Kinetics of Hypochlorite Oxidation of Polyvinyl Alcohol

Indian Journal of Textile Research Vol. 3, March 1977. pp. 20-23

Kinetics and Mechanism of Hypochlorite Oxidation - of Polyvinyl Alcohol

G. C. AMIN & S. D. WADEKAR Parekh Brother's Science College, Kapadwanj 387 620

&

H. U. Mehta

Ahmedabad Textile Industry's Research Association, Ahmedabad 380015 Received 4 October 1976 ; accepted 4 February 1977

The kinetics of oxidation of polyvinyl alcohol, FH-17, with sodium hypochlorite has been studied. The effects of ~ buffer concentration, pH, initial oxidant concentration and temperature of reaction on the rate of reaction have been

investigated. The experimental data indicate that the reaction is first order with respect to hypochlorite concentration.

Additional data on change in the rate of reaction with pH lend support to the results of earlier studies of this type on carbohydrates and confirm the participation of hydroxyl group in the reaction. A mechanism based on the observed data has been proposed explaining the rate retarding effect of increasing alkalinity or acidity of the medium.

P OLYVINYL ALCOHOL contains three types of polyvinyl alcohol was taken in a weighing bottle functional sites for reaction: (i) hydroxyl group, and kept in a vacuum desiccator over calcium chloride (ii) methylene group, and (iii) -CH group. The and phosphorous pentoxide. It was weighed to polymer molecule has C-C chain skeleton, which constant weight. The drying took 72-108 hr.

can also form the site of depolymerization Preparation af 10% solution of polyvinyl alcohol-

r"1 An accurately weighed sample (10.52 g) of c polyvinyl

HO.HsC- I CHs-CH- I -CHs-CHs.OH. alcohol was taken in a 100 ml volumetric flask. ) I I Distilled water (60-70 ml) was added and the flask

l OH J was kept in boiling water-bath till the sample

n dissolved. Finally, the contents were diluted to Sodium hypochlorite is a non-specific oxidant. It 100 ml after cooling to room temperature. In all attacks the hydroxyl functions and ether bonds and cxperiments, 10% solution of polyvinyl alcohol (20 g) cart also cleave C-C and C-H bonds. Its reaction was used.

depends markedly on the pH of the reaction Kinetic runs-Kinetic runs were carried out in

medium. a standard joint 250 ml conical flask coated with a

Hypochlorite oxidation of poly alcohols like thick black paint outside and placed in a thermostati- cellulose, starch and certain gums has been studied cally controlled water-bath maintained at the desired extensivelyl-6. However, the oxidation of polyvinyl temperature. Polyvinyl alcohol solution (10%, 20 g) alcohol has not been investigated in detail, though and 30 ml of 0.375 M buffer solution were shaken some studies have been reported6-8. In the present occasionally and allowed to equilibrate at the desired

study, hy.pochl.orite oxidation. of polyviny~ alcohol temp~rature.. Then t~e requ~red amount of hyp~- '1 has been InvestIgated systematIcally. The Influence chlorIte solution was dIluted wIth water to make It

of variables like period and temperature of reaction, 100 mI. Hypochlorite solution and water were pH of the reaction medium, oxidant concentration, equilibrated at the desired temperature before mixing.

etc. on the rate of reaction has been studied. At intervals, aliquots (1.0 ml) were withdrawn quickly

Materials and Methods and diluted to about 10 ml with distilled water. The

Polyvinyl alcohol-Alcotex-17 F-H, a high mole- unreacted sodium hypochlorite was determined after cu1ar weight completely hydrolysed grade poly- adding acetic acid (50 %. 1 ml) and potassium iodide vinyl alcohol, was used. Its analysis showed 5.24% (20%, 2 ml) and titrating against sodium thiosulphate :moisture and 0.54% acetyl content. (0.005 N). The sodium thiosulphate solution was

Sodium hypochlorite solution-Calichlor, a ready- prepared daily from the stock solution (1.0N). The made hypochlorite solution (Chemical Division, diluted solution was standardized using potassium Ahmedabad Manufacturing and Calico Co. Ltd, dichromate.

Ahmedabad) was used. It contained 39 g per litre of

available chlorine. Buffers-The buffer systems used were the same Results and Discussion E.fJect of buffer concentration on the rate of -'' as those employed by Patel et al. "1°. reaction-The effect of buffer concentration on the

Determination ~f moisture content in polyvinyl rate of oxidation was studied at pH 10 and 37°C.

alcohol-An accurately weighed sample (, 1 g) of The first order rate constants derived graphically for

20

/

AMIN et al. :KINETICS & MECHANISM OF HYPOCHLORITE OXIDATION OF VOLYVINYL ALCOHOL

180

pH10 Temp:37°c

o 0006~

I I

100 200 300

Initial Chlorine (m-moll?sl/PVA unit Fig. 2 - Plot of rate constant versus initial oxidant concen-

tration (pH, 10; temp., 37°C)

oxidant concentration, The same family of curves are obtained at 47°C using the same initial oxidant concentrations. At high temperature, the consump- tion of available chlorine per PVA unit at all initial oxidant concentrations is faster than that obtained at 37°C.

Effect of initial oxidant concentration on the rate of reaction at pH to and

37°C-The first order plots of In

CoIC

against t, where Co is the initial oxidant concentration and C, the oxidant cODcentrationafter time tare linearll (Fig. I). The first order rate constants were determined from the slopes of the lines. Plots of the rate constants against the oxidant concentrations at 37° are shown in Fig. 2; At 31 m moles/PV A unit, the rate is low. At oxidant con.

centration 62 m moles and above, the rate does not change appreciably with increase in the initial oxidant concentration. At 47°C, the overall values of rate

'040III E

::J

~ 30o U

Go20

o~ 1:/0

U

C

::J70

<t :>

-%.60 III

Go

'0 E50 E

80

0-- I --0

I ----0

I ---

C0 004 / - - - - ~

o

I --

iii I

§

^{0003 :}

u ,

a. I

_ 0

00.002 I

a::: II

I 0.001 ~ /I

/ /

20 40 60 80 100 .20 140 I~

TIME ',"minI

Fig. 3 - Plot of oxidant consumed versus period of reaction at different pH levels (oxidant cone., 62 m moles/PYA unit; temp.,

37°C) 310 m·mOlK.

0.0063 0.0058 0.0080

Polyvinyl alcohol precipitates . Rate contant

min-1

o

Buffer cone.

m moles/litre 0.114 0.227 0.340 0.454

10

-

~ 160

~

~140

Ul Go

'0E 120 E '0 100

Go

E

a

80

s

U

~ 60

;::

o

:c 40

u

TABLe 1 - EFFECT OF BUFFER CoNCENTRATION ON THIl RATS OF REACTION

[pH 10 ; oxidant cone., 124 m moles CI./PV A unit and 0.455 mole PVA/litre]

the consumption of hypochlorite by polyvinyl alcohol are given in Table 1. In unbuffered system, as tbe reaction progressed, the

pH

drifted gradualIy towards the acidic side. Therefore, the optimum concentra- tion of the buffer to maintain the

pH

at

^to

was determined. At as low as 0,12

M

buffer concentra- tion, the rate remains almost constant;

with increase

in buffer concentration up to 0.34 M, it shows slight increase, Above this buffer concentration, polyvinyl alcohol gets precipitated from the solution, In all subsequent experiments, 0.12

M

buffer concentration was employed throughout this study.

Effect of initial oxidant concentration on the rate of consumption of available chlorine at pH to and

37°

and

47°C-At

pH

10, the initial available chlorine con- centration was varied from 31 m moles to 310m moles, maintaining the polyvinyl alcohol concentra- tion 0.455 unit mole/litre. Plots of chlorine consum- ed in m moles per PVA unit against

t

at 3JDCare shown in Fig. I. The plots show that the consump- tion of available chlorine increases with time at any oxidant concentration. The consumption is faster in the initial stage and slows down with increase in the reaction period. The consumption of available chlorine also increases with increase in the initial

~~

~.

0-0 31

^m-molH

.~_ ,_ a

. IEJJ 't' I I I I I

60 80 /00 /20 140 '60

TIME' 't',mil))

Fig. 1 - Plot of oxidant consumed versus period of reaction at different initial oxidant concentrations (pH, 10; temp., 37°C)

21

INDIAN J. TEXT. RES., VOL. 2, MARCH 1977

0.03 a.ctivation energy is very low. The changes in activa-

'/ x tlon energy suggest that the rate of the hypochlorite

i~ 0.025 oxi~ati.on reaction does !lot depend solely .on ~he

E -~62 m.mOle~ activation energy; otherwIse at the lowest actIvatIon

~ PVA unit energy, the rates of reaction would have been the

-0.0 highest. However, this is not the case at pH 5.5. There-

5 x f?re, apart from. the activation energy for the forma-

iii 0 015 tI<>:n of. the actIvated complex, other factors like

g OrIentatIon of the molecule or symmetry of formation

I-' of the activated complex also affect the rate of oxidation.

-G: 0.01 These factors are governed by the entropy of activa-

& tion. If the entropy of activation is more negative,

0005 it would require greater orientation and order before

: the reaction takes plac~. The compensation effect

: and entropy consideratipns are discussed else-

J 6 7 where"lo. .In fact, the ~echanism of oxidation postu-

9 ') 10 lated envIsages repulsIon between two oppositely

pH charged ions, requiring higher order in the activated }..

Fig. 4 -Plot m moles/PVA unit; of rate constant versus pH (oxidant temp., 37°C) conc., 62 complex. The activatIon energy at pH 7.0 IS as hIgh as that obtained in similar oxidation reactions in the case .of

starch and cellulose9'lo.

TABLB 2 -ACTIVATION ENBRGY AT DIFFERENT pH LEVE~ Mechanism of oxidation O

if Pol y vin y l alcohol ith

AND OXIDANT CONCENTRATIONS w

sodIum hypochlorzte -The above kInetic data show

~E, kcal that the specific rate constants are independent

-of the initial oxidant concentration, and the

pH 31 m moles 62 m moles oxidation is a first order reaction. The plot of

100 2784 -rate constant (K) versus pH shows that the rate of

8:5 19:85 19.80 reaction is highest at pH 8.5. In alkaline medium (pH

7.0 21.56 21.58 10) and in acidic medium (pH 5.5 and 4.0), the rates

5.5 19.78 18.93 are very low. Therefore, there appears a distinct rate

4.0 27.25 22.22 re ar a Ion rn t d t

..

th~ presence 0 aCI s an f .d d a ales. lk 1

.

A l , similar trend was observed in the hypochlorite oxida-

constants are higher than those at 37°C ; however, tion <>:f starches3"'lo, simple carbohydrates, polysa- the rate does not change appreciably with initial cchandes and-gums.

oxidant concentration up to 310 m moles oxidant In the cas.e of st.arch, ~atel et .a1.3~"lO adv~nced

concentration. the hypothesIs that rn alkalIne medIum, OCl- IS the

Effect of change in hydrogen ion concentration- oxidant and it reacts with the OH of starch forming

The consumption of hypochlorite in the presence of i 1-

polyvinyl alcohol was determined at pH levels 10, 8.5, H -~ -OH + NaOH -+H -~ -DNa + HaO 7.0, 5.5 and 4.0 in buffered condition. The con-

s~mption. of available chlorine per PV A unit a~ainst H -C -DNa ~ H -C -<> + Na+

time at dIfferent pH levels and the same oxIdant I I

concentration (62 m moles per PV A unit) are shown --I -

in Fig. 3. The plots show that the consumption of 2H -C -0 + 0 CI---+C = 0 + HaO +CI

the oxidant. is maximum at pH 8.5 ; it is lo.wer at 1 I '-!

pH 7 and still lower at pH 10. The consumptIon at Scheme 1 (Alkaline medium)

pH 5.5 and 4.0 is the lowest and is almost equal at I

these two levels. H -C -OH -HOCI -+H -C -OCI +

The plot of rate constant against pH at 37°C and I I

62 m molesjPV A unit concentration level is shown I

in Fig. 4. It is seen that in a highly alkaline medium, HaO -~ -0 + HCI

the rate of reaction is very slow. At pH 8.5, the rate

is maximum acidic range. and it again In acidic medium, falls with the rate is extremely pH in the more H -I C -OH + OCI--+, -C = 0 + HIO + CI

low. The same reaction was studied at several Sch 2(N t I ed

.

.

d

.

^{3 d o d h eme} eu ra m Ium)

OXI ant concentratIons at 70 an 47 C an t e same

fami!y of curves with maxima at pH 8.5 were H -C -OH + CI- CI-~~H C -0 -CI + HCJ

obtaIned. I I

Frqffi the rates of reaction obtained at two Slow ~

different temperatures, the activation energy (6 E) H -C -OCI .C = 0 + Ha

was calculated. The results obtained at different pH I I

levels are given in Table 2. The activation energy Scheme 3 (Acidic medium)

varie~ with the .initial oxidant concentration and in Fig. S -Schemes showing mechanism of reaction under certaIn cases wIth pH also. At pH 8.5 and 5.5, the alkaline, neutral and acidic conditions

22

AMIN et al. : KINETICS & MECHANISM OF HYPOCHLORITE OXIDAnON OF POLYVINYL ALCOHOL soda starch, which further ionizes into sodium cation buffer concentrations, initial oxidant concentrations, and starch anion. Polyvinyl alcohol can also form pH levels of the reaction medium and temperatures.

sodium salt with hydroxyl groups. As observed in the The rate, first order with regard to hypochlorite con- case of starch, the repulsion between the similarly centration, is found to vary with pH of the solution.

charged ions (OCI- and negatively charged poly- It is highest at pH 8.5 and decreases with increasing vinyl alcohol) determines the retarding effect on acidity or alkalinity of the medium. A mechanism the rate of oxidation under alkaline conditions. In based of hydroxyl group ionization is advanced to acidic medium, chlorine molecule attacks the hydroxyl explain the observed effects of pH on the rate of groups of polyvinyl alcohol to form hypochlorite oxidation.

ester and molecule of hydrogen chloride. The ester

further decomposes to give hydrochloric acid and Acknowledgement

ketone. In the second step, hydrogen is removed as The authors thankVr P. C. Mehta Director Shri proton in the acidic medium. Such a phenomenon S. S. Trivedi, Head, Chemical Techn~logy Di~ision will hinder the rate of the forward reaction by law of and Dr H. C. Srivastava, Deputy Director and Head, mass action, because the liberation of proton in an Chemistry Divison of A TIRA, fQr affording facilities acidic medium will be difficult. In a neutral medium, to carry out part of the wo,k at A TIRA laborato- hypochlorite is partly dissociated. The undissociated Ties.

oxidant WQu.ld lead to the formation of water and

hypochlorite ester. Ester would give the oxidised prQ- References

duct and a molecule of hydrochloric acid. The action 1. RADLEY, J. A., Starch and its deriv4tives (Chapman & Hall of hypochlorite anion on undissociated polyvinyl Lt9, Londo?), 1968. .

alcohol would not be ver y different from that of the 2. Starc~, chemIstry and technology, Vol. II, e~lted by R. L.

Whistler and E. F. Paschall (Academic Press, Inc., undlssoclated oxIdant and, therefore, as m poly New York), 1967.

alcohols like starch and cellulose, the rate of Qxida- 3. PATEn, K. F., Kinetics and mechanism of oxidation of starch tion is high near neutrality. The mechanism schemes with hypochlorite, Ph.D. thesis, Gujarat University, 1970.

in Fig. 5 illustrate this. 4. BIRTWELL, C., CLIBBERNS, D. A. & RIDGE, B. P., J. Text.

I ...Inst., 16 (1925), T 13.

t IS noteworthy that wIth polyvinyl alcohol, the 5. SCHMORAK, J. & LEWIN, M., J. Polym. Sci., A-I (1963), highest rate is obtained in the slightly basic medium 2601.

at pH 8.5 and not at pH 7.0. It appears that struc- 6. MAKOTO SHIRAISHI & ¥,,:SAKAZU MATSUMOTO, US Pat.

tura! components adjacent to hydroxyl functions 3,052., 662 (to Kurashlkl Rayon Co. Ltd, Japan), Sept.

d

.

^{h t". .1962,} Chem. Abstr., 58 (1964), 594.

etermme t e pH Jor the hIghest rate of reactIon. 7. MAKOTO SHIRAISHI & MASAKAZU MATSUMOTO, Kogyo The electron donating effect of -CHI in polyvinyl Kagaku Zasshi, 6S (1962), 1430-3; Chem. Abstr., 58 (1964),

alcohol thus shifts the pH maximum for 7.0 observed 5496. .

for carbohydrates and polysaccharides to 8.5 for 8. OGIWARA, V. & UCHIYAMA, M., J. Polym. SCI. A-I (7)

I .(1969), 147.

po yvmyl alcohol. 9. PATEL, K. F., MEHTA, H. U. & SRIVASTAVA, H. C., J. appl.

Polym. Sci.. 18 (1974), 389.

Conclusion 10. PATEL, K. F., MEHTA, H. U. & SRIVASTAVA, H. C., Die

Th k . t. f .d ' f f II h d I d Starks. 25 (1973), 266.

.e me ICS 0 OXI atl.on o. u y y ~o yse 11. FROST, A. A. & PEARSON, R. G., Kinetics & mechanism polyvInyl alcohol has been Investigated at dIfferent (John Wiley & Sons, Inc., New York), 1961.

..

,.,

: ..

23