The electrochemical preparation of magnesium perchlorate from magnesium chlorate employing a platinum anode and a rotating stainless steel cathode is described. The effect of electrolyte con,=en-tration, cathode and anode current densities, pH and temperature of the electrolyte and cathode rotation on current efficiency for the preparation of magnesium perchlorate was studied. A maximum curren t efficiency of 65-72% was achieved. Based on the results obtained on the laboratory scale, a 100A cell was designed, fabricated and operated

K C Narasimham

S Mohan

S Pushpavanam

S Vasudevan

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JOURNAL OF APPLIED ELECTROCHEMISTRY 22 (1992) 877-882 
Electrolytic preparation of magnesium perchlorate 
S. V A S U D E V A N ,  S. M O H A N ,  S. P U S H P A V A N A M ,  K. C. N A R A S I M H A M  
Central Eleetrochemieal Research Institute, (CSIR),  Karaikudi, 623 006, India 
Received 23 July 1991; accepted 10 February 1992 
The electrochemical preparat ion of  magnesium perchlorate from magnesium chlorate employing a 
platinum anode and a rotating stainless steel cathode is described. The effect o f  electrolyte con,=en- 
tration, cathode and anode current densities, p H  and temperature of  the electrolyte and cathode 
rotation on current efficiency for the preparat ion of  magnesium perchlorate was studied. A maximum 
curren t efficiency of  65-72% was achieved. Based on the results obtained on the laboratory  scale, a 
100A cell was designed, fabricated and operated. 
1. Introduction 
Among the various known methods, the electrolytic 
route is economical for the manufacture of a number 
of inorganic chemicals and especially perchlorates [1]. 
A variety of perchlorates, such as ammonium, 
sodium, potassium etc., are well known and their 
applications have been reported previously [2]. But 
work on electrochemical methods of preparing alka- 
line earth metal perchlorates are scanty. Because, in 
the case of alkaline earth metal salts, the precipitation 
,of the corresponding metal hydroxides causes great 
problems, leading to a loss in current efficiency. 
Further, heavy deposits of magnesium hydroxide on 
the cathode prevents continuous electrolysis. This dif- 
ficulty has been obviated by using the rotating cathode 
technique in this work and this paper presents results 
for the preparation of magnesium perchlorate using a 
platinum anode and a rotating stainless steel cathode. 
Anhydrous magnesium perchlorate (anhydrone) is 
a well known powerful desiccant. Another potential 
application of magnesium perchlorate is its use as 
electrolyte in high energy density batteries [3-5] and 
Mg/MnO 2 batteries [6]. 
2. Experimental details 
'The electrolytic cell consisted of a 500 cm 3 Pyrex tall 
:form beaker fitted with a PVC cell cover with slots to 
:introduce the electrolyte, pH sensor, thermometer and 
electrodes. The anode was either platinum foil or 
platinum coated titanium [7] of size 25 mm (b) x 
g5mm (h). Cylindrical stainless steel rods of 90ram 
height with diameters of 10, 15 and 25mm were 
employed as cathodes. The cathode was fitted to a 
:rotating assembly and positioned at the centre of the 
,::ell at an interelectrode distance of 5 mm. The cell 
assembly was placed in an outer vessel containing 
glass spiral surrounding the cell assembly and 
immersed in water. This entire assembly was kept in a 
l:hermocole box. The temperature of the water and 
consequently the temperature of electrolyte in the cell 
was controlled to the desired value with a variation of 
:___ 1 ~ C, by adjusting the rate of flow of thermostatic- 
ally controlled water through the glass spiral. 
0021-891x/92 9 1992 Chapman & Hall 
The magnesium chlorate solution obtained from the 
electrolysis of magnesium chloride was used as elec- 
trolyte. 
Electrical connection to the cathode was through 
the mercury contained in a cup attached to the top end 
of the rotating shaft driven by a 0.5 h.p. motor. A d.c. 
current was supplied from a rectifier (25A0 0-12V). 
The electrolyte pH was monitored using a pH probe 
and regulated by adding either acid or base. Each 
experiment was continued until the theoretical charge 
had been passed. 
The constituents of electrolyte, viz., Ct-, C103, 
C1Oz and Mg(II) were analysed periodically [8]. 
The cumulative current efficiency was calculated for 
each experiment. The effect of various parameters on 
current efficiency was determined. 
3. Results 
3.1. Effect of initial concentration of magnesium 
chlorate on current efficiency 
The results of the experiments carried out with dif- 
ferent initial concentrations of magnesium chlorate 
ranging from 1.0 to 3.0 M (and maintaining other 
parameters constant), are given in Fig. 1. The current 
efficiency for perchlorate formation increases signifi- 
cantly up to an initial concentration of magnesium 
chlorate of 2 M and above this value it decreases 
rapidly. 
3.2. Effect of pH on current efficiency 
Table 1 shows the effect of electrolyte pH on the 
current efficiency for the formation of magnesium 
perchlorate. The current efficiency was found to be 
maximum around pH 6.0. It is also well known that 
lower pH ( < 5.0) can lead to the evolution of chlorine 
gas when pH is adjusted with dilute HC1. However at 
higher pH (> 7.0) the precipitation of magnesium 
hydroxide is more pronounced under these conditions. 
3.3. Effect of anode and cathode current densities on 
current efficieney 
The current efficiency was found to increase with 
877 
! 
BO-O 
7 7~0 
@ 
o 
_~ 6/.-0 
s 
560 
300 
I I I I 
5.00 7.00 
E lec t ro l y te  pH 
1202 SHORT COMMUNICATION 
120 
r 
c 
o_ 
E 
100 
>~ 
o- 
Ld  
Fig. 2. Variation of current efficiency and energy consumption with 
electrolyte pH. Conditions: Electrolyte concentration: 185 g dm -3; 
temperature: 333 K; anode current density: 15.0 A dm -2; cathode 
current density: 14 .0Adm -z and cathode peripheral velocity: 
40 m min -1" 
90 
o 80 
r -  
IO 
70 
L 
o 
0 
14 t -  
10 E 
> ,  
2 
c 
Ld  
I I I I I 
s0 10 20 30 40 
Cathode cu r ren t  dens i t y lA  dm -2 
Fig. 3. Variation of current efficiency and energy consumption with 
cathode current density. Conditions: Electrolyte concentration: 
185gdm 3; electrolyte: pH6.0; temperature: 333K; anode current 
density: 15.0 A dm -2; cathode peripheral velocity: 40 m rain - 1. 
concentrations the precipitation of magnesium hyd- 
roxide is increased, thereby reducing the current effi- 
ciency. 
3.2. Effect of p H  and cathode current density 
Figures 2 and 3 show the effect of electrolyte pH 
(ranging from 4.0 to 7.0) and cathode current density, 
respectively, on current eƒ Unlike the alkali 
metal chloride reactions, the pH and cathode current 
density play a significant role in determining chlorate 
formation as they are directly connected with hyd- 
roxide formation. 
Attempts have been made to measure the cathode 
surface pH, [pH(s)], adopting different methods by 
various authors [10]. The change in pH(0 (ApH) is 
related to the bulk pH [pH(b )] and current density. At 
lower current densities ApH is not appreciable whe- 
reas at higher current densities ApH increases signifi- 
cantly, even when pH(b) is lower [10]. In this case 
current efficiency is found to be maximum around 
pH 6.0. But when pH(b ) becomes 7.0, pH(~) is more 
alkaline, thereby favouring the precipitation of mag- 
nesium hydroxide and releasing an equivalent amount 
of free chlorine with a loss of eff• [11]. Below 
pH(b) 5.00, it is well known that the efficiency decreases 
due to evolution of free chlorine It]- Figure 2 shows 
this trend; the current efficiency was maximum around 
pH 5.5 to 6.0. 
Kurdrayavstev et aL [12], found that in the case of 
KC1 solution, using a platinum electrode, the surface 
pH increased with increasing current density. Starting 
with an initial pH of 4.8 the pH becomes 5.9 and 14.0 
at current densities of 0.1 and 100 A m - 2 respectively. 
At still higher current densities the increase in pH(o is 
higher. In the present instance, with high current den- 
sity the pH(s) can easily reach >/14, thereby becoming 
alkaline, resulting in the precipitation of metal hyd- 
roxide on the cathode surface. 
According to the Mg-H20 Pourbaix diagram [13], 
magnesium hydroxide formation is predicted by 
Mg 2+ + 2OH- ~ Mg(OH)2 (2) 
log[Mg 2+] = l o g K -  2pH (3) 
Table 1. Effect of  anode current density on current efficiency for the preparation of  magnesium chlorate 
S. No. Anode current Voltage/V Final Loss of  Current efficiency 
density/A dm -2 concentration Mg2+/gdm -3 for Mg(Cl03) z 
/gdm - ~ .formation/% 
Cl-  CIO; 
Energy consumption 
for Mg(ClOa)~/ 
kWhkg  -I 
1 5.00 3.9 15.0 
2 10.00 4.0 11.0 
3 15.00 4.2 9.7 
4 20.00 4.3 7.1 
242.0 10.2 64 10.2 
260.0 8.7 69 9.8 
275.0 7.9 73 9.7 
301.0 6.8 78 9.3 
Concentration of MgCI2: 185.0gdm-3;  electrolyte pH6.00. 
Temperature: 333 _+ 1 K; cathode current density: 40A dm -2. 
Cathode peripheral velocity: 40 m ra in-  l. 
Theoretical quantity of electricity passed: 273 A h. 
1204 SHORT C O M M U N I C A T I O N  
Acknowledgement 
The authors wish to express their gratitude to Prof. S. 
K. Rangarajan, Director of the Central Electrochemi- 
cal Research Institute, Karaikudi, for his kind en- 
couragement and permission to publish this paper and 
to Council of Scientific and Industrial Research 
(CSIR), New Delhi, for the award of Senior Research 
Fellow to S.V. 
References 
[1] N. Ibl in (eds) J. O. M'Bockris, B. E. Conway, E. Yeager and 
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E L E C T R O L Y T I C  P R E P A R A T I O N  O F  M A G N E S I U M  P E R C H L O R A T E  881 
A 
I 
I 1 . . . .  
- - k .  . [ --4 --i-- , t  
, I', I 1 '! I 
290 
]Fig. 4. A sketch of 100A magnesium perchlorate cell. (Not to scale: ali dimensions in millimetres). (1) SS tank, (2) SS cathode, (3) anode, 
14) electrolyte, (5) pulleys, (6) V-belts, (7) PVC cover, (8) copper bus bar, (9) mercury cup and (I0) 0.5 h.p. motor. 
6. Conclusion 
Table 4. Results of 100 A cell for the production of magnesium 
perehlorate 
1. Electrolyte Mg(C103)2 
2. Volume of the electrolyte 7.5 dm 3 
3. Initial concentration 
C1- 0.0gdm -3 
C10; 352.0 g dm-  3 
ClOg 0.0 g dm -s 
4. pH 6.00 
5. Temperature 333 K 
6. Cathode peripheral velocity 82mmin -~ 
7. Final concentration 
C10~ 5.4 g dm 3 
CIO~- 451.0 g dm -3 
8. Current passed 100A 
9. Anode current density 30 A dm -2 
10. Cathode current density 20Adm 2 
11. Total quantity of electricity 2247 A h 
t2. Average voltage 6.00V 
13. Current efficiency for magnesium 
perchlorate formation 72.0% 
14. Energy consumption for magnesium 
perchlorate 3.40 kWh kg- 1 
The electrochemical oxidation of magnesium chlorate 
to perchlorate can proceed efficiently, when 2M 
Mg(C1Os)2 (pH 6.0) is electrolysed at an anode current 
density of 30 A dm -2 and a cathode current density of 
20Adm -2 with a cathode peripheral velocity of 
82 m min 1 to achieve: current efficiency of 72% corre- 
sponding to an energy consumption of 4.5 kWh kg- 1 
of Mg(ClO4)2. 
Acknowledgement 
The authors wish to express their sincere gratitude to 
Prof. S. K. Rangarajan, Director of the Central Elec- 
trochemical Research Institute, Karaikudi, for his 
kind encouragement and permission to publish this 
paper and to Council of Scientific and Industrial 
Research (CSIR), New Delhi for the award of Seniol 
Research Fellowship to SV. 
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Electrolytic preparation of magnesium perchlorate

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Electrolytic preparation of magnesium perchlorate

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