_-KOLTHOFF ASD V. A. STEKGER, I'oLzrmeCvic_-lmziysis,K'ol. II, Interscience Publishers, New York. 1947, p_ 2S3_ York, 1955. 2 J_ J_ L~G_XBIE. Elecivoanaly~ical Chemistry, 2nd cdn.. Interscience Publishers, New Chap. XX;. 3 C. N. BILLEYXND \X-.\V. PORTERFIELD, drzul. CBem.. 15 (1936) 4+3_ -i B. MZLLER AND D. N. HUME, _~m-zZ.Chem.. 33 (1960) 524. 5 J.-J_ LIXGAKE, _-lzaZ.Chezn., 26 (1954) IOZI. Interscience Publishers, New York, Igs.5, p_ 3.4. G L. >&ITE.S, Polarogiraphic Techxiqttes. 7 B. S. EVANS, .-inaEysi, 62 (1937) 363. /_ Eiedvoanab.
Chel~Z.. 3- (1961) 3-37-z+
COULOMETRIC FRED
CHEMISTRY
337
TITIt4TIONS WITH ELECTROGENERATED CYANIDE ION* C. ANSON.
CaZifomzia
Instiirtle
KARL
H. POOL
of Tec?rtzoZogy.
JOM
ABID
PasAGtza.
(Received September zznd,
RI. LVRIGHT
Calif.
(U.S.A.)
rg6o)
INTRODUCTION The numerous
titrations
of metal
cations
with cyanide
ion which
have been developed1
all suffer from the difficulty of maintaining standard solutions of potassium cyanide. One of the chief advantages of constant-current coulometric titrations is that unstable reagents from which it is difficult to prepare standard solutions can be generated electrolytically as needed*_ This led us to try to develop a procedure for generating cyanide ion electrolytically. The first method which suggests itself.for generating cyanide ion is one in which a metal ion in the form of a stable cyanide comples of fised stoichiometry is electrolytically reduced to the metallic state, thus releasing the cyanide ions which were in the comples ion. Analogous methods have been developed by REILLEY AND PORTERFIELDS for the electrogeneration of ethylenediamineof sulfhydryl tetraacetic acid and by MILLER AND HUME -2 for the electrogeneration compounds. The cyanide compleses of both mercury(I1) and silver were investigated but the silver complex gave more reproducible and accurate results, so that only the silver complex was employed for carrying out titrations_ It is possible to generate cyanide ion w-ith IOOO/~ current efficiency in a solution of potassium dicyanoargentate according to the electrode reaction: Ag(CN)s-
+ e-
+ Xg + 2 CN-
(I)
A fine-textured. adherent deposit of silx-er is formed on the platinum the generation, and the freed cyanide ion thus formed can be used cations. Titration of silver, nickel and gold will be described.
cathode during to titrate metal
EXPERIMENT_4L _4ppamt1is The constant current source was similar to that of LINGANE~. The generating electrode was a platinum gauze electrode having an area of approximately 50 cm2. The auxiliary electrode was a coil of platinum wire placed in a IO-mm sintered glass tube wkich was contained in a 30-rnm sintered glass tube. This double insulation of the auxiliary electrode was found to be necessary in order to insure that none of the osygen which is evolved at the anode gets into the cathode comp&tm&t. The solution in both the anode and cathode compartment& of the cell was tieed of o%gen and kept under an atmosphere of pre-purified-nitrogen which had been p&sed through two. * ConGbution
No_ ~627 from the Gabs
and
Crellk
L&d&tories
J-
of Ch&&try.
EZectroatid_
Cheqz_.
4
(1961)
237<3:41-
23s
F-C.
AKSOX:,
K.H.
POOL,J.
BI. WRIGHT
washing towers containin, Q a solution of -r-anadous chlorideG_ The a.Gode compartment was filled with a 0.2 F TiKOa solution which was also O.OI-_F-in NaOH. The titration ceh was a ~~o-ml beaker with the electrodes, nitrogen inlet tube, and sintered glass tubes mounted in a closely-fitting rubber stopper. About 50 ml of solution was used for a titration; the solution was stirred magnetically_ Potentials were measured ets. a saturated calomel electrode \;iith a Leeds and Northrup Model 7664 pH-meter. _ The indicating electrode was a coti of pure silver w-ire through which a constant cathodic current of 4-5 pA1 was continuously passed. Another silver electrode in the same cell compartment was used to complete the circuit. The passage of this small current through the indicating electrode greatly increased the rate of potential establishment, thus decreasing potential drift. Reagents
aizd
sokffio~zs
A3.I chemicals were reagent grade and were used without further purification. Standard solutions of silver nitrate were prepared by \\veight. Standard solutions of ~o~d(I~r) were prepared by d.ksofving pure gold wire in aqua regia and evaporating with hydrochloric acid to remove nitrate. Stock solutions of nickel nitrate were standardized with a solution of EDTA wh.ich had been standardized against pure zinc metal. The generating electrolyte was a 0.25 F solution of potassium dicyano argentate which was also 0.01 F in sodium hydro_xide. The generating electrolyte was prepared by dissolving o. 5 mole of potassium cyanide in ca. IOOO ml of 0.01 F KaOH. Approximately 0.~5 mole of silver nitrate was added, and the solution tvas stirred overnight or until all of the precipitate had dissolved. After equilibrium was attained small quantities of solutions of s&Fer nitrate and potassium cyanide were added to the solution until a shght turbidity due to solid Ag*Ag(CN)2 persisted in the solution. The presence of the precipitate of Ag-Xg(CN)_ Z)in the sofution insured that the sohrtion contained. no excess of either silver ion or uncompleted cyanide ion. To insure complete removal of oxygen both the generating electrolyte and the elecfrolyte in the anode compartment were made 0.1 F in NazSOa in most titrations.
Fifty ml of the generating electrolyte are placed in the cell, and the anode electrolyte is placed in the anode compartment. Both solutions are made 0.1 F in sodium sulfite (escept as noted below). Nitrogen is passed through the cell, and both compartments are maintained under a nitrogen atmosphere. The generating sohrtion is pretitrated to a potentiometric end-point by cathodic generation_ The pretitration serves to convert the small c&amity of solid Ag,-Ag(CN)z-present in_ the electrolyte into Xg(CN)a-. The sample is pipetted- into the cell, (If sulfite-has not been added -this results in the formation of a precipitate of Ag*Ag(CN)z.) The cathodic generation is then continued ro a second potentiornetric end-point_ Pot&ial measurements are mad~_=iit.h the generating current off. Although the presence pf-snlfite ion and the priss&eof a-few r&roampeGs through the indicating electrode &crease the rate of establi_shment .of _potential- equilibrium, some_ drifting of the -Pot&tials is obseived i;l -%e ’ ti&it$--of the ~~~c$oi+. LBy’ Feko&ng t‘he pdtential 1 dz a fixed _time ~_ (e.g. _ 15 sec)_foiloivingeach c&rrerit&c?ement I’ie-Arthe end-point, reproducible and accurate r*&u.hsare~o~&&ed. :-P&&s of potential & -time in -th-e-v&&y of -the end-p_oin& for-
COULOMETRS
\VlTH
ELECTROGENERATED
CYANIDE
239
ION
the pretitration and titration of-the sampIe were made, and the interval between the curves at a selected potential was taken as the titration time. In case of over-titration the current can be reversed and-the-excess cyanide back titrated lvith electrogenerated silver ion. RESULTS
AND
DISCUSSION
The results of titrations of silver, nickel, and gold(III) are summarized in Table I. Gold(II1) consumed esactly four cyanide ions per atom of gold when the titration was performed in the absence of sulfite. In the presence of sulfite, however. only TABLE
I
.X. TITRATION
5-435 5.435 40.43
4
120.75
3-050 15.29 30-57
I
2z.0;
5-433 5-4’5 40-36
51-33
h-43
;
99.96
80.98
+o-15
120.88
_tO.II
.o
f&t??&
106.16
3 -I 4
51.33
fO.12
3.005
IO.+.2
TITRXTION
--0_01
4.18 ---o-I7
15.2s
-1.5 *.07
30-5-l
--o-IO
IO.11
OF
GOLD
Per ce?zt *IDI
CUTW7d fm.-ll
Titrations
2
a-393
2
3
C.
rmgl
SILVER
1o.og
I
60.36 So.86
Au
OF
+1.18
IO-33
22.1X
6
20.36
22.11
2
31.16
22.11
I
2g.89
4
i-o.32
to-14
49-5s 3-l-33
4-o-27 +0.27
about two
+ Au(CN)i-
= Au(CN)a=
Au(C
f
(2) (3)
C&z
In the presence of sulfite, however, positive errors result if any of thk red&ion Au(III) proceeds. by reaction (3) rather than bji reaction (4.’ Au[CN)a-
+
& SOS=-
=_ Au(ChZ~-I.+
2 CN-
f
$ Sc)J’-
J. Elecivoianil.
Zhe-+.,:a
of (4)
(rg6r)_-2~3TT241
F. C. ANSON,
240
XC_ H.
POOL,
j.
M.
WRIGHT
Positive errors of I--2”/b were obtained in titrations- of gold(III) attempted in the presence of sulfite. These errors most likely result because both reactions (3) and (4) _ take place. For this reason all the titrations of gold reported in Table I were performed without sulfite. In order to avoid mechanikrl loss of some of the precipitate of Ag - Ag(CN) z w?&31 is formed when gold(I~I) is added to the generating solution, approximately 90% of the stoichiomeWc quantity of cyanide ion was generated before the gold(II1) was added to the generating solution_ The current efficiency for the generation of cyanide ion according to reaction (I) was established by titrating with standard silver nitrate a solution of KAg(CX)z had been passed, and by w-eighing the through which a known number o f coulombs deposit of silver on the cathode. The latter method wa.s more precise, but both pracedures indicated that cyanide ion could be generated with rooO;, current efficiency so long as the generating solution was maintained completely o_xygen-free_ Any oxygen in the solution was co-reduced at the electrode and rapidly o_s.idized the silver’ on the cathode in the presence of cyanide, thus reducing the current efficiency. The double insulation of the anode described in the expexime~tal section was found to be necessary to avoid contamination of the generating solution with some of the osygen evolved at the anode. No upper limit to the amount of metal which could be accurately determined was encountered_ There is, however, a lower limit to the quantity of metal which can be determined accurately. The change in potential of the silver-indicating electrode in the vicinity of the end-point becomes smaller and smaller as the concentration of metal taken and the generating current used decrease, until, with currents smaller than about 5 rnA, the end-point is too poorly-defined to be useable. Fig. I shows titration curves for the titration of silver at two different currents. Curve I resulted from a titration of silver at a current of 104 n-A. Cur-x-e 2 resulted from the titration of silver at a current of 26 mA. The curves are essentially independent of the quantity of silver titrated. Cu.rx~es3 and zgshow the effect of the addition of sulfite ion on the titration curves with currents of 26 and IO_+ mA, respectivel_v_ Sulfite ion complexes silver ion strongly enough to d.issol\rethe precipitate of Ag-Ag(CN)a which is otherwise present up to the end-point in the titration. As a result, the difference in the concen-
Fig_ 1. Poten_titimetic curve
3, 26 mA;
yithout
tztratipn cur& for t&e titration of silver: cur& r, 104 m-4. without sulfite; curve 3. a6 -4, &Zth sulfit&, 0. I f;; curve &ro_i ‘III& with 0-r F,
sulfite;
suffite.
cOUL&rETRY
WITH
ELECTROGENER-4TED
CYANIDE
ION
231
trations df_ free silver ion before
and after ~the equivalence-point is not so great as without sulfite and the titration curve is less steep i&the vicinity o-f the end-point. With currents of 35 II-LA or more the potential change in the vicinity of the endpoint is sufficiently sharp to give accurate titrations with sulfite present. In the absence of sulfite the potential inflection is considerably sharper but +ifts in the potential result which make the end-pdint more difficult to detect. Sodil.un suLfit.e was used in all thk titrations where the generatin g current being used was large enough to give a distiict end-point. For currents below 25 mA no sulfite was used. An attempt was made withput success to follow the titration amperometrically with two silver electrodes. Large currents resulted at all applied voltages presumably because the anode reaction prior to the end-point (5) _.g + Xg(CN)z- = _Ag-_Ag(CX)s + etakes place very close to the potential end-point (6) occurs. Xg f- 2 CKTzl~ation
of other
at which =
hg(CK)2-
(5)
the anode reaction f
following
e-
the (6)
iizetak
but potential drifts made end-PO&t The titration of copper(I1) was attempted, detection very difficult in the absence of sulfite. In the presence of sulfite the end-point was not sharp. Cobalt (II) rapidly reacts with Ag(CN)zto give silver metal. The resulting Co(II1) species appears to contain more than five but fewer than sis cyanide ions and the stoichiometry is not reproducible. This behavior corresponds to that reported by El-ANS~ in the \volumetric titration of cobalt(I1) with potassium cyanide. ACK?XOWLEDGEJIEST
Estensix-e undergraduate participation in this work was made possible from the California Corporation for Biochemical Research_
by a grant
Cyanide ion can be generated electrolytically with roo”h current efficiency according to the reaction Ag(CN)zj- e- = Xg +- a CN-. Constant current coulometric titrationsofsilver, nickelandgold(III)havebeenperformedwithanaccuracy ofafew parts per thousand_ Attempted titrations of copper(I1) and cobalt(I1) were unsuccessful. REFERENCES 1 I. RI_-KOLTHOFF ASD V. A. STEKGER, I'oLzrmeCvic_-lmziysis,K'ol. II, Interscience Publishers, New York. 1947, p_ 2S3_ York, 1955. 2 J_ J_ L~G_XBIE. Elecivoanaly~ical Chemistry, 2nd cdn.. Interscience Publishers, New Chap. XX;. 3 C. N. BILLEYXND \X-.\V. PORTERFIELD, drzul. CBem.. 15 (1936) 4+3_ -i B. MZLLER AND D. N. HUME, _~m-zZ.Chem.. 33 (1960) 524. 5 J.-J_ LIXGAKE, _-lzaZ.Chezn., 26 (1954) IOZI. Interscience Publishers, New York, Igs.5, p_ 3.4. G L. >&ITE.S, Polarogiraphic Techxiqttes. 7 B. S. EVANS, .-inaEysi, 62 (1937) 363. /_ Eiedvoanab.
Chel~Z.. 3- (1961) 3-37-z+