Prec. Indian Acad. Sci. (Chem. Sol.), Vol. 91, Number 3, June 1982, pp. 279-287. (~) Printed in India. Periodic precipitation of cobalt(II) oxinate in agar gel : factors influencing the flocculation N KANNIAtt, S AMBROSE, F D ONANAM and P RAMASAMY* A C College of Technology, Perarignar Anna University of Technology, Madras 600 025, India MS received 2 February 1982 ; revised 18 May 1982 Abstract. The influence of concentration, ageing and pH of the gel medium on the periodic precipitation of cobalt oxinate in agar gel is reported. The results are explained on the basis of Shinohar~t's revised flocculation theory. The flocoulation value (F) increases with increase in the gel concentration whereas it decreases with increase in ageing and pH of tke gel. The raise in temperature of the gel increases the solubility of the sparingly soluble substance and hence the F value. The effect of additives on the periodic precipitation of oobalt(II) oxinate is reported. Keywords. Periodic precipitation ; cobalt oxinato ; floceulation value ; gel oonc,en- tration ; ageing. 1. Introduction Many compounds that form insoluble precipitates in a counter diffusion system exhibit the Liesegang (1896) phenomenon in which a series of concentric rings are produced rather than a continuous precipitate. Few quantitative data are available despite over 800 publications on this subject (Stern 1967). The formation of Liesegang rings has been influenced by various factors lille concentration of the reactants, concentration, ageing, temperature and pit of the gel medium and the amount of additives present. We have recently published the experimental conditions for obtaining the Liesegang rings of cobalt(II) oxinate in agar and the influence of the concen- tration of the reactants on the periodic precipitation (Kanniah et al 1981). In this paper the influence of the concentration, ageing, pH and temperature of the gel and the effect of additives on the formation of Liesegang rings of cobalt oxinate are discussed in detail. The periodic precipitation of cobalt oxinate has been explained on the basis of the revised coagulation theory of Shinohara (1970). As the outer electrolyt~-- * To whom correspondence should be ~le. 279 280 ]~ l(.~iah et al cobalt nitrate--diffuses into the ~:I impregnated with oxine, cobalt oxivate is formed as a sol at the contact plane of the two reactants. This bouv.dary l~.owl,. as sol-front advances spreading the sol region. As more and more outer electro- lyte diffuses, the ionic concentration reaches a characteristic valtm F which triggers th: flocculation of the sol. F is expressed by the equation F = Cao E1 G (kl)'] (z) where Ca0 is the concentration of cobalt nitrate 1 1 G(R) ffi ~)z,, / cxp.(-~/ )d~/, (2) @ kz ffi kip and p --- x,+__~t, (3) x, p is l~aown as the spacing CoefQcient. x,+z and x~ are the positions of the (n + 1)th and nth rings from the g~l boundary, k is called the front constant which is estimated using the Adair's equation. The cov.ecntration of the super- saturated solution of the prodaot (Cm) formed just before the formation of sol is given by ,., exp ( - k~t2) (4) The flocculation value F is calculated as Fffi + r. (5) 3.1. Effect of gel concentration 2.1~ grams of analytical grade oxide w9 dissolved in mil,.imum amount of 2N acetic acid. This solution was mixed with hot agar agar solction and the final volume wss made up to 300 mL The pig of the solution was adjusted to A.25. Thus 1"0~ agar agar solution impregnated with 0.05 mole/lit oxine was prepared at 4.25 pig. 50 ml of this solution was poured into a coming tube of 20 ram diatmter and allowed to set. After 3 hr 10 ml of 1"031 mole]l cobalt -nitrate was take-n over the gel. To study the t,ff~t of ~1 concentration of the periodic precipitation of cobalt oxinate, the I~1 covcvntration was varied from 0-4~ to 2.0~. In all the experiments the concentration of the inner electrolyte and that of the outer eltctrolyte was kept as 0.05 and 1-031 mole/l respectively. 3.2. E:O'ectof agat~ of the gel 1~ agar agar gel impregaateA with 0"05 mole/l oxitm was prepared as before. 1-031 mole/l cobalt nitrate was taken over the set gel at different time. The ageing of the ~1 was varied from 2 to 27 hr. Periodic precipitation of cobalt (It) oxinate 281 Figure 1. The Liesegang rings of oobalt oxinate in agar gel in presence of additives potassium sodium tartarate, potassium thiocyanate and potassium chloride. Periodic precipitation of cobalt(II) oxinate 283 3.3. Effect of temperature The periodic precipitation of cobalt oxinate was normally carried out at room temperature (30 ~ C). The intluence of the temperature was studied, by keeping the gel at different temperatures. The glass tubes containing the hotgel solution with oxine were immersed in a thermostat kept at the required tempemtu/e. The temperature of gel was varied from 30 to 42 ~ C. In all these experiments the concentrations of the gel, oxine and cobalt nitrate was 1~o, 0'05 mole/l and 1"031 mole/l respectively. 3.4. The effect of pH of the gel medium Ordne was dissolved in minimum amount of 2N acetic acid and the hot agar agar solution was mixed. The pH of this solution was adjusted with aqueous ammonia to 4.25. Similarly the pH of the gel solution with oxine was varied from 3.7 to 4" 7. Oxine is precipitated, when the pH is above 4.7. When the pig was below 3' 7 gel set was not observed. The pH of the solution was measured at 60 ~ C. The gel solutions were allowed to set at room temperature (30~ and cobalt nitrate solution was taken over tt,e set gel after 3 hr. The concentrations of gel, oxine and cobalt nitrate were the same as in the previo~ experiment. 3.5. Influence of additives To study the effect of additives on the periodic precipitation of cobalt o~inate, suitable additives were taken along gith oxir.e in tb.e gel medium. The amount of additive was varied from 0.001 to 0.026 mole/l. When the gel containing oxine and adclitive was set, cobalt p.itrat~ was taken as the outer electrolyte. The concentrations of the gel, inner electrolyte and outer electrolyte were kept as 1,0~., 0.05 and 1"031 mole/l respectively. Potassium sodium tartarate, potassium chloride and potassium thioeyanate are the additives taken along with oxir.e in the gel. In all the experiments, sharp brown coloured disc-like precipitate rings demar- cated by clear void spaces were obtained within a week (tigurr 1). The interspac. ing between su:cessive rings increases with the number of ring (n) from the gel boundary. At the lower rings, small crystals of cobalt oxinate were observed. The distance n~.~asur,~ments were made with cathetometer. The IBM 1130 com- puter was used for the calculation of flocculation values using Shinohara's coagulation theory. 4. Results and discussion 4.1. Effect of concentration of the gel medium Matalon and Paekter (1955) have established that the gel has a great influence oa the periodic precipitation of insoluble salts and that the gel interacts apprec- ciably with the precipitated substance. They have modified Waguer's relation (1950) and derived the following equation: B p - 1 = RC~ + C1---~ (6) 284 lq Kanniah et at where Co = concentration of the gel~ R = a constant and it is a measure of the interaction between the gel and the substance precipitated. B -- a constant which is a n~.asure of supersaturation of sparingly soluble substance; m = a constant with integral value ; and Clo -- outer electrolyte concentration. From the above equation it i~ very clear that the spacing coefficient (p) will increase with increase in the gel concentration. As the concentration of gel increases the interaction of the gel with the precipitated substance increases. I-Ence the sol of the sparingly soluble substance is well protected. The stability of th~ sol increases. Therefore the amount of the outer electrolyte required to floccu[at: the sol will be more. T/m~ the floccttlation value must increase with increase in the gel concentration (figure 2) leading to an increase in the spacing coefficient (p). 4,2. Effect of ageing of the gel D~ingageing of the gel the micellae and the intervening capillary spaces become coarser. This will decrease the solubility of cobalt oxinate leading to rapid precipitation. Hence the floeculation value (figure 3) decreases with iv.crease in the ageing of the geL 4.3. Effect of temperature The increase in temperature of gel leads to a progressive diminution of the total volume of the re;celiac (Clayton 1932). Hence the pore size increases. This in turn l,.~ads to an increase in the diffusion coefficient. Moreover the solubility of 7.75 w t..~_ O 7.5o 0 U. 7.25 -- I i I i 0.5 1.0 1.5 2.0 Cg (%) Figure 2. Effect of gel r on the flocculation value (F). C~0 = 1.031 mole]l ; C~o •ffi 0.05 mole/l. Periodic precipitation of cobalt (H) oxitlate 7.3 I+_ -5 E ~M o "7-2 5 15 25 Ageing (h) Figure 3. Effect of ageing on the floeuulation value (F). CIo ~- 1.031 mole/l ; C~o = 0"05 mole/l. C o = 1%. the sparingly soluble product increases with increase in temperature.