STUDIE$ ON THE KINETICS OF THE DECOMPOSITION OF MUREXIDE IN ACID SOLUTIONS BY N. A. RAMAIAH, S. L. GUITA AND VISHNU (Department of Phy~lcal Chemiatry, Indlan Instituto of Sugar Technolog.v,'Kanpur) R.r March 5, 1956 (Communicatcd by K. S. G. Doss, V.A.Sc.) INTRODUCTION IN re~r ycars, murexido of ammonium salt of purpuric acid (D is being •xtcnsivcly uscd as ah analytical rcagcnt for many mctaUic ions, cspccially / OH o\ /~~ c\ q ~~\ O-,C C--N"C C-,,,O O 0 I of Ca~. Bcck1 r murexidc for the dctcction of So, Zn, Th and other raro carth and raro mctals; Lconid Kul'bcrg 2 us•d the sarao as rcagcnt for hcavy mr Schwarzenbach and Gysling ~ dcsc¡ in dota¡ tho uso of muroxidc for quantitativc cstimation of Cae. Sincc thcn it proved to bo an cf¡ rcagent for dotcnnination of calcium-contcnt in pharmaccutical products, 4 milk ultra filtrat~,4 etc. s Estimation with caso of Ca~" in sugar- cano julcr is problcmatical in sugar tcchnological laboratorios. Du¡ a se¡ of cxperimcnts on this invcstigation employing murexid•, a f•w intcrost- ing rcsults on the dccomposition of murexidc in acid solutions wcro obtained. It is known7. 8 that murexidc in solutions of low pH undergocs dccomposition lcading chicfly to tho production of uramil (II) and aUoxan (III). No data HN CO }IN ,,CO OCI CH,NH,I OCI COI II IIl in the litcraturc on the kinctics of this rcaction. The prcscnt co~muni= cation rr for the first time the kinctics of the dccomposition of murexidr 286 The Kinetics of the Decomposition of Murexide in Acid Solutions 287 investigated by employing the characteristic absorption spectra of the sub- staner in the visible region. EXI'Eg/ME/ql'AL The eommercial (B.D.H.) sample of murexide was purified by the method described by Davidson. s One grato of the crudo substance was dissolved in 900 c.e. of pure distilled water. The filtrat• was salted out with 60 gro. of ammonium chloride. The preeipitated substance was washed with absolute methyl alcohol, to be free from chlorido and later d¡ at 120 ~ C., when fine reddish brown crystals possessing a green reflex were obtained. Absorption spectra of a solution of known concentration of murexide with an equal volume of a buffer solution os desired pH, were studied with Unicam S.P. 350 D.G. spectrophotometer; this last had on• cm. effective light path. '~. .oif /~ ~~ / N ~o,t /j~.~ N :o.,1 //" "~\ 400 440 480 520 560 600 640 680 ra/el WAVE LENGTH FIo. 1. Absorption Spr of murr in the visibl9region. (Curvr 1 refr to the concentrar/on of 0"125 and Curvc 2, to 0.0625 mM of Murexide; pH = 6.0.) RESULTS ~ DIS~~JSSION A typical series of results on the characteristic absorption spectra of murexido in the rango 400 to 700 m/~ aro 8ivon in Flg, I; in this, curro I refers to a eoncr of 0. 125 mM while eurve 2, to 0.0625 mM of mur- exidc at pH ----- 6-0. It was interesting to note that the absorbancy of mur- exidc increased progressively to a maximum followed by a decrease with increas• in the wave-length. Thus, e.g., it was 0.35, 1.13 and 0.2 at ;~ = 460, A3 288 N. A. ~H A~ 620mp respectively. Murexide has maximum absorption at a = 530m/~ in accordance with the data obtained by other workers? Further, this wave- length ~ was unaffected by a change in the concentration or/and the pH value of the solution. At a fLxed pH, incraase of the concentration of the murexide solution especiaUy in the rango 10-a to 10-e M enhanced the maximum absorbancy. Thus, e.g., the absorbancy at ;~ = 530 m/~ was 0.17, 0.65 and I'. 13 at 0.016, 0-063 and 0.125 mM of murr Further, ir was interesting to note that the absorbancy increased sensibly linearly with the concentration (see Fig. 2) in accordance with Beer's law; the corresponding constant hada value of 1-04 • I0r at 25 ~ C. (Tabla I). |.2 O 0.8. ‰ 0.4_ ,,~ O ! nm | nu in O 0.4 0.8 1.2 x IO- CONCENTRATION OF MURE.XIDE FIo. 2. Variation of absorbancy with the concentration of murexide. (x = 530 m~ ; pH •, 6.0) The Kinetics of the Decomposition of Murexide in Acid ~olutions 289 T~Im I Variation of maximum absorbancy at A = 530m/z with concentration of Murexide Concentration Optical Beer's Constant M x 10 3 density (10"~) 0.125 1.135 0.91 0.10 1.03 1.03 0.0833 0-87 1.04 0.0625 0.655 I. 05 0.0417 0-44 1.06 0.0313 0.342 1.09 0.0157 0-171 1.09 Mean Value == 1.04 The data #ven in Figs. 3 and 4 represent the va¡ of the absorbancy at ~----530 m~ of murexide solution with time, at pH----2-2 and 2.6 res- pectively. In these curvcs 1 and 2 refer respectively to the above data with initial concentrations of 0-125 and 0.0625 mM of murexide. These data were obtained as follows: A known volume of the murexide solution was mixed with equrd volume of a buffer and the absorbancy was recorded at different intervals of time at ~ = 530 m~. It was interesting to observe that the absorbancy at ), = 530 m/z characte¡ of the substance decreased pro- gressivdy wit.h time. Thus for an initial concentration of 0.125 mM and at pH = 2-2, the optical density was 0.72, 0.42 and 0.22 at 1, 5 and 9 ruin. after the solution was mixed with the buffer. With further increase in time, it decreased to a negligibly small value (see Fig. 3). Essentially similar data were recorded at oth•r pH values (Fig. 4). The decrease with time of the absorbancy was accompanied by the gradual disappearance of the characteristic pink-red colour of the mdrexide. Furthermore, the above change in the absorpUon or the colour, appeared permanent and irreversible in the sense that when the acid-reacted solution was made alkaline, the charaeteristic colour was not restored, and the absorption at ~ = 530 m~ was not exhi- bited. This is illustrated clearly by the data given in Figs. 5 and 6; these refer to the expe¡ in whicix during the progress of the reaction, the 290 N. A. ~ ~'D oT~ O.9_ O.B 0.7 >06 U I £ , U'J m <0.3. 02 O'lI 6 IO 20 30 TIME IN MINUTES Fxo. 3. Decomposition of murexide with timo at pH -- 2.2. (Curve 1 ,= 0"125 and Curve 2 m 0.0625 mM of murexide ; ~. - .~30 m~) The Kinetics of the Decomposition of Murexide in Acid Solutions 291 1.0- _ 0.9. 0.8. 0.7 0.t~ ZO.5 eCO. 4 0 11) m 0.3. 0.2 0.1 C 0 I0 20 30 40 TIME IN MINUTES F1o. 4. Dr of murexidr with time at pH ffi 2.6. (Curvr 1 = 0-125 and Corvo 2 = 0"0625 mM of Murexid ~ =530 m~) pH of the solution has been increascd to 8 by addition of 0-2 c.c. of 4 N soclium hydroxide solution, which caused negligible dilution of the murexide solution. It was interesting to note that as soon as the pH was raised, the reaction leading to the decrease in the absorbancy of murexide was quenched as cvidenced by the observation that the absorbancy of the solution recorded 292 N. A. ~ ~ orma~ just before addition of alkali remained later sensibly constant (see Figs. 5 and 6). These data not only demonstrate the irreversible nature of the decomposition, but also reveal the marked dependence of the reaction on the hydrogen-ion concentration of the solution. !.0. 0.~ 0.8 0.~ 0.~ A ~0.5 co O0.4. LO ti3 <0.3. B O.2. O.I. O. 0 2 4. 6 8 I0 12 14 16 TIME IN MINUTES ]:lO. 5. Effect of addition of alkali to the acid reacted murexide ~olution. [[nitial pH of the solution = 2"2 (Curro B), pH after addition (at the instant indicatr by the arrow) of the alkali = 8-0 (Cm've A). ~ = 530 m~,.] The Kinetics of the Decomposition of Murexide in Acid Solutions 293 •O.8 ~)O.6. 0.~ 0,2 , .. , , 0 I0 20 30 4,0 TIME IN MINUTES FTO. 6. l~ffr of addition of alkali to the acid reactcd mur9 solution. [[nitia! pH of the solution ---- 2 "8 (Curve B), pH aftcr addition (at the in.~tant indicatcd by the a,'~row) of the alkali-- 8-0 (Curro A). ~ = 530 m#.] The data in Figs. 3 and 4 further show that larger the pH of the solu- tion, the greater is the timo necessary for complete decomposition of a known concentration of murexide. Thus the absorbancy at ~ ---- 530 m# decreased to undetectably smaU value within 25ruin. at pH = 2.2 (curro 1, Fig. 3), while the same at pH ---- 2.6 decreased to zero value by about 40 ruin. Table II gives the results on the time necessary for half decomposition ofa known initial concentration of murexide, recorded at different pH values. These were employed for the determination of the order (n) of the reaction under investigation. The figures in the last column gave an average value of 0-93 of ,~ 1 for n. Ir is for this reason that the authors considered the 294 N. A.