Indian Journal of Chemical Technology Vol. 1 0. March 2003. pp. 1 50- 1 53

A rticles

Influence of humic acid on i onisation constants of sulphurous acid: Calcium sul­phi te precipitation during clarification of cane juice

Mahendra Prasad & Kaman Singh*

Physical Chemistry Department, National Sugar I nstitute, Kanpur 208 0 1 7. I ndia.

Received 1 Novell/bel' 2001; revised received 27 Novell/ber 2002; accepted 5 January 2003

The presence of humic acid seems to decrease the rate of CaSOJ precipi tation during clari fication of cane juice i n the

sugar manufacturing. The first and second ionisation constants of H 2SOJ were found to decrease by 8 and 1 3% at RT in presence of humic acid i n solution, respectively. However, the first and second ionisation constants of the acid were af­

fected in the range 1 9- 1 6% respectively across the temperature range 50-90°C. In the presence of humic acid the activation

energy was found to increase. The nature of posit ive slope of Arrhenius plot i ndicates the di ffusion controlled r.::action in the precipitation reaction which is consistent w i th low value of activation energy of ionisati ion of sulphurous acid.

The Indian sugar industry employs two processes for clarification of cane juice for the manufacture of crystal sugar. These are carbonation and sulphi tation processes in which the cane juice is treated with l ime, carbon dioxide and sulphur dioxide, respectively. The clarification of cane juice by either of the processes is pri mari ly controlled by maintaining optimum pH. During the passage of S02 into l imed cane juice, the most predominate chemical reaction other than decol­ourisation is the precipitation of CaSO). The stoichi­ometry ' of the reaction i s as follows:

Ca (OHh � Ca++ + 20H-S02 + H20 � H2S03 � W + HS03-HS03- � H+ +S03

2-

2W +20H- � 2 H20 I Ca++ +S03 -- � CaS03 .. The chemistry of cane juice clarification involves the removal of non-sugars. The removable could be or­ganics, i norganics, precipitates, colourants or even col loids, etc. All of them are primarily governed by ionic reaction andlor an adsorption

2 mechanism. In

the present contribution, the influence of a cane ju ice colourant; humic acid

3-5 on ionisation constants of

sulphurous acid during precipitation of calcium sul­phite is investigated.

Experimental Procedure Humic acid was i solated from clarified cane juice

by the method described elsewhere3-5 . I ts isolation

was confirmed by IR and UY absorption spectrum of the isolated product. The absorplion peak at 3080 cm- '

indicated the present of C-H groups. Other peaks at 1 660 and 1 590 cm- ' suggested the presence of quinol and >C=C< groups respectively. In order to observe the effect of humic acid on ionisation constants of sulphurous acid a series of pH- metric titrations of sulphurous acid with Ca(OHh solution were per­formed. Fused CaO ( 1 5%) was used for l iming of juice and li med juice was sulphited from S02 gas supplied by SPUN MAD ICCSE- 500 N 546. The supernatant solution of l ime slurry was fi ltered

through Whatman # I fi lter paper. EDT A di-sodium sal t (M/56) was prepared in deionised water. Erio­chrome black T (Merc) and starch solution ( I %) were used as indicators for the estimation of CaO and S02,

respectively. The concentration of S02 and Ca++ ion in solutions were determined iodimetrical ly and by EDTA titration, respectively.

Calibration of pH meter The pH meter employed (CP-90 1 -P) was calibrated

with buffer solutions of known pH values of 4.0 and 7.0 respectively using 0.2 N 9.0 mL CH3COONa + 0.2N 4 1 .0 mL CH3COOH (by standardisation) and 1 2.40 g H3B03 + 0.93 g NaCI in one l i tre. Once lhe i nstrument is cal ibrated to give the known pH of the buffer solutions. pH of experimental solutions can be obtained directly without any calculation . This pH meter measures the pH in the range of 0- 1 4 with an

accuracy of ± 0.0 1 pH.

Prasad & Singh: I nfluence of humic acid on ionisation constants of sulphurous acid Articles

pH-metric titrations

In 20 mL sulphurous acid (S02 + H20 solution) containing 10 kg/L S02 was titrated with Ca(OHh solution by measuring the pH as function of Ca(OHh. In another four sets, 10 mL of sulphurous acid was di luted with 8, 6, 4, and 2 mL of deioni sed water and 2, 4, 6 and 8 mL of (0. 1 %) humic acid solution was added to each solution, respectively keeping volume equal to 20 mL i n each case. Each solution was t i­trated with Ca(OHh solution and change i n pH was recorded as a function of volume of alkali using a digital pH meter.

Results and Discussion

All pH ti tration curves are shown in Fig. I in which curve I refers to the pure sulphurous acid titration whereas curves I I , I I I , IV and V correspond to the titration of H2SO) i n presence of 1 00, 200, 300 and 400 ppm humic acids, respectively. At a glance it could be noticed from Fig. I that the presence of hu­mic acid in (S02+H20) solution has marked i nfluence on the ti tration curve showing i ncrease in consump­tion of Ca(OHh for neutral ization and i ts consump­tion appears to be proport ional to the concentration of humic acid. I t is observed further that pH i ncreases sl ightly in the range 1 .5-3.0. However, beyond pH 3 .0, the value increases abruptly to 6.0 on addition of only 0.044 mi l l imole Ca(OHh solution showing the first inflection in the t i tration curve. The second in­flection was observed when l ime just exceeds 0.325 mi l l imole.

S ince in the above observations the effect of humic acid on both the ionisation constants of sulphurous acid was fai rly establ ished. It was, therefore, decided to observe the effect of temperature on the ionisation constants of pure H2SO) in presence of humic acid. In order to observe the effect of temperature of sulphur­ous acid with Ca(OHh solution were also performed

at higher temperatures viz. 50, 70 and 90°C i n absence and presence of 250 ppm humic acid i n sulphurous acid . The datu are shown in Figs 2 and 3. I n Figs 2 and 3 curves A, B, C and 0 correspond to the titra­

tions data at 25, 50, 70 and 90°C, respectively. Fig. 2 demonstrates that the temperature has marked effect on the ti tration curves. These ti tration curves were uti­l ized to determine the ionisation constant of sulphur­ous acid in each case by the half equivalence method.

Half equivalence method Let the ionisation and neutral isation of H2SO) be

considered.

I2.U .. I'un- U,SO,

lII,n _ 0 I I,SOJ -t- IOU 1111111 humit, IIdt! • I I,SO, .. 2bh Itlull humic ll(i�1 X II.SO,+ Jnnlllllll hllmi(' ari�1 !Xl H;SOJ + "till rrm hUlllie- IIfill

'.0

� (;,0

2.0 �...-_

[!.IIK2 11.11." II.H(, 1I,32K nAIO (Un n.�7", 11,(,5" 11.7311

Fig. I -p H metric titration of H�SO, and H2SO, contai ning hu­

mic acids wi th Ca(OH)2 at room temperature.

1 1 . u l--------------------:-1 /1. 2!" C (i) �U· c • 7U· C 10.0 )i 90' ( '

11.11

'.0

1l.!ll tu.. O.I �(. n.211 0.2MI 11.31 1 11.3(." O A l l, II.�/'" II.�ll) C;t(OIl ),milli mul('

Fig. 2-pH metric titration of sulphurous acid with Ca(OH h in absence of humic acid at d ifferent temperatures.

. I

Fig. 3-pH metric titration of sulphurous acid wi th Ca(OHh in presence of humic acid at different temperatures.

1 5 1

A rticles I nd ian J. Chern. Techno! . , March 2003

Table I -First and second ionisation constant of H 2SO.1 and H2SO) contai n i ng humic acid of d ifferent concentration

Sample p K I p K2 K I K2

H�SO) 1 .80 6.40 1 .585 x 10-2 3.98 1 X 1 0-7

H2S02 + 1 00 ppm 1 .83 6.45 1 .479 x 1 0-2 3.548 X 10-7

H2SO) +200 ppm 1 .86 6.55 1 .390 x I (r� 2.8 1 8 x 10-7

H�SO) +300 ppm 1 .89 6.60 1 .288 x 1 0-2 2 .5 1 2 x 10-7

H2SO) +400 ppm 1 .95 6.65 1 . 1 22 x 1 0-2 2 .239 X 1 0-7

Table 2-lonisation constant of sulphurous aci d at d i fferent temperatures

Temp. In absence of humic acid In presence of 250 ppm humic acid

25

50

70

90

1 .47 X 1 0-2

9.33 X 1 0-.1

6.60 X 1 0-)

5 . 0 1 X 1 0-)

2 .88 X 1 0-7

2.23 x 1 0-7

1 . 86 x 10-7

1 .58 X 1 0-7

1 .20 x 1 0-2

7 .58 X 10-)

5.37 X 1 0-.1

3.98 X 10-.1

2 .5 1 X 1 0-7

1 .86 X 10-7

1 .5 1 X 1 0-7

1 .25 X 10-7

Ca(OHh -. Ca++ + 2 OH -

[ HS03" l or pH = pK + log - · 00

I [H 2S0 3 l undissociated

at half equivalence point the Eq. (4) reduces to

pH = pKI

For 2nd ionisation, that is

K, HSO - ---:.. H+ + SO 20

J 3

and l ikewise at half equivalence point pH = pK2

( 1 )

(2)

(3)

. . . (4)

. . . (5)

. . . (6)

Both ionisation constants of sulphurous acid in ab­sence and presence of humic acid of different con­centrations are given in Table 1 which il lustrate that the presence of humic acid in sulphurous acid influ­ences both ionisation constants of sulphurous acid.

1 52

The first and second ionisation constants of sulphur­ous acid are decreased by 8% and 1 3% in presence of 1 00 ppm of humic acid in solution, respectively.

Ionisation constants of sulphurous acid at different temperatures i n absence of humic ac id are given in Table 2 . The first and second ionisation constants of

sulphurous acid at 25°C are reported6 to be 1 .47 x 1 0-2 and 2.88 x 1 0.7 respectively. These values differ only by 4.05% and 5 .73% respecti vely from literature

value. At 25°C both ionisation constants of the acid in

presence of humic acid are 1 .20 x 1 002 and 2 .5 1 x I 0-7, respectively which differ by 1 0 and 1 3% from that of pure sulphurous ac id. Both the values of ionisation constants in absence and presence of humic acid at 50, 70, and 90°C differ by 1 9, 1 7, 1 9, 1 3 and 20% re­spectively. Therefore, on an average i t is approxi­mated that at al l temperatures the first and second ionisation constants of acid in absence and presence of humic acid are affected by 1 9% and 1 6%, respec­tively. Ionisation constants of sulphurous acid at dif­ferent temperatures ei ther in pure form or in presence of humic acid solution employed for Arrhenius plot in both cases gave positive slopes. Activation energy of first and second ionisation of the acid in presence and in absence of humic acid were calculated by the slopes equal to -£.12.303 R form the Arrhenius plots shown in Figs 4 & 5. The energy of activation of first and second ionisation of sulphurous acid in absence

of humic acid were 3 . 5 1 and 2 .01 kcal mole- I , respec­tively while i n presence of 250 ppm of humic acid

Prasad & Singh: lnlluence of humic acid on ionisation constants of sulphurous acid A rticles

- 1 .7

- 1 . 9

£ e -2.1

OJ) o

-2.3

-2.5 2.75

• Pure H2S03 @ 1-12SD, + H u llIic :lcid (250 ppm)

2.92 3.90 3.36 3 .53

Fig. 4-Arrhenius plot for first ionisation of sulphurous said.

these values increased to 3 .66 and 2.28 kcal mole- I , respectively .

In the present case the A rrhenius plots demonstrate a posit ive s lope. However, the precipi tation of CaSO] in water and in sugar solution in the absence of humic acid7 i s governed by a mechanism which is different from that i n the presence of humic acid . The slope of the A rrhenius plot was found negative i ndicating that precipi tation reaction is control led by an act ivation process. The nature of posit ive slopes of Arrhenius plots (Figs 4 & 5) indicate the diffusion control led reaction in the precipitation reaction which is consis­tent with low value of activation energy of ionisation of sulphurous acid. Thus, i n general the presence of

-6.2 • Pure R2S03 ® H2SO, +Rumic :lcid (250 ppm)

-6.4

-£ o -6.6

on .£

-6.6

-7.0 2.75 2.92 3.90 3.36 3.53

Fig. 5-Arrhenius plot for second ion isation of sulphurous acid.

humic acid seems to decrease the rate of precipi tation reaction.

References

Prasad M. Chandra R & Nigam G D. 1111 Sligar J, 9 1 ( 1 989)

49.

2 Honig P, Prillciples ()l Sligar Techllology (Elsevier, London), 1 992.

3 Fleming M , Parker K N & Wil l iams J C, 1111 Soc Sligar Calle

Tech ( 1 968) 1 780.

4 Prasad M & Dubey Y, 46th Proc COIIV Sligar Tech Assoc

( india) CI ( 1 982) I .

5 Singh K, lilt Sugar J. 1 02 (2000) 355.

6 Atkins P W, Physical Chemistry, 6th ed. (Oxford Univ. Press, Ox ford), 1 998.

7 Prasad M & Chandra R, lilt Sligar J, 87 ( 1 985) 3 1 .

1 53