Controlled Potential Electrolytic Separation and Determination of

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Controlled Potential Electrolytic Separation and Determination of Copper, Bismuth, lead, and Tin JAMES J. LINGAKE AND STANLEY L. JOYES Harvard Lhiversity, Cambridge 38, Mass. The optimuni conditions for the successive separation and determination of copper, bismuth, lead, and tin from a tar’trate solution by automatic controlled potential deposition onto a platinum cathode hare been established. The influences of such important variables as pH, total tartrate concentration, teniperatnre, and cathode potential have been investigated. All four metals may be determined without any intermediate treatment of the solution, except acidification for the determination of tin, in a

T

H E aim of this investigation was to devise a pi,ocedure, utilizing the techniques of controlled potential electrolysis, whereby copper, bismuth, lead, and tin could be deposited successively onto a platinum cathode from the same solution merely by appropriate regulation of the cathode potential, with little or no intermediate treatment of the solution. This aim has heen realized by using a tartrate electrolyte of controlleci pH. The separation of copper from lead and/or tin, and of lead from tin, presented no particular difficulties. The chief problem was the discovery of conditions that’ would permit the separation of copper from either large or small amounts of bismuth, a separation which has heretofore not 1)eenachieved. Sand (8)reported the separation of large amounts of copper from very small amounts of bismuth from a hot tartrate solution ( p H and other critical conditions not specified), but he R-as unable to achieve separation when more than traces of bismuth were present,. In the present investigation the influence on the deposition potentials of copper and bismuth of such variables as the p H of the tartrate solution, the total tartrate Concentration, the temperature, and the presence of hydrazine and hydroxylamine, was studied systematically. The data thus obtained define the optiinuni cathode potential and optirnuni solution conditions for the separation. Conditions conducive t,o a maxirnal rate of deposition were established by autoniatically recording current-time curves during the electrolyses. These curves also reveal that the deposition of bismuth on copper (or platinum) proceeds autocatalytically. Hydrazine and hydroxylamine have been used from the earliest period of electroanalysis as “anodic depolarizers,” but the literature contains no information on their relative efficiencies for this purpose and they are commonly specified interchangeably. From nieasureinents of the potential of the platinum anode during electrolysis it was found that hydrazine is by far the more effective anodic depolarizer. The oxidation products of hydrazine and hydroxylamine at a platinum anode at controlled potential have been identified in this study. From the current-time curves and from coulometric measurements of current efficiency, it \vas discovered that hydrazine plays a more iniportmt role in the copper deposition than merely depolarizing the anode. Partially by reducing + 2 copper to the cuprous state, and partially by complexation with cupric copper, hydrazine greatly accelerates the copper deposition and aids in its separation from bismuth. From these and other data a procedure has been developed that permits the successive determinations of copper, hismuth, lead, and tin, in a total elapsed time of less than 4 hours from the time t’he solution has been prepared, and with an expenditure of only

total elapsed time of about 4 hours (1 hour of actual operator time). Very few other elements interfere. The beneficial effect of hydrazine and hydroxylamine as addition agents has been investigated, and their oxidation products at a platinum anode have been identified. Hydrazine in the presence of chloride ion is a much more efficient anodic depolarizer than hydroxylamine, and it plays a more important role by reducing +2 copper to a chloro cuprons coniplex which greatly accelerates the copper deposition.

about I hour of actual operator time. The accuracy is equal to that obtainable by more laborious classical methods, arid very few other elements interfere. EXPERIMENTAL TECHNIQUE

A sclieniatic diagram of the electrical circuit used is sho\\-n in Figure 1. The electrolysis cell itself was of conventional design, comprising a 300-ml. tall-form lipless beaker, a cylindrical platinum gauze cathode ( 5 em. in diameter, 5 em. high, area 160 sq. cm.), a helical platinum wire anode (area 8 sq. em.) centered inside t,he cathode, and a mechanical stirrer to provide efficient stirring (3). The potential of the cathode was control led^ against a saturated calomel reference electrode, the tip of the salt bridge being placed close to the outside of the cathode near its niitidle. .The initial volume of solution was usually 200 ml.

I +

STANDARD RESISTOR

I

I

1

REFERENCE ELECTRODE

CELL

COULOMETER

Figure 1.

I

,

Schematic Electrical Circuit

The potentiostat used to control the applied voltage autoniatically to maintain any desired constant cathode potential has been described ( 7 ) . The control sensitivity was kO.01 volt. A hgdrogen-oxygen coulometer (6) in series with the cell was used to measure current efficiency. Current-time curves during electrolysis were recorded by a Leeds & Sorthrup Micromas potentiometer recorder in terms of the iR drop generated by the electrolysis current in the precision resistor (0.1 to 10 ohms) in series with the cell. (The coulometer, standard resistor, and recorder are, of course, unnecessary in routine analytical applications of the final procedure.) After each deposition the cathode n-as lvashed with a sinall volume of water, then with pure acetone, placedin an oveii a t 110’ C. for 2 minutes, and then allowed to cool in air before weighing. The wash water, but not the wash acetone, was added to the residual electrolysis solution. JVashing is best accomplished by lowering the electrolysis beaker away from the eiectrodes without 1798

V O L U M E 23, N O , 1 2 , D E C E M B E R 1 9 5 1 1ire:iking the cii cuit :ind iinnit~tliutelydelivering the wad! \v:itcI' from a \\-;ish i w t t l e a i n u i i t i the upper edge of the cathode. Quirk nti:iI t o avoid IOSP of lead and t,in deposits by resolution :i.q :i result of' a i r oxidation. Even under the best contiitioiis a h u t 1.5 nig. of lead usually are lost into the wash water ( 5 ) . The c:ithotle \vas not c1e:ined between each metal deposition, but aril!. :iItcii' thc last metal had been deposited-Le., copus per \vas depoktrd o n I)latiriuni, hismuth on the p i ~ ~ i o copper co:it, lead on t h e iiiaiiiuth, and tin on the lead. The soilium t:irtixtca tlihydrate used was of analytical reagent of' t l i r s solutions was measured rvith the glass itijurtcd to the desired value bj- addition of hydrochlorir :iciti or sodium tiyclroxide solution. c tlihydrorhloride was used n-ithl-ellow laliel hydroxyl:iniine lfate were purificd heforc use hy

b ir El O N P T W i T M U I O R A Z I N E i El ON C U

e -0.6

e i ON

C U ON PT W I T H H Y D R A Z I N E

1 C,ON

-G'{

BISMUTH

tu w i T n w D a * z I N E

PT

J C i i O N PT WITH HYDRAZINE

T 0

I I

I

2

3

-

I

4

PH

5I

61

7I

8I

Figure 2. Deposition Potentials of Copper and Bismuth from 0.25 M Tartrate Solutions as a Function of pH at 25" A.

Bi-muth on copper or platinum w.ith or without hydrazine dihydrochloride

H. Copper without h>dra.sine C.

Coppcr in pre-enw of h \ drarine dih>drochlorirlr

RESULTS AND DISCUSSION

A tartrate electrolyte \vas used because polarogi,:ij)hic c ~ p e r i eiice (4)showed that it offered the best hope of a successful yeparation. As will be shown in a separate comniunication, c~onip1ex:rtion of metal ions with tartrate involves the replacement of the hydrogen of the hydroxyl groups by the metal ion-Le., hytlroger~ ion is a product of the complexation reaction. This is thv furidamental reason why the stnhilities of tartrate complexes, and consequently their electrochemical behavior, are so great,ly dependent on pH. Close control of pH is the most important single factor in tletei,mining the success of any type of metal seliaration (eleotrolJ.sis, ioii cxch:inge, rxstraction, etc.) which employs taitrnte or citrate niedia. The polarog.rri1iliy of' the t:tl,tr:itc cvml)lcser o f c ' o l i l ) i ' 1 ' nntl tiisniuth (,$) indicBatetl that the optimum p H for the reparation shoultl he between 4 anti 6. Because the amalgam formation involved in the polarographic reductions creates a considerably different therniotiyn:imic situation than in reduction to the solid metals, and because deposition of the solid metals proceeds less reversibly than reduction to amalgams, it was necessary to verify this prediction by determining the deposition potentials under the actual conditions extant in electrolysis with the platinum cathode. This \vas done by polarographically recording current-roltagc, curves of copper and bismuth separately in tnrtrate solutioris of various pH, using a platinum microelectrode. The platinum microelectrode consisted of a 3-nun. length of 0.6-nun. diameter platinum v4re sealed into the end of a glass tube. The so1ut)ions were 0.25 .If in total tartrate and they contained 50 mg. of the metal ion and 1 gram of hydrazine dihydrochloride per 100 ml. The cell used n-as provided \vith a saturated calomel anode and a low resistance salt bridge. Air wns removed from the test solutions with nitrogen, :md efficient stirring n-as provided by a magnetic stirrer. The deposition potential \vas measured arbitrarily by extrapolating the nearly linear steeply rising curve back to intersection with the cstrapolated residual current. The results obtained are summarized iu Figure 2. Tlic deposition potential of bismuth increases regularly from -0.10 volt US. S.C.E. a t pH 1.5 to -0.80 volt a t pH i . 5 . .It a given pH the bismuth deposition potential on a copper-plated cathode n-as the same as on platinum, and it \vas also the smie with or kithout h>.tlixzirio ~irescaiit. Tlic. ~li~l)o.~itioii potc1iiti:il o f m i i 1 w i ' rh:ingw

4.5 f

\

Figirre 3.

Current-Time CurTes f o r Copper Deposition at pH iallif35f r o m $5to 6.3

A t each pH 10 mg. of copper w e r e present i n 160 ml. of solution containing 0.5 M total tartrate and 0.06 M h r drazine dihydrochloride. potential -0.30 volt 1 s . S.C.E.

Cathode

ANALYTICAL CHEMISTRY

1800 much less than that of bismuth with changing pH. In the absence of hydrazine (curve B ) the deposition potential of copper is more negative than that of bismuth a t p H values below 2.3, but more positive a t higher pH's with a maximal difference between pH 5 and 6. Partial reduction to cuprous oxide was observed when the pH esceeded about 7. 0.20r

Figure 4. Characteristics of Copper Deposition in Presence of Hydrazine Hydrochloride as a Function of pH In each case 10 mg. of copper in 160 ml. of solution containing 0.5 M total tartrate and 0.06 M hydrazine dihydrochloride. Cathode potential -0.30 volt us. S.C.E. See Equations 1 and 2 for significance of k value

Curve C demonstrates the great influence of hydrazine hydrochloride on the deposition potential of copper; in the joint presence of hydrazine and chloride the deposition potential is shifted to a more negative value a t p H below 4.5 but to a more positive value a t higher pH values. Below p H 4.5 hydrazine compleves cupric copper (shown by a change in color) and this complex is more stable than the tartrate complex and thus is reduced a t a more negative potential. Above p H 4.5 in the presence of chloride hydrazine reduces + 2 copper to a chloro complex of the cuprous state and the reduction potential of this complex is more positive than that of the cupric tartrate complex. Above pH 6.0 hydrazine rapidly reduces the cupric copper all the way to the metal. These data indicate that the optimum pH range for the copperbismuth separation is 5 to 6 and that hydrazine hydrochloride should be distinctly beneficial. These predictions were tested by a series of experiments in which solutions of 10.14 mg. of copper in 160 ml. of 0.5 Af total tartrate a t various pH values were electrolyzed with the potential of the platinum cathode automatically maintained a t -0.30 =t0.01 volt us. S.C.E. In each experiment current-time curves were automatically recorded. The quantity of electricity consumed was simultaneously measured with the hydrogen-oxygen coulometer in series with the cell in order to evaluate the current efficiency. A typical set of current-time curves is shown in Figure 3. At each p H the current decays virtually to zero according to the relation =

io 10-hl

! i=

DA 0.43 V6

(2)

where D is the diffusion coefficient of the reducible ion, A is the electrode area, V is the solution volume, and 6 is the thickness of the diffusion layer, which is governed by the rate of stirring. k is easily evaluated by plotting log it against time, and it serves as a convenient criterion of the efficiency of the electrolysis. The influence of pH on the value of k , on the initial or masimum current, on the time required for the current to decay practically to zero (3 ma. or less), on the completeness of the copper deposition, and on the current efficiency, with and without hydrazine dihydrochloride present is sun~marizedin Figures 4 and 5. Hydrazine dihydrochloride not only decieases the electrolysis time but also extends the pH range over which complete deposition of the copper is obtained. Without hydrazine (Figure 5) the current efficiency is considerably below 100% because of the siniultaneous reduction of oxygen produced a t the platinum anode and originally present as dissolved air. In the presence of hydrazine (Figure 4) the apparent current efficiency exceeds 100% when the pH is greater than 6 because a considerable fraction of the cupric copper is reduced to a chloro cuprous complex by the hydrazine. When the pH exceeds 6.5 in the presence of hydrazine the latter reduces the cupric copper partly to cuprous oxide, which accounts for the incomplete deposition above this pH. Without hydrazine present a fairly accurate coulometric determination of as little as 10 mg. of copper can be achieved at a pH between 4.5 and 5.0 by multiplying the final "residual current" by the electrolysis time and subtracting this quantity of electricity from the coulometer reading. This correction is relatively small (equivalent to only 2 to 3 mg. of copper) and is virtually independent a t a given pH of the quantity of copper deposited, so that results accurate to a few tenths of a milligram are obtainable.

PH

1,

when t is time (minutes) from the beginning of the electrolysis, io is the initial current (milliamperes) a t zero time, and it is the current a t time t. This is the expected relation for a diffusion-controlled current and the constant k is theoretically given by ( 3 )

(1)

1. -f

4

c

T

Figure' 5. Characteristics of Copper Deposition without Hydrazine Conditions exactly ae in Figure 4, but no hydrazine present

V O L U M E 2 3 , NO. 1 2 , D E C E M B E R 1 9 5 1 An evactly similar series of egperiments was made with hydroxylamine hydrochloride, with the results shown in Figure 6. Although hydroxylamine behaves like hydrazine in extending the p H range over which complete deposition of copper is obtained, it has relatively little influence on the electrolysis time and therefore is much less efficient than hydrazine. Furthermore, the final residual current is large when hydroxylamine is present, and the current efficiency correspondingly is small, because hydroxylammonium ion is reduced a t the cathode probably according to SH,OH,+

+ 2H+ + 2e = SI&++ H2O

The beneficial effect of hydrazine dihydrochloride results from the simultaneous presence of both hydrazine and chloride. With chloride absent hydrazine (added as hydrazine sulfate) has no effect whatever, and chloride without hydrazine is also completely ineffectual. The effect of the concentration of hydrazine on the initial or maximum current a t pH 5.55 a t a constant chloride ion concentration of 0.09 ill is shown in Figure 7 , and Figure 8 shows the effect of chloride ion concentration a t a constant hydrazine concentration of 0.044 M . The optimum concentrations correspond to 1 to 2 grams of hydrazine dihydrochloride per 200 ml.

1801 hydrogen ion and accumulation of tartrate ion both operate to cause a relatively large increase in p H in the diffusion layer a t the electrode surface. As shown in Figure 5, such an increase in pH inhibits the reduction of the cupric tartrate complex. The reduction of the cupric tartrate complex by hydrazine eliminates this inhibitory effect.

= L Z 400

/

loo+

: 1 1 1 1 , , , , 1 ' 0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 C O N C E N T R A T I O N OF H Y D R A Z I N E M/ L

Figure 7 .

Influence of Hydrazine Concentration on Rate of Deposition

40 mg. of copper from 215 ml. of solution containing 0.23 M total tartrate and 0.09 M chloride (as potassium chloride) at pH 5.55 and cathode otential of -0.30 volt us.

s.c.Ej:

75r-

PH

Figure 6. Characteristics of Copper Deposition in Presence of Hydroxylamine Hydrochloride I n each case 10 mg. of copper in 160 ml. of solution containing 0.5 M total tartrate and 0.06 M hydroxylamine hydrochloride. Cathode potential -0.30 volt us.

S.C.E.

Hydrazine in the presence of chloride ion reduces the cupric tartrate complex to a chloro cuprous complex. As shown in a following section, the chloro cuprous complex has a much larger diffusion coefficient than the cup1ic tartrate complex, and this leads to an increased current because of the increase in k in Equations 1 and 2. However, this accounts for only part of the beneficial effect of hydrazine and the remainder may be interpreted as follows. The reduction of the cupric tartrate complex liberates a t the electrode surface a tartrate ion which lacks either one or both of the hydrogens of its hydroxyl groups because these had been replaced by cupric ion in the complex. This "deprotonated" tartrate ion combines with hydrogen ion (from the ionization of water and/or hydrogen tartrate ion). The removal of

As shown in Figure 9, increasing the concentration of total tartrate a t a p H of 5.5 with 0.044 .If hydrazine dihydrochloride present decreases the initial mavimum current and hence increases the required electrolysis time. A moderately large concentration of tartrate is required to prevent the precipitation of bismuth and a concentration of 0.2 to 0.5 ,If appears to be about the best compromise to achieve this aim and still obtain satisfactorily rapid deposition of copper. Data obtained in trial depositions of bismuth onto a copperplated cathode from 0.5 kf tartrate solution a t various p H values are shown in Figure 10. The open circle points were obtained with the cathode potential a t -0.40 volt us. S.C.E. and the solid circle points a t a cathode potential of -0.50 volt. The deposition proceeds rapidly a t pH's below about 4.5, but the deposit tends to be loose and the're is danger of mechanical loss during vashing. At a pH between 4.5 and 5.5 very good smooth bismuth deposits are obtained when the cathode potential is -0.40 to -0.50 volt. I n the optimum pH range of 4.5 to 5.5 the deposition of bismuth onto either platinum or copper proceeds autocatalytically, as shown by the typical current-time curves in Figure 11 obtained a t a pH of 5.25 and with the cathode potential a t -0.40 volt 1's. S.C.E. At 25' (curve A ) the current is initially small, but it gradually increases to a maximal value and finally undergoes a normal exponential decrease. The mavimal current occurs a t about the time that the cathode becomes completely coated with bismuth. With increasing temperature the induction period decreases and the maximum current increases until a t 50' (curve D ) the current-time curve corresponds to a normal diffusioncontrolled electrode reaction. When the cathode is previously plated with bismuth a normal current-time curve with no induction period is obtained a t 25". On a platinum or copper-plated cathode a t a p H between 5.2 and 6.0 no deposition of bismuth occurs even after several hours a t a cathode potential of -0.30 volt us. S.C.E. However, under the same conditions the deposition of bismuth proceeds readily a t -0.30 volt if the cathode is previously plated with bismuth. I t is evident that the success of the copper-bismuth separation

1802

ANALYTICAL CHEMISTRY

primarily depends on the inct that the deposition of bisiiiuth on copper involves a large overvoltage or activation energy. Skepticism about the reliahilitp of a separation involving unfavorable thermodynamics arid resting solely on favorable kinetics proinptetf an estensive systematic study of the separation of both large and small amouiits of copper from large and small amounts of hisinuth under a variet>-of conditions simulating those encountered in practical amlysis. The separation \vas tested in the presence of many, other metal ions commonly associated tf-ith copper and Iiismuth, to be sut'e that none of these vitiated the separation by catalyzing the l h i i u t h deposition. Esperience thus gained tlemonstrates that the sepal ation is entirely reliable, provided tho following conditions are maintairied.

0

i

3 200 j r i :

X

of the hydrogen succinate ion-succiiiate ion system is a t pH 5.6 in the optimum range. Addition of 1 gram of succinic acid (or 2.3 grams of sodium succinate hesuhydrate) per 100 mg. of copper is adequate to prevent the p H from dropping below 5.2 from an initial value of 5.8 when as much as 500 nig. of copper is cleposited from 200 nil. of solution. The accuracy and reliability of the copper-bi~niuthseparatioii :ire demonstrated by the data in Table I, which were ohtained under the optimum conditions tiesci ibc4 above. The n i i ~ t u r e s w r e composited 1)y dissolving the weighed :inlourits of the pure metals in 10 nil. of warm 1 to 2 nitric acid in the electrolysis beaker. Mixtures containing metallic tin or antimony were dissolved in hydrochloric acid x i t h the aid of a minimal amount of nitric acid. To this solution were theti added 1 gram of urea (to reduce oxides of nitrogen), 50 nil. of 1 -11sodium tartrate, 2 grams of hydrazine dihydrochloritle, and 1 gram of succinic acid for each 100 mg. of copper present. The solution \\-as diluted to about 190 ml. and its p H was adjusted to 5.9 + 0.1 (glass electrode) by addition of sodiuni hydroside solution. The solution was cooled to 25' or below and immediately electrolyzed. Copper r a s deposited a t -0.30 volt U S . S.C.E. After it was weighed the copper-plated cathode was replaced in the solution and the bismuth \vas deposited a t -0.40 volt. The pH after the copper deposition was in the optimum range of 5 . 2 to 5.4 and 110 further adjustnient \vas necessary before the bismuth n-as cleposited. The deposition of each metal required about 45 miiiUtes. I n each case the electrolysis was stopped after the current had decreased to a constant minimal value for 10 minutes. The final constant current in the case of copper is u,w;iily about 3 ni:i. or less, and in the case of bismuth it ~v:I.\ 10 to 12 1ii:i. The separations arid deterniiii:itioiia of copper :~ritl1)isiiiutli are to be very satisfactory in the presence of :is much us 1 gram of tin, antimony, cadmium, :iiitl nickel. llangmese, zinc, and inagiiesium up to 200 nig. each, l e d up to 50 nig., and ferric iron u p to 40 mg. do not interitre. I,:irget, ainouiit,. of tii:uig:ineie, swii

6

o0 01 ~ 02 " 03 l 04 " 05 l 06 l 07 l 08 l 09 l I'O lI I l12 CONCENTRATION

OF

KCI

13

MOLES / L I T E R

Figure 8. Influence of Chloride Ion Concentration on Rate of Deposition 40 mg. of copper from 215 ml. of solution containing 0.23 M total tartrate and 0.044 II hydrazine (as htdranine sulfate) at pIf 5.55 and cathode potential -0.30 volt LS. S.C.E.

The quantity of bizniuth present must not exceed about 400 mg. per 200 ml. The tartrate concentration should be between 0.25 and 0.5 JI. The initial riH should not exceed 6.0 and the final uH must not be lower than'about 5.2 when large amounts of bismuth are present. The optimum initial p H is 5.8 to 6.0. The temperature should not be above about 30" to avoid the decrease of the bismuth overvoltage that occurs a t higher temperatures. Approximately 2 grams of hydrazine dihydrochloride per 200 ml. should be present to accelerate the copper deposition. For the copper deposition the cathode potential must not be more negative than -0.30 volt cs. S.C.H. to prevent codeposition of bismuth and it should not be much below -0.28 volt to obtain satisfactorily rapid and complete deposition of the copper. For the subsequent deposition of bismuth the initial p H should be in the range 4.5 to 5.5 and the cathode potential should be -0.40 volt VS. S.C.E. \\-hen lead is present, the cathode potential must be limited to -0.40 volt, but when lead is absent it may be as negative as -0.50 volt, although there is very little gain i n using the larger value. At the optimum starting p H of 5.8 to 6.0 the buffer capacity of the tartrate system is rather small. Consequently, when large amounts of copper are deposited the correspondinglj- relatively large amount of hydrogen ion generated by the oxidat'ion of hydrazine at the platinum anode causes the pH to drop below the permissible value of about 5.2. The buffer capacity can be increased by increasing the total tart,rate concentration, hut this is undesirable because it decreases the rate of copper deposition (see Figure 9). The evident answer to this problem is the addit,ion of another buffer system with constituents that do not complex copper or bismut'h or otherwise influence the copper and bismuth depositions. For this purpose succinic acid (pXi = 4.2, pK2 = 5.6) is very satisfactory; the maximum buffer capacity

0.2+

T 0

I

I

I

I

I

I

0.1 0.2 0.3 0.4 0.5 0.6 CONCENTRATION OF T A R T R A T E

Figure 9.

I

I

0.7

0.8

M /

L

Influence of Total Tartrate Concentration on Rate of Deposition

40 mg. of copper from 215 ml. of solution of pH j.S.5 containing 0.044 . ! hydrazine dihydrochloride. Cathode potential -0.30 volt L.S. S.C.E.

zinc, mid lead precipitate as tal trates, and larger amounts of magnesium cause a pondery bismuth deposit. Iron (present as n ferric tartrate complex) undergoes cyclic reduction a t the cathode and oxidation a t the anode and thus decreases the current efficiency in the copper and bisniuth depositions arid increases the final "residual current." This causes no difficulty, provided the amount of iron does not eweed about 50 mg. per 200 nil. Aluminum inhibits the reduction of both copper and bismuth

V O L U M E 23, NO. 12, D E C E M B E R 1 9 5 1

1803

:ind nccessitates the use of a more negative potential for their

deposition. However, amounts of aluminum u p to 200 mg. can I)P tolerated, provided the cathode potential is increased to -0.40 volt 1's. S.C.E. for the deposition of copper and to -0.60 volt for the deposition of bismuth. The reason for this peculiar effect r ( ~ ~ n a i to n s he explained. Seither selenium nor tellurium can be tolerated 1)c~auseboth :ire reduced t'o the elemental states by hydrazine. Silver and mercury also interfere because they codeposit with copper. IIotlrr:~te:itiiounts of nitrate or sulfate are not. objectionable.

\

lOOOr

Table I. Successive Determinations of Copper a d Bismuth in Presence of Other Metals CoiJyer. AIg. Taken Found Diff.

Risinuth. Alg, Taken Found Difi.

34.7 400.2 404.5

404.3 401 3 398.0 401.0 402.1 400 0 401.3 402.1 395 i 398.1 398.5 398.0 398.7 398 3 399.2 -101 0 60.7 402 0

.i4.6 400 1 404.3 4 0 0 . 5 400.8 403.3 4 0 3 . 0 399 0 398.9 399 2 399 4 401.1 400.1 396.8 396.6 398 I 388 I 397 8 397.6 3 9 8 . 1 397.6 400.4 4 0 0 . 2 396 Li396 3 398 .j 398 6 399 7 399 6 66 7 66 4 101 8 101 8 '1

'1

c

-0.1 -0.1 -0.2

+0.3 -0.5 -0.1

+0.2

-1.0 -0.2 0.0 -0.2 -0.8 -0.2 -0.2 +0.1 -0.1 -0.3 0.0

404.8 + 0 . 3 401.5 + 0 . 2 398 1 + 0 . 1 400 3 - 0 . 5 401.8 - 0 . 3 100.0 0.0 401.6 T O . 1 402 2 +o 1 395.9 + 0 . 2 397 9 - 0 . 2 398.4 - 0 . 1 397.4 - 0 . 6 398 4 - 0 . 3 398 7 + 0 . 2 399 3 +o. 1 401.4 +o 4 (i0.4 - 0 . 3 402.2 f 0 . 2

Other I I e t a l s Present, Ilg Sone None None Sone 1000 Sn 1000 Sb 1000 Cd 1000 Xi 200 S I 200 Sln 200 Zn 200 AIg

.50 P h

40 I2e 200 h l " b

850 Sn; 17 Si]

Cii d r p w i t e d a t -0.40 volt and Bi a t -0.60 v o l t . 100 each Sh Zn Sn Cd IIe SIn \-i, 40 each Ph a n d Fe. 200 t:arh Ph: Zn: Cd; 100 Sg; .id Pb'; 40 Fc.

SUCCESSI\-E DETERMIRATIO3 01;COPPER. BISJICTH, LE 4 D , AND TIN

P" Figure 10. Characteristics of Bisniuth Deposition onto CopperPlated Cathode as a Function of pII In each case 118.3 m g . of bismuth i n 160 m l . of Eolution containing 0.5 W total tartrate and0.06 M h>drazinedih) droohloride. (hthode potential -0.40 volt LS. S.C.E. for oprn circle points and -0.50 volt for solid circle points

I

100 M A .

IO MIN.

Data in Table I show that Iiiainuth cim be separated iroin lencl a t a potential of -0.40 volt, :nit1 it. is also knon.ri ( 6 ) that lead can he deposited a t a potential of -0.50 to -0.60 volt from a tal'trnte solution of pH 4 to 5 , Thew is therefore 110 Iirotileiii in tieterniiiiing lead in the solution i,emniiiiiig from the depositioii of bismuth. The onl>- limitation is 1h:it the aniount of lead must not esceetl about 100 mg. per. 200 nil. :it a pH of :illout 5, or aliout 50 m g . per 200 1111. a t a pH of : h u t 4, I)ec:iuit. I:~rger. xiiouilts precipitate as the tartrate or hydrrigt,Ii t:wtr:itr. Tin is coiiiplesed so strongly tiy t:irtrate t h t it cannot lie tic,posited from a tartrate medium o! pH greater than :il)out 2 . Hou-ever, simply by strongly acidifying with 111-drochloric: acid the solutioii remaining from the tlcposition of lead, the stnniiic tartrate c*omples is decoinposctl ant1 tlie tin niay tie clepositc.tl Casily at a cathode potential of -0.60 to -0.65 volt 2's. S.C Typical result,s obtained in tlir iucwssive determitiations of coplit~i~, t)isiiiuth, lead, and tiii :IW + h ( i \ \ - i i iii Tuhle 11.

4

Figure 11. Current-Time Curies f o r Deposition of 100 3Ig. of Bismuth onto Platinum Cathode From 200 m l . of wlution of pH 5.25 containing 0.25 41 total tartrate and 0.05 7.1 hydrazine dihydrochloride. D 50' C. Cathode potential -0.40 volt CS. S.C.E. in each case

Temperature 4 2 5 ' , B 303, C 35', and

1804

ANALYTICAL CHEMISTRY

Table 11. (T

=

Successive Determinations of Copper, Bismuth, Lead, and Tin taken: F = found: D = F - T. All weights in milligrams) OthPr

None

None

97.1 95.9 -1.2

200.4 200.6

Sone

401.1 401.0 -0.1

100.7 99.3 -1.4

200.3 199.3 -1.3

Xone

63 4 63.3 -0.1

60.1 59.8 -0 3

52.9 52.1 -0.8

246.2 246.2 0.0

None

101.1 101.1 0.0 203.7

101.4 102.4 +1.0

48.4 48.1 -0.3 47.4 45.4 -2.0

105.9 106,6 +0.7

Copper

Bismuth

Lead

T F

200.0 200.7 +0.7

200.0 200.4 +0.4

100.0 98.2 -1.8

T I:

403.3 404.5 +1.2

502.8 501.7 -1.1

-2.1

301.7 301.2 -0.5

201.8 202.7 t0.9

400.0 399.9 -0.1

D I1 T

I.’

D T 1; I>

T F

D T

F D T

F

Q

Tin Sone

Cletals Present None

204.5

200.6 200.8

52.1 50.0

D +0.8 4-0.2 100 each Cd, AI, M g , Zn, Mn; 40 Fe.

decreases the anode potential by about 1.2 volts. Hydroxylamine has relatively little influence until most of the copper has deposited (ca. 15 minutes) and the current density has decreased to a relatively small value. The rapid increase in the efficiency of hydroxylamine after 15 minutes was reproducible. It was thought that this might reflect the formation of hydrazine as one of the products of the hydroxylamine oxidation, but tests for hydrazine in the solution were negative. When the concentration of hydroxylamine was increased fivefold (to 0.25 M ) i t maintained the anode potential below $0.75 volt from the start.

+0.2

I

205.1

..%I \

\

206.9 +1.8

The solution conditions were the same as in the determinations of copper and bismuth-vie., 200-ml. volume, 0.25 M total tartrate, initial pH 5.8 to 6.0, 1 gram of succinic acid per 100 mg. of copper, and 2 grams of hydrazine dihydrochloride. Copper was deposited a t -0.30 volt us. S.C.E., bismuth a t -0.40 volt, and lead a t -0.60 volt, without any treatment of the solution between the depositions. At the beginning of the bismuth deposition the pH was 5.2 to 5.5 and after bismuth had been deposited it was 4.9 to 5.1. After the lead was deposited the solution was acidified with 20 ml. of concentrated (12 M ) hydrochloric acid, 2 grams more of hydrazine dihydrochloride nere added, and the tin was deposited a t -0.60 to -0.65 volt. The accuracy of the copper, bismuth, and tin determinations is very satisfactory. The consistent negative error in the lead determination has been proved to be caused by loss during washing and not by incomplete deposition ( 5 ) . This loss averages 1.5 =k 0.6 mg. and is sufficiently reproducible so that it c i n be added as a correction to obtain lead results which generally will be correct to well nithin 1 mg. ANODIC DEPOLARIZATION BY HYDRAZINE AND HYDROXY LAWINE

An anodic depolarizer is a substance which, by being osidized in preference to water or other constituents of an electrolyte, maintains the potential of an anode (usually platinum) a t a relatively low value, and thus prevents undesirable anodic reactions. Such undesirable reactions include evolution of chlorine when chloride solutions are electrolyzed-with attendant dissolution of platinum from the anode and its subsequent deposition on the cathode-the deposition of higher oxides of certain metals (PbO,, MnOs, Bi206),and the oxidation of anions such as acetate, tartrat,e, etc. Although both hydrazine and hydroxylamine have long been favorite anodic depolarizers in electroanalysis, the literature contains practically no information on their relative efficiencies or their oxidation products a t a platinum anode. The anode potential-time curves in Figure 12 demonstrate the great efficiency of hydrazine as an anodic depolarizer and its pronounced superiority to hydroxylamine a t equimolar concentrations. These data were obtained by direct measurement of the potential of the platinum anode against a saturated calomel reference electrode during the deposition of 50-mg. quantities of copper from 200 ml. of a tartrate solution initially adjusted to pH 6.0. Without any depolarizer present (curve A ) the anode potential remains a t a relatively large value, and in addition to oxygen evolution there is considerable Oxidation of tartrate. Hydrazine

n

:.:I 0.6

IO TIME

15 IN

20 MINUTES

25

30

Figure 12. Relative Efficiencies of Hydrazine and Hydroxylamine as Anodic Depolarizers Data obtained during deposition of 50 mg. of copper from 200 ml. of solution of pH 6.0 containing 0.5 M total tartrate. For curve A multiply time scale by 4

The Oxidation of hydrazine under these conditions was found to be NzHs+

=

N2

+ 5 H + + 4e

This was established by collecting and analyzing the gas evolved a t the anode, which proved to be mainly nitrogen with about 3% carbon dioxide and 1% oxygen. The volume of nitrogen (remaining after the removal by alkaline pyrogallol of the small amount of carbon dioxide and oxygen) agreed to 1 to 3y0 with the volume expected from the foregoing reaction and the measured quantity of electricity passed during the electrolysis. The oxidation of hydrosylaniine a t the platinum anode proved to be more complicated. To determine the Oxidation products, solutions containing a known amount of recrystallized hydroxylamine hydrochloride in 1 Jf sodium chloride as supporting electrolyte were electrolyzed with the potential of the platinum anode controlled a t +0.95 to +1.00 volt us. S.C.E. and the quantity of electricity passed was measured with a coulometer in series with the cell. Sodium chloride was used instead of sodium tartrate as supporting electrolyte because the oxidation of the latter prevents 100% current efficiency in the oxidation of hydroxylamine. The large platinum gauze electrode of 160 sq. cm. area (normally used as cathode in metal depositions) was used as anode and a helix of platinum wire served as cathode. The cathode was isolated from the anolyte by placing it in a chamber containing 1 M sodium chloride which made electrolytic contact with the anolyte through a fine porosity sintered-glass disk (3-cm. diameter). A slow stream of nitrogen was passed over the surface of the anolyte to exclude air.

V O L U M E 2 3 , NO. 12, D E C E M B E R 1 9 5 1

1805

I +o I

0.1

-0L to.,

-03 0

r0.1

-03

-0.2

-01

.3.2 ro.1

-0 I

I

I

I

-oZ

.0.3

I

I

I

I

-0.3

0

-0 i

Figure 13. Polarograms Showing Extent of Reduction of Cupric Tartrate Complex by Hydrazine Dihydrochloride After A 2, B 15, C 30, D 45, and E 60 minutes. Solution was 2.66 millimolar in respect to total copper, and contained 0.23 M total tartrate, 0.1 M hydrazine dihydrochloride, and 0.01% gelatin. Temp. 25 C . pH 5.7

millimoles of nitrate and 0.25 millin~oleof nitrous acid. trate is formed by the 6-electron osidation

+

SHZOHZ+ 2H20 = S O 8 -

The ni-

+ 8Hf + 6e

and 3.67 millimoles of nitrate correspond to 2120 coulombs. The nitrite results from the 4-electron oxidation

+

XHZOHZ+ HzO

zb u -2p

'

o!

' ' io

TIME

eb

IN

I

I

Id0 l!O MINUTES

'

!lO

'

led.

=

+ 5 H + + 4e

and 0.25 millimole corresponds to 96.5 coulombs. The amount of nitrate plus nitrous acid thus accounts for the oxidation of 3.92 millimoles of hydroxylamine and 2217 coulombs, leaving 21.55 3.92 = 17.63 millimoles of hydroxylamine and 5580 - 2217 = 3363 coulombs to be accounted for by some other reaction. The unaccounted 3363 coulombs correspond to 3363/96.5 = 34.8 me. per 17.6 millimoles of hydroxylamine, or 34.8/17.6 = 1.96 equivalents per mole. This clearly demonstrates that the third and predominant reaction in the anodic oxidation of hydroxylamine is the 2-electron-per mole oxidation to nitrous oxide ZNHzOHz+ = N?O

4_

HSOZ

+ H20 + 6 H + + 4e

REDUCTION O F CUPRIC TARTRATE BY HYDRAZINE - 5 L

Figure 14. Rate of Reduction of Cupric Tartrate Complex by Hydrazine Dihydrochloride at pII 5.3 at 25" Initial concentration of cupric copper was 2.66 millimolar in 0.21 M total tartrate and M hvdraaine dihvdmrl - ~-~ ~ - ..iloride ., .~~ ~~~.~ _ _ _ 0.40 .... ~. -,-~ A . Cathodic diffusion current . B . Cathodic reflection of curve C C . Aniodic diffusion current ~~

~~

During the electrolysis the current decreased from an initial value of 0.4 to 1.4 amperes (depending on the hydroxylamine concentration) to 5 ma. or less after 2 to 5 hours. The fact that the current decays practically to zero shows that the hydroxylamine is oxidized with 100% current efficiency. Tests of the final solution showed that no appreciable amount of hydroxylamine remained. There was no gas evolution a t the anode, and this excludes nitrogen as a product. Qualitative test,s showed considerable amounts of nitrate and nitrite in the final solutions: the presence of nitrite excludes ammonia or ammonium ion as a final product. The amount of nitrate in the final solutions was determined (after removing nitrous acid by reaction with a small excess of sulfamic acid) by the Kolthoff, Sandell, and Moskovitz method ( 2 ) , and nitrite was determined by the Lunge method as described by Kolthoff and Sandell (1). In a typical experiment 21.55 millimoles of hydroxylamine in a volume of 200 ml. required 5580 coulombs (57.8 me.) for exhaustive osidation. The final solution was found to contain 3.67

Observations during the deposition of copper described in a preceding section indicated that hydrazine in the presence of chloride in tartrate media of p H above about 5 reduces the cupric tartrate complex to a chloro cuprous complex. On standing for several hours the reduction proceeds further to produce a reddish brown precipitate which appears to be mainly a mixture of metallic copper and cuprous oxide. To obtain further information on the rate of reduction to the chloro cuprous complex, the progress of the reduction was followed polarographically with the dropping mercury electrode. The tartrate supporting electrolyte containing a large excess of hydrazine dihydrochloride and 0.01% gelatin was adjusted to the desired pH, and then placed in the polarographic cell and freed from dissolved air by a stream of nitrogen. A measured volume of standard cupric sulfate solution was then added and polarograms were recorded after various time int,ervals. A typical set of polarograms is shown in Figure 13. The polarogram after only 2 minutes (curve A ) already shows a small anodic wave of the chloro cuprous complex, which increases with time and finally approaches a constant value. The total cathodic diffusion current (above the galvanometer zero line) results from the simultaneous reduction of both +2 and + l copper. It remains nearly constant because the much larger diffusion coefficient of the chloro cuprous ion approsimately compensates for the decreasing concentration of the cupric tartrate complex. The concentration of the chloro cuprous complex may be computed directly from the anodic diffusion current, and that of the remaining c u p i c tartrate complex from the dif-

1806

ANALYTICAL CHEMISTRY

ference betlyeen the total c:tthotlic diffusion current and the anodic diffusion current. The rate of accumulation of the chloro cuprous complex is shown by curve C in Figure 14. The data in this figure were obtained at a pH of 5.3 a t 25". Fiom the difference between curve A (the total cathodic diffusion current) and curve R (the cathodic counterpart' of curve C) thr equilibrium concentration of the cupric tartrate complex is calculated t o he 1.13 millimolar compared to its initial value of 2.66 millimolar, and the equilibrium concentration of the chloro cuprous complex was 1.53 millimolar -i.e., 5i.5yO reduction. At a higher p H value the reduction proceeds more rapidly and more cornpletely, and when the copper concentration is greater than about 5 millimolar cuprous chloride precipitates. From the observed diffusion currents the id/Cm2/3t' l 6value for the cupric tartrate complex under the conditions of Figure 14 is 2.:38 and that of the chloro cuprous complex is 1.i5. Correspondingly, from these data and the Ilkovic' equation, the diffusion coefficient of the cupric complex is 0.39 X 10-5 and that of the cuprous complex is 0.83 X 10-5 sq. cm. per second. The niuch larger diffusion coefficient of the cuprous complex accounts for about a third of the greatly enhanced current observed when copper is deposited on a platinum cathode from a solution containing both hydrazine and chloride. SURZMARY

The optimum conditions for the separation and successive determinations of copper, l)isniutli, lead, and tin from a tartrate

solution by controlled potential deposition onto :I plntinu~ncathode have been established. The influence of such impoitant variables as pH, total tartrate concentration, temperatut e, and cathode potential have been investigated systematically. All four metals may be determined without any intermediate treatment of the solution, except acidification for the determination of tin, in a total elapsed time of about 4 hours (1 hour actual operator time). Very few other elements interfere. The beneficial effect of hydrazine and hydroxylamine as addition agents has been investigated, and theii oxidation products a t a platinum anode have been identified. Hydrazine in the presence of chloride ion is a much more efficient anodic depolarizer than hydroxylamine, and it plays an even more important role by reducing + 2 copper to a chloro cuprous complex which greatly accelerates the copper deposition. LITERATURE CITED

Kolthoff, 1. hI., and Sandrll, E. B., "Textbook of Quantitative Inorganic Analysis," p. 602, Sew York, hfacmillan Co., 1947. (2) Kolthoff. I. M., Sandell, E. B., and hloskovitz, B., J . Am. Chem. (1)

SOC.,55,1454 (1933).

(3) Lingane, .T. J., Anal. Chim.Acta, 2, 584 (1948). (4) Lingane, J. J., IND. ESG.CHEM., ASAL.ED.,16, 147 (1944). (5) Ibid., 17, 640 (1945). (6) Lingane, J., J . Am. Chem. Soc., 67, 1916 (1945). . 22, 1169 (1950). (7) Lingane, J. J., and Jones, S.L., A N ~ LCHEM.. (8) Sand, H. J. S., Trans. Chem. SOC.,91,373 (1907). RECEIVED March 19, 19.51.

Rapid Photometric Determination of Aluminum in Zinc and Steel LUTHEK C. IKENBERRY AND tRBA THOMAS, Armco Steel Corp., Middletown, Ohio

It is frequently necessary to determine small of acid-soluble aluminum amounts (0.002 to 0.10~"c) in zinc and steel. The conventional gravimetric methods are long and tedious. -4 simple photometric method has been developed based on the reaction of aluminum with eriochromecyanin R to form a \ iolet-red colored complex. Its reproducibility is at least equal to that obtained with the gravimetric method. Aluminum may be determined in zinc in 20 minutes as compared to 4 hours by the gravimetric method. .4 determination of acid-soluble aluminum in steel requires on11 3 hours as compared to 8 hours by the gravimetric method.

T

HE increasing use of sm:ill additions of aluminum t o zinc in the production of zinc coatings and the ever-increasing uee of aluminum in the production of steel has made it. highly desirable that a simple, rapid method be developed which would givr accurate values. The gravimetric methods are long and t ctlious and must be carpfully performed by a n-ell-t'rained analyst in order t o obtain accurate values, especially when the nluminum content is under 0.05%. The literature contains numerous references-nIanJ- of which are given by Sandell (6)-on the use of organic reagents for the colorimetric determination of aluminum. Most of these are I)ased on the formation of a st,rongly colored lake x i t h a suitable organic reagent. These lakes have been regarded as internal complexes of aluminum that esist in colloidal solution or as

colloidal aluminum hydroxide on Tvhich the organic compound is adsorbed with a pronounced change in color. Some of the more common reagents which have been used are ammonium aurintricarboxylate commonly known as "aluminon," alizarin S, eriochromecyanin R, and hematoxylin. The most widely used reagent in this country is aluminon (4, 7 , fl), while eriochromecyanin R has found great favor in Europe ( 1 , 2 ) . The simplicity and ease of developing the color with eriochromecyanin is advantageous, but the dark color of the dye itself in the alisence of aluminum is the greatest handicap in its use. I n the development of the colored lake with eriochromecyanin R most investigators (1, S) have added the dye t o the solution containing the aluminum, adjusted the acidity, added the buffer solution, and then allo\yed from 15 t o 30 minutes for color development. Seuthe (8) found that if the dye were added to .a dilute acid solution containing the aluminum and allowed to react for 5 minutes, the maximum color would develop ininiediately when the solution was buffered a t a pH of 6. The final pH of the solution affects the color of the dye as well as the intensity of the aluminum lake. Khile most investigators have used a pH of 6, some have used a lower pH (4.6-5.6). APPARATUS

The Kromatrol photometer was employed in most of this ~ o r k , using the test tube supplied with the instrument as the absorptiorl cell. The absorption cell has an effective light path of approximately 12 nim. Bromatrol filter KO. 5 , vihose maximum transmission is 525 millimicrons, was used. The industrial model of the Klett-Summerson photoelectric photometer with Klett filter KO. 54, whose maximum transmission is 540 millimicrons, was also used.