Electrochemical synthesis of tin fluoborate

ELECTROCHEMICAL

SYNTHESIS C. J. CHEN and

Tzu-Chiang

Research (Received

Institute, Tsing-Hua

17 December

OF TIN FLUOBORATE

C. C. WAN University,

Hsin-Chy

Taiwan,

1984: in revised form 12 March

R.O.C.

1985)

Abstract-An electrochemical process to synthesize stannous fluoborate by means of ion-exchange membranes was established. The anodic limiting current could reach 6 A dm - * and the current efficiency was over 95 %. A galvanostatic method was adopted to study the kinetics ofanodicdissolution of tin in fluoboric acid. The exchange current density was evaluated to be 4.57 rnA dm-‘. Polarographic study of the cathodic reaction of stannous ions also confirmed that the process was diffusion controlled.

NOMENCLATURE current efficiency of anodic dissolution of tin standard equilibrium electrode potential anode potential saturated calomel electrode potential polarographic electrode potential when i = 4 il polarographic electrode potential when i = 4 il polarographic electrode potential when i = 3 i, Faraday constant current density exchange current density limiting current density when surface concentration of active ion approaches zero number of BF: which are attached to stannous ion in the rate-determining step

E0x1

concentration

of oxidizing species

Ox], surface concentration of oxidizing species [Red1 concentration of reducing - species _ duration of electrolysis temperature volume of anolyte volume of catholyte amount of tin deposited on the cathode stannous ion concentration in the anolyte stannous ion concentration in the catholyte total tinconcentration (Sn’+ plus !W’)in theanolyte stannic ion concentration in the anolyte number of charge transferred in the rate-controlling

step transfer coefficient activation overpotential

dissolution is the preferred approach resulting in fast reaction, high efficiency and few by-products [ 1,2]_ In our study we use an anion-exchange membrane to improve the separation effect between cathode product and anode product. The operating conditions of this process were determined and its electrochemical kinetics were also studied by means of transient techniques.

THEORETICAL The electrochemical cell used in our synthesis reaction was separated into two compartments by means of an anion exchange membrane. This membrane prevents the dissolved stannous ion in the anolyte migrating into the cathode compartment and allows the fluoborate ions in the catholyte to migrate into the anolyte compartment. The main net reactions are Anode: Cathode: 2BF;

Sn-tSn”+ 2HBF,-

-+2e-

(1)

+ 2e- + Hz (g) + ZBF,

(catholyte)-2BF;

(anolyte).

(3)

The net result is the generation of Sn (BF& anolyte until it reaches the desired concentration taken out as a solution.

in the and is

The kinetic relation of this dissolution assumed to follow the Tafel equation: q = ( - 2.303 RT/ZaF)

INTRODUCI’ION Fluoboric acid and stannous fluoborate are widely used in tin plating and solder plating systems since they can sustain high current density and current efficiency, and the alloy composition can be varied in a wide range without much difficulty. The system has received wide acceptance in the manufacture of printed-circuit boards, cables and in the surface treatment of other electronic devices. However, relatively few publications deal with the process and chemistry of synthesizing stannous fluoborate. Since tin cannot be chemically dissolved in fluoboric acid easily, it is known that electrochemical

(2)

ie

reaction

log i, + (2.303 RT/Za)

q =a+blogi

was

log i (4) (5)

where q is the overpotential and i is the current density. The parameters LI,bean be evaluated from q vs i data. A polarographic method was also used to study the mechanism of the reaction Sn2+ + 2e-

Supposing

the limiting

with a saturated 1307

*Sn.

(6)

step of this system

Ox+ze-

*Red

calomel

electrode

is (7)

as the reference

C. J. CHEN AND C. C.

1308

neglected under very small currents and by measuring E at various BF;, we can obtain

(.I&), the anode potential E, can be written as E,+E,,,

= E”--

0.059

log

-lLRedl

(9)

cox3s

where [OxL is the surface activity of the Ox ion. In a polarographic system [3] i = K ([Ox]

WAN

-[Ox],)

(9)

i, = k [Ox]

AE/Alog

(11)

then ; 0 r

If we take i = id/2, then E,,,,i, potential E1,3. Hence El/= + E,,

0.059 ---log& 2

becomes

zd - i’ 112)

the half-wave

= E” -

U 3)

and E = E,,,

--

(14)

For an irreversible reaction (3) (15)

E = El/z and E 314

=

h/z

EXPERIMENTAL 1. Synrhesis

Yield (wt 7;) = (X, VI) 3

(16) (17)

E I,4 -- EL,Z

where E,,, and E,,, are electrode potentials when the current densities equal ti, and $i, respectively. Combining Equations (16), (17) 0.916 RT -----log9 UZF

=

em~Sr?‘+n(BF.)~

Given Sn(BF,)z-“,

where I is the current and Lis the electrolysis duration in seconds. The molar concentration of stannic ion in the anolyte Z, is z, = Y, -x,.

(23)

Finally the current efficiency is

(18)

(19)

where n is the number of BF, which may be attached to the stannous ion in the reactant species, then E = E”-EE,,--

(22)

2X,-V,+4(Y,-Xx,)V,+2X2V2

= - 7.

Knowing a from polarization information, then z can be determined from Es,, - E,,, according to Equation (18). For the stannous ion-tin system, if the limiting step is Sn (SF,):

of stannous jhoborate

50% HBF, was used as catholyte. The anolyte was prepared with varying concentrations of HBF4 and with boric acid as stabilizer[rl]. The ion-exchange membrane (Ionac MA 3457) was pretreated with 50% HBF4 solution (immersion for 6 h at 50°C). Graphite panels were used as (he cathode and pure tin panels (99.9%) were used as the anode. Different agitation methods were applied to investigate their effect. During electrolysis, samples were taken every 12 h to analyze the dissolved metal content and the amount of stannic ions which were formed due to oxidation of the stannous ions. Since only stannous ions were regarded as the real product, the yield and current efficiency were calculated as follows: Assuming that molar stannous ion concentration is X, and the total tin ion (Sn’+ + Sn4*)concentration is Y, in the anolyte with a volume of V, dm3, also assuming that in the catholyte with a volume of Y, dm’, the stannous ion concentration is X1 and the total tin ion concentration is Y2and the amount of tin deposited on the cathode is W2 g, then

-

0.916 RT =E 112--log azF

E w-&14

(21)

(10)

i = K, [Red]

E+E,

= Fn.

This equation can give us information about the indentity of Sn (BF,):-” and the effect of BF; on the reaction mechanism.

where id is the limiting current when [Ox], becomes zero. Knowing that the concentration of reduced species is proportional to the current,

0.059 = E”--+og

[BF,]

[BF;]” [Sn*-“1 0.059 z log [Sn (BF4)f-“] the variation of [Sn2-‘1

‘20) can be

2W, +918.69 2. Analysis

I[ 1 F Ir’

(24)

of products

The stannous ion was quantitatively analyzed by titration with permanganate solution and the stannic ion by a polarographic method. The free FB; and boric acid was also measured polarographically at - 1.68 and -1.74V with 0.1 NKNOs plus 10% manitol as supporting electrolyte. Other metal impurities were measured by atomic spectroscopy and anions such as Cl-, SO:were determined by ion chromatography (Dionex model 10).

Electrochemical 3. Electrochemical

synthesis of tin fluuoborate

1309

kinetic studies

The polarization data of this anodic dissolution reaction was studied by a galvanostatic technique with a saturated calomel electrode (E,) as reference electrode. The transient response was recorded on an oscilloscope. The galvanostatic method can cleady indicate the individual contribution of IR drop, activation overpotential and concentration overpotential[S, 61. The basic set-up is shown in Fig. 1. In this study we are interested in the activation overpotential which is the chief variable controlling the charge-transfer reaction. A typical galvanostatic response is shown in Fig. 2. A dc polarographic method was also used to measthe ure %. ELI2 and EIJ4 in order to determine reaction mechanism based on Equation (18). Different concentrations of BF; were used to study the form of its existence in the charge-transfer step as shown in Equation (19). Both galvanostatic measurements and polarographic studies were carried out at 25°C. RESULTS

AND

DISCUSSIONS

Fig. 3. Applied voltage as a function of HBF,, concentration

at four different anodic current densities. 1. Process conditions The applied voltage is strongly influenced by the fluoboric acid content in the electrolyte as shown in Fig. 3. Presumably, in the initial stage, the fluoboric

C

A

Fig. I. Schematic diagram of the circuit for measuring A, anode; C, cathode; P, reference electrode (see); B, 12 V. 150 Ah lead-acid battery as power source; R, variable resistance

acid accounts mainly for the conductivity. A higher content of HBFI naturally offers better conductivity. But as the concentration of Sn(BF& increases in the electrolyte, the situation becomes more complex. The electrolytic conductance comes from the migration of of H+, Sn2+ and BFL. When the concentration Sn (BF4)1 reaches 50 % in Fig. 4, the interaction of ions become so great that the conductivity actually decreases when the remaining fluoboric acid concentration is increased from 1 to 3M. In order to obtain a product solution with minimal resistance, the recommended final fluoboric acid content in the stannous Auoborate solution is 1%. The effect ofcurrent density on the yield and current efficiency is shown in Fig. 5. The current density can reach over 6 A dm- ’ without noticeable undesirable effect. Mechanical agitation (300 rpm) was used in this case. The total electrolysis time was over 30 h. From the production point of view, high current density is preferred. However a compromise must be reached when a high production rate can be maintained

panet; T&.,1 ohm precision resistor; S,, switch; r,, 1 ohm precision test resistor.

V (mv) t 200

i

150 100

----

----

I T’ 17--J%-

Jc _ one

_--_

50 0 0

1

2

3

2

tl msl

cl

0

Fig. 2. Typical

potential-time transient.

curve

of

galvanostatic

I

’ 1

2

3

4

CC”,

Fig. 4. Specific conductivities in aqueous Sn (BF& solution.

1310

C. J. CHEN AND C. C. WAN

Fig. 5. The yield and current efficiency of anodic dissolution

without the generation of excess by-products. The yield and current efficiency are over 100 % in the low current density range. This is because the tin anode dissolves chemically in addition to the electrochemical dissolution. The agitation of the electrolyte is another important operation variable. Table 1 shows the result of the process under various agitation conditions. It is apparent that vigorous agitation tends to improve the production rate. However too much agitation also induces rapid oxidation of the stannotts ions, which is undesirable. Stannous ions are easily oxidized to stannic ions, but it was also found that stannic ions tend to be reduced back to stannous ions in the presence of metallic tin and fluoboric acid. It is shown in Fig. 6 that this reduction phenomenon becomes apparent at high concentration of Sn (BF&_ In an auxiliary test, we found that if we put tin sheet into a 0.01 NNazSnOj-1 N HBF, solution for 24 h, the stannic ion concentration dropped from 0.01 to 0.008 M, probably due to the reaction Sn4+ +Sn + 2Sn2+. (25) It is shown in Fig. 5 that the ratio of Sn2+ to Sn4+ was also affected by the current density. A possible explanation is that Equation (25) may be influenced by the electrochemical state of the metallic tin. When the tin is strongly polarized, as in the electrolytic cell, Equation (25) is retarded and a low SnZ+ /Sn4+ ratio is obtained. Table 1. Agitation effecton theelectrolysis of tin acid (C.D. = l.0Adm-2,

Agitation mode Yield ( %) Sri’+ /Sn4+

of tin in 1 % HBF solution.

2. Kinetic

Table 2. Concentration VW

time = 3 h)

Nitrogen

studies

A galvanostatic method was used to study the polarization of the anodic dissolution of tin in fluoboric acid. Pure tin sheets were used as both the anode and the reference electrodes so that direct overpotential reading can be obtained. The fluoboric acid concentration was controkd at 0.25 M. The activation overpotential in response to various applied current densities was recorded in Fig. 8. The experimental result was fairly reproducible. Four polarization measurements were made under the same condition and the maximum deviation was only 1.5 %. From Fig. 8, it was noted that the process follows the Tafel equation at high current density. Furthermore concentration overpotential was not detectable within our experimental conditions. This may be one of the reasons why the fluoborate system is widely used in high speed plating processes. When we fitted the polarization data into the Tafel equation, the transfer coefficient was evaluated to be 0.66 and the exchange current density to be 4.57 mAcmm2. A dc polarographic method (Princeton Applied Research, Mode1/384) was used to study the reaction mechanism of the system. When the stannous ion concentration was fixed at O.ooo8 M the El ,I and E,,., - EL,, values were evaluated to be around - 0.248 V and 47.6 mV respectively. The results, recorded in

in fluoboric 0

Stirring ([pm)

10-5

w=ginf5

300

900

1960

89.4 17.5

91.4 11.75

92.7 2.64

92.4 1.18

Time = 24 h.

5 x 10-S 10-d 5 x 10-4 10-g 5 x 10-a

of BF,

E,,, W) - 0.247 -0.244 - 0.246 - 0.249 - 0.248 - 0.247 - 0.248

E 3,4-E,,* (mV

48.1 47.7 46.5 47.5 46.2 47.0 48.0

Electrochemical

Fig. 6. The ratio of Sn2+/Sn4*

‘synthesis of tin fluoborate

and the concentration

1311

of Sn4+ at different concentratmns

of Sn (BF,),.

Table 2, were found to be practically independent of the concentration of Auoborate ions. Based on Equation (18). the charge-transfer number of the rate determining step was found to be 2. In addition, since E 1,2 is practically constant, AE,,JAlog [BF; ] = 0. Hence according to Equation (21), there is no fluoborate ion involved in the charge-transfer reaction. The stannous ion exists in a completely dissociated state.

1 t

CONCLUSIONS

Fig. 7. The change of the ratio of Sn2+/Sn4+ anodic current density.

Fig. 8. Anodic

as a function of

polarization

The electrochemical dissolution of tin metal in Buoboric acid with the aid of an anion-exchange membrane was found to be a satisfactory process in the preparation of stannous Iluoborate solution as a raw material for the tin plating and solder plating indus-

cume of tin in 0.25 M HBF,

solution.

1312

C. J. CHEN AND C. C. WAN

tries. The only major area which needs further improvements is the development of a new ion-exchange membrane which can sustain higher current density with a lower IR loss. The practical anodic current can reach 6 A dm- ’ and the current efficiency is over 95 *A. The current density could be increased with intense agitation but probably at the expense of current efficiency. By means of a galvanostatic method, the reaction was found to follow the Tafel equation and the exchange current density was evaluated to be 4.57 mAcm_‘. Polarographic study shows the stannousltin reaction to be diffusion controlled and no fluoborate ions were involved in the charge-transfer reaction.

Ackwwledgeme~rs-The Petroleum Chemical appreciated.

financial assistance of the China Development Corp. is sincerely

REFERENCES 1. T. M. Seiko, U.S. Patent 4038586 (1977). 2. H. P. Wilson, U.S. Patent 3795595 (1974). 3. D. Sawyer, J. Sawyer and Roberts, Experimental Electrochemistryfor Chemists. Witey, New York (1974). 4. J. A. Harrison, Eiectrochim. Acta 25, 1165 (1980). 5. L. Karasyk and H. Linford, J. electrochem Sot. 110, 895 ( 1963). 6. P. Danna and H. Linford, Plot& 55, 456 (1968). 7. J. D. Kanakara, Metal Finish. 73, 25 (1983).