Effects of ageing on the polarograms and the ultraviolet absorption spectra of aqueous solutions of tannins-香川大学学術情報リポジトリ

Vol.. 18, No.. 1 (1966) 27

EFFECTS OF AGEING ON THE POLAROGRAMS AND THE ULTRAVIOLET

ABSORPTION SPECTRA OF AQUEOUS SOLUTIONS OF TANNINS

Yoshiaki FUKUI

(Laborattory of Forest Products Chemistry)

Introduction

Polarography of an aqueous solution of tannin was first studied by VLCEK et al.(')

,

who observed the polarographic wave of a solution of quebracho and suggested that the waves are due to oxidation of quebracho to fisetin. BROWNING et al.(') also investigated the polarogra-

phic behavior of various tannins and showed that t h e reduction wave a t -1 .Ovolt (us.S.C.E.) was due to the oxidized structures. Recently, VAGO et a l . o reported the polarographic

behavior of tannin as a surface-active agent. However, no experiments has yet been made to clarify the relationship between the wave height and the ageing time of tannin solutions.

On t h e other hand, the spectrophotometric investigations of tannin solutions have been made by many workers and i t is well known that the ultraviolet absorption spectra of the condensed- type tannins have maxima around

280

mP and minima around 260mC1, while those of the hydrolyzable-type ones have neither maximum nor minimum. It was also reported that the intensity of absorption of tannin solutions was risen and no lateral shift of the peak was caused by the oxidation of tanninc4'. However, effects of ageing on the absorption spectra of tannin solutions are not completely clarified.

The present investigation was undertaken to clarify the effects ef ageing on the tannin solutions. The direct curr ent (D .C .) polarogr ams, the ultraviolet (U .V .) absorption spectra, and the preliminar y square- wave (S .W .) polarograms of aqueous solutions of tannins were measured during ageing and the obtained results are disccussed in this report.

Experimental 1. Materials

Five tannin extracts used in this investigation were two hydrolyzable-type tannins, chestnut and myr obalane, and three condensed

-

type ones, quebracho, gambier

,

and wattle. These tannins were obtained through the courtesy of MI. Hattori, Director of the Hyogo-ken Leather Research Institute, and Mr. Kambara, The Kawamata Co. These tannin extracts were actually the same as industrial tanning materials.

2. Polarography of Aqueous Solutions of Tannins

Aqueous solutions of tannins (2.5mg/ml) buffered at pHs 4.7, 7.2, and

9.8

with N/20

Luss

buffer were aged i n an air-bath at 2 5 t 0 . I 0 C . A little amount of the ageing solution was taken a t intervals of one day. The dissolved oxygen i n the solution was removed by passing hydrogen gas for 15 min, and then D.C. and S.W. polarograms were measured

28 Tech Bull Fac Agr

.

Kagawa Univ

by a Yanagimoto Shoji polarograph Model PA-202. All measurements we:e made in a water thermostate a t 25+0.I0C.

3. Ultraviolet Absorption Spectra of Aqueous Solutions of Tannins

Aqueous solutions of tannins (5mg/ml) were aged in an air-bath at 25-t0.l0C. A little amount of the ageing solution was taken at intervals of one day and the solution was dilu- ted so as to make a solution containing 0.02 or 0.05mg of tannin per 1 ml with deionized water

.

The U .V

.

absorption spectra of the diluted solutions were measured immediately after the dilution by a Hitachi Seisakusho photoelectric spectrophotometer Model--139.

Results and Discussion

When the aqueous solutions of tannins were kept at. pHs 4 . 7 and 7.2, all the solutions tested gave no polarographic wave both

I with and without ageing after the pre-

paration of the solutions. At pH 9.8 the solutions of chestnut and myrobalane showed no wave immediately after the preparation, while the solutions of que- br acho, gambier

,

and wattle gave weak waves at -0.95-1 .0 511 ( u s . Hg pool anode) and the solution of gambier gave the second wave at -1. 77 v . Since it took about 30 min before measurement of po- larograms, the obtained weak first waves seemed to be due not to the real fresh tan- nin solutions but caused by the effects of short ageing.

VLCEK

et a l . reported that the solution of quebracho showed two cathodic waves a t -1.07 and -1.57 v at pH 9.6, and BROWNING et al. repo- rted that all the freshly prepared solu-

I I I I

o 5 1 0 15 2 0 tions of quebracho, wattle, mangrove,

1:d I1 1 -!"It I \ 1 1 ~ ~ 0 0 1 _{chestnut, and myrobalane showed the}

reduction waves at -1.0 v a t pH 8-9.

Fig 1 Changes in the D C. polarograms of

chestnut solution during ageing These observations seemed not to be the The solution contains 2 5 mg of chestnut fact f ~ o m the point of view mentioned extract in 1 ml and is buffered with LUBS above. The height of waves at -0.95

buffer at PH 9 . 8 . -1 .(I5 v of each tannin solutions markedly

Sensitivity : 0 04 mA/mm increased in 1-5 days of ageing. As an

Ageing time : a, no ageing; b, I day; _{example, the polarograms of chestnut} c , 2days; d , 3days; e , 5 d a y s . _{solution are shown in Fig. 1. The wave}

heights of each tannin solutions were determined by an usual manner and

Vol, 18, No. 7 (1966)

in 1 ml and are buffered with LUBS buffer a t Ageing time (day)

pH 9,8 Fig 3. Changes in t h e wave heights of tannin

Measurement : gate 3 7 ; time const

,

5 5; solutions a t pH 9 8

S W.volt, 20 mv; a m p sens

, 1/20;

_{Sensitivity: 0 04mA/mm}

record sens ,O 08 _{a , myrobalane; b, chestnut; c , quehracho;} a , wattle aged for 1 day; b, quebracho aged _{d , gambier ; e , wattle}

for 1 day; c , myrobalane aged for 2 days; d , gambier aged for 1 day; e , gambier aged for 5 days

plotted against t h e ageing time as shown in Fig.3. Further, some examples of S.W. polarograms are shown in Fig. 2 , although they have not so well defined. Analysis of the waves of each tannin solutions was made according to the theoryc6) for a single ion. Fig.4 shows the results obtained for quebracho as a typical example. From the slope of the plot of

log [ z / (id-z)) us. Ea , p . (Fig.4) the average number of electrons involved in the reduction

of tannin, n , could bc estimated and the half -wave potentials, E1/2, were determined from the potentials corresponding to the value of zero on the log term. The values of n and E1/2

Tech.. Bull. Fac. Agr. Kagawa Univ.

Ea p (-volt us Hg pool)

Fig 4 . The plot of log ( i / ( i d - i ) vs Ea p for the quebracho waves

The queb~acho solutions (pH 9.8) are aged for : a , 1 day; b, 2 days; c, 3 days; d , 5 days.

Ageing time (day)

Fig. 5. Changes in the E1/2 and n of the Polarograms of tannin solutions during ageing

As will be seen from Fig. 3 , the wave heights of a l l the tannin solutions marked- ly increased during t h e ageing of 1

-

3

days, but when the ageing time increased to 3-5 days, the wave heights of some solutions I emained constant or decreased.

The results that t h e heights of the f i r s t waves increased by the ageing a r e essen- tially identical with those of BROWNING et , but they have observed no decrease in the wave height during the longel age- ing. Gener ally speaking, the increasing rates in the wave heights obtained for the condensed type tannins, gambier, que- bracho, and wattle, were higher than those of the hydrolyzable-type ones, chestnut and myrobalane. Although this difference would be partly due t o a difference in the tannin content, these results seemed

t o suggest that the condensed-type tannins are xnor e easily oxidized by atmospheric

Vol.. 18, No. 1 (1966)

oxygen than the hydrolyzable-type ones at pH 9.8.

Although the structures of oxidation A - A - A - - d A--A--A-

products of tannins cannot be presumed

the four tannin the absor

-

Fig.6 Changes in the absorbance a t t h e maximum

bances gradually increased with the i n - _{absorption of tannin solutions during ageing} from the polarograms, since the values ₀₅₅

of half-wave potential and peak potential

'3

for a l l the aged tannin solutions were

_.$

essentially similar one another, the oxida- 050 tion products seem to be analogous which-

ever tannin solutions are aged. However

,

_{0 4 5}

the oxidation products would not be iden- tical as stated below.

0 4 0

Fig.

6

shows the absorbances at the

- -

-

crease of the ageing time a s the Roux's(~) _{a , wattle (0.005%, a t 278mp) ; b, gambier} observation on a wattle solution. How- _{(0.005%, a t 278 mp); c, myrobalane (0.002}

-

*A*-

*#5'y

JyX-x

'--.,

-

-

/%

Oa-o--o----~-

-

! O t

.

t 1 I

ever, when the ageing time was too lon- %, at 274mp); d, quebracho (0 005%, a t

ger the absorbances tended to der crease 280mp).

in some tannin solutions seen from t h e

results given in Fig. 6. This results are similar to the finding reported by NAKABAYASHI(~).

He showed, when a D-catechin solution was titrated with permanganate, the absorbance of the solution increased in the beginning and then turned to decrease at a certain point of titration. Therefore, the changes in the absorbances during ageing also may be risen accor

-

ding to the oxidation of tannins by air -oxygen

.

However, the changes of the absorption intensities given in Fig. 6 were different. Therefore, it would not be considered that all the tannins formed a same oxidation product.

The changes in the U.V.absorption spectra during the ageing were reasonably related with those in t h e heights of polarographic waves of the corresponding tannin solutions. The solution of gambier containing much monomeric D-catechin indicated the highest and continuous increases in the wave hights of D.C.polarogram and t h e absorbance during ageing. Moreover, its n value changed from 2.2 at 2 days' to 1.

4

at 3 days' ageing. Further, as seen froin the S.W.polarograms, d and e in Fig. 2, the peak of t h e solution aged for a day appeared a t -1.4 v a s the other tannin solutions, while in the 5 daysf ageing this peak disappeared and two new peaks appeared a t more positive potentials. Therefore, the first oxidation products of gambier would change to t h e second products which were more reducible a t a dropping mercury electrode and had a n absorption in the U.V.region.

absorption maxima for each tannin solu- I 0 * 1 2 3 4 5

tions plotted against the ageing time. I n A e l n g tlme (day)

I n t h e case of wattle and myrobalane, the wave heights in the beginning of ageiog increased to maxima and then decreased sharply. This results a r e similar to the polarogra-

32 Tech.. Bull.. Fac. Ags. Kagawa Univ;

phic behavior of pyrogallol ('1. Therefore, t h e oxidation products may be similar to

pyrogallol.

The wave heights of the solutions of quebracho and chestnut reached to maxima a t 3

days and then remained almost unchanged. Then their oxidation products seemed to be rather relatively stable against air -oxygen. However, the oxidation product of quebracho might change particularly in the beginning of ageing because the values of n and E1/2 of the solution varied significantly with the ageing time.

Summary

1 . The changes in the heights cf D.C. polarograms and in the absorbances at the maximum

U .V .absorption of the five tannin solutions during ageing were determined.

2. At pHs 4.7 and 7.2, all the tannin solutions showed no polarographic wave even after the ageing of 5 days.

3. At pH 9.8, t h e f i r s t wave heights of all t h e tannin solutions markedly increased during the ageing of 1-3 days, but when the ageing time increased to 3-5 days, the wave heights of quebracho and chestnut remained constant and those of myrobalane and wattle decreased. 4. The increasing rates in the wave heights obtained for the condensed-type tannins, gambier, quebracho, and wattle were higher than those of the hydrolyzable-type ones, chestnut and myrobalane.

5. T h e values of n and E1/2 of the first waves were determined from the plots of log

[i/ (id-z)] us.Ea.p..

6 . The changes i n the U.V. absorption spectra during t h e ageing were resonably related with those in t h e heights of polarographic waves of the corresponding tannins.

7. The obtained preliminary S

.

W. polarograms were shown.

8.

T h e obtained results were discussed f ~ o m t h e view point of the oxidation of tannin.

References

(1) VLCEK, A K

,

MANSFELD, V

,

KRKOSKOVA,

D : Collegzum, 1943, 231, (CA, 40, 6279 (1946))

(2) BROWNING, B L

,

PIGMAN, W .W

,

LEAF, P L

,

STEINERT, G , WE'IHERN, J. : J Am

Leather Chemists' Assoc., 44, 30 (1949)

(31 VAGO, G

,

GEHER, J : Tenszde, 2, 142 (1965) (CA, 63, 3182(1965))

(41 Roux, D G : Wattle Tannzn and Mzrnosa Extract, edited by the Leat Ind Res Inst Grahamstown, South Africa, P 153 (1955)

(5) KOLTHOFF, I M

,

LINGANE, J T : Polaro- graphy, 1 P 194, New York, Intercience Pub (1952)

(6) NAKABAYASHI, T : Nzppon Shokuhin Kog yo

Gakkaishz, 9, 313 (1962)

(71 VLCEK, A K

,

SPALEK, E., KRATSKY, L., HAVLICEK, E : Chem Obzor, 25, 81 (1950),

Vol. 18, No. 1 (1966) 33

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