June, 1057
~’OLhROURAI‘€IIC BEHAVIOR O F CHLORANILIC A C I D
765
POLAROGRAPIIIC BEHAVIOR OF CHLORANILIC ACID’ BY JOSEPH WEISSBART~~ AND PIERRE VANRYSSELBERGHE~~ Department of Chemistru, University oj Oregon, Eugene, Oregon Receiuad December 86, I066
On tlic b:isis of scvcral criteria of reversibility it is fonnd that chloranilic acid behaves reverclibly a t the dropping mercury electrode. A plot of lo ( ( i d - i)/i) us.fotentia1 gives the required value of 2 for the number of electrons involved in the electrode proccss. Catkodic, anodic an mixed anodic-cathodic waves exhibit practically the same half-wave potential a t a given p H . The plot of the half-wave potential us. PH consists of three lincar segments connected by smooth curves. From t,he intersections of the linear Regments the dissociation condants K I and Kr are obtained for the chosen total ionic strength of 1.0. The diffusion current is found to be pro )ortional t o concentration over a 500-fold concentration range. The p H of the solution appears to have no effect on the diffusion current,. By using the Ilkovic equation it is found that the average diffusion coefficient for the various reducible forms of chloranilic acid A i 1.04 X lo-’ cm.2 sec.-l
Introduction
measurement of the excess chloranilic acid after precipitation of zirconium chloranilate) and on account Thc oxidation-reduction potential of chloranilic of the uncertainty as to the values of K1 and K z we acid (2,5-dichloro-3,6-dihydroxy-p-benzoquinone) was first measured by Conant and Lutz.* They undertook the polarographic investigation reported dissolved small amounts of the acid in buffered here. Breyer* has observed the reduction of chlorsolutions, added enough reducing agent to convert anilic acid a t the dropping mercury cathode (d.to 4 X M , while approximately one-half of the acid to its reduced m.e.) in the range of 9 X Breyer and McPhillipsg used chloranilic acid in the form and measured the corresponding potential of a indirect determination of calcium based upon the noble metal electrode. Conant and Fieser4redetermined this potential in aqueous solutions and in 95 wave heights of the remaining acid. They reand 50% ethyl alcohol-water mixtures. Schwarzen- ported well-defined waves in the range 1 X lo-* to bach and Sutcld determined the empirical “redox 3.3 x 1 0 - 4 ~ . Having recognized early in our work that the bepotential” of chloranilic acid in buffered solutions a t constant ionic strength and obtained the acid disso- havior of chloranilic acid a t the d.m.e. was e w n ciation constants K I and K z on the basis of the tially reversible we were able to carry out a detailed change of this empirical “redox potential” with pH, examination of the variation of the half-wave poa potentiometric titration with a reducing agent be- tential with pH, to establish the corresponding values of p K 1 and pKz, which are compared with preing carried out for each pH value. vious in Table I, and in addition to deterChloranilic acid solutions exposed to air are mine values the concentration and pH ranges within stable, the acid being then entirely in its oxidized which wave height and concentration are proporred-colored form. Upon chemical reduction, by tional to each other. sodium hydrosulfite for example, the acid is deTABLE I colorized but rapidly reverts to its oxidized form, COMPARISON OF VALUE6 OF pKi A N D pKa FOR CHLORANILIC unless care is taken to prevent contact with oxyACID AT 25’ gen. Applying to chloranilic acid a spectrophotoSchwarzenbach Tharncr Preaent metric method for the determination of overlapping and Siiter’ and Voigto paper dissociation constants of dibasic acids (valid for Irengthof 1.0 which we used regions. This is discussed in the next section in in niost of our m r k . Tlic pH vnlucs wcrc nicasured with a connection with t,he det,ermination of the acid disglafis clcctrode-Benkman Modci CI pH-mct)er combination. Purified nitrogcii was passcd through the cclls to remove sociation cons tan ts, Acid Dissociation Constants.-The determination dissolved oxygen. Procedure.-The valucs werc detcrmincd by intcr- of oxidation-reduction potentials as half-wave polation between thc init'ial and final potciitials of the wave, potentials of polarographic waves is particularly these being measured by meam of an external potentiometric circuit. Thc voltage ~ c a l cwas cxpanded to approximately convenient in the case of a quinone system such as 2.5mv. per division 011 the recording paper. Thc vnlucs chloranilic acid. The usual assumption about the were reproducible within 1 niv., the requircd Ri correction equality of tthe diffusion coefficients of oxidized and being made in onch casc. In experiments carried out at a reduced forms has of course to be made and the pH of or near 3.6 for the purpose of determining the diffusion current id gelatin in amounts varying from 0.01 to activi tjy coefficients, made practically constant 0.0570 had to he uwd to suppress n ma,xirnuni in the waves. through the use of a constant total ionic strength, Residual currcnt corrections werc innde in the i d determiria- are incorporatcd into the El/, values. Let 11s reptions and, on account of the slow decomposition of chloran- resent chloranilic acid by the formula HzQ. At the ilic acid occurring on standing, t h i d values were all ob d.m.e. the reduction taincd with freshly prepared solutions. HzQ 2 H + 2 ~ IT HqQ (1)
Results and Discussion Reversible Behavior.-The reversibility of a polarographic oxidation-reduction reaction can be established by determining the E l / , values of cathodic, anodic and mixed anodic-cathodic waves observable in a given medium (with application of the proper Ri corrections) arid verifying their coincidence. When half of the total conacntratioii of the substance under study is in its oxidized form and the other half in its reduced form no Ri correction is necessary since i d is then exactly, or close to zero. This method furnishes the most reliable criterion of reversibility. The corresponding equations have been given by Kolthoff and Lingane," and by others. Anodic waves were obt'ained after first reducing the chloranilic acid with an excess of sodium hydrosulfite. Mixed anodic-cathodic waves were obtained by adding enough sodium hydrosulfite exactly to dccolorize the solution and exposing the solution to the oxygen of the air for different time intervals before taking the polarograms. The E l / , values of cathodic, mixed anodic-cathodic and anodic waves of chloranilic acid were found to coincide. For a concentration of 1 X loF3 M and a pH of 3.61, the E l / , value was found to be -0.098 0.003 volt V S . S.C.E. We can regard this test of reversibility as satisfactory. We also used the test based on a plot of log ( ( i d - i ) / i )against the potential of the d.m.e. M The recording of the polarogram for 2 X chloranilic acid ,in a buffer of pH 4.71 was intcrrupted a t nine points along the rising portion of the wave, the correspouding potential being measured with the external potctitiomctric circuit. The average current reading a t each point was corrected for the residual current. An excellent linear plot was ohtained with a slope corrcsponding to 1.97 0.05 or 2 electrons for the reduction of one molecule of chloranilic acid, as expected. The proportiotialit8y of t,he diffusion currcnt8 t o the square root of the cwrrcctd height of tthe mer-
*
(10) J. J. L i n g a n r , "Eloatnmnalylicnl Glirinistry," Intwnciciice ~ ' i i h l i ~ l i e r nN, c w York, N. Y..19.58. (11) I. M Kolthoff and .I. .I. Lincnno. "l'olrrrournliliy," 2nd orlition, Interscience PllbliRhcrR, New Yiirk, N. Y . , 1952.
+
+
occurs. The dissociation of the very weak id H4Q is neglected, but H2Qdissociates as
JI + HQ+ Q--
HzQ H+ HQ- _J H +
(2) (3)
According to the treatment of Clark and Cohen,12 representing by (Ox) the sum of the concciitrations of the reducible species HzQ,HQ- and Q--, and by (Red) the concentration of the oxidizable species H4Q,the Neriist equation takes the form E = Eo
+ (RT/PF) In [(Ox)/(Red)] + ( R T / 2 F )In (H+)' - ( R T / 2 F )In
+ K d H + ) + K&l
[(H+Ia
(4)
At the half-wave (Ox) = (Red) and we have
+
( R T / 2 F ) In (H+)*( R T / 2 F )In [W"a KdHY
EllZ = E " I / ,
+
+ KXzl
(5)
in which Z i l and Kz are the acid dissociation constants corresponding to equations 2 and 3. M chloranilic Polarograms were taken for acid in buffers in the pH range from 0.40 to 7.11, t8hetotal ionic strength being 1.0 in all cases. The Et/Zvalues reported in Table I1 were found. When plotted against pH (Fig. 1) they are seen to align themselves along three linear segments and smooth connecting curves. Straight lines of slopes equal, respectively, to 0.059, 0.089 and 0.118 were drawn through the experimental points. The first two intersect a t a pH of 1.22 equal to pK1, while the last two intersect a t a pH of 3.01 equal to pK2. These values are reported in Table I. Analytical Applications.-We found the diffusioii current id of cathodic waves of chloranilic acid to be proportional to concentration over a 500-fold to 1 X l o + M . range extending from 5 X The corresponding expcriments were carried out in 0.1 N H2S04and in a buffer of pH 3.62. The rcsults are reported in Table 111. Different capillaries were used in the two series of measurement's, tJhe ratio id/Crn'/at'/s being found to be 4.05 in the GLSC of t,he 0.1 N H2804solutions and 4.03 in the case of tho buffer of pH 3.62. A possible effect of pII on thc valtic of i d for a givcn coiiccntrattion of chloranilic acid was follnd to (19) W. hT.
-300
2 -400
-500 0
I i,22 2
3.01
4 5 6 7 PH. Fig. l,-Viuhtion of Ellz wit,li pH for 0.001 A4 chloranilic acid in buffers of ionic strength of 1.0.
CONCENTRATION id/C
Deviation
In 0.1 N Hz80,, with m'/at'/8= 1.59 rng.'/' scc.-I/' 2.0 12.75 6.37 -0.08 1.0 6.43 6.43 - .02 0 . to 0.645 6.45 . 00 .02 .I20 6.45 .00 .01 .Oti6 6.55 .10 __ Av. 6.45 k0.04
+
+
I n citric acid NnZHPO4 buffer of pH 3.62, with l'I6 = 1.68 rng.'/' set.-'/* 5.0 32.80 6.75 -0.16 2.0 13.37 6.60 - .02 .04 1.o 0.75 6.75 0.5 0.342 G--.84 -_ .13 Av. ti.71 f O . 00
+
+
be non-existent, tis is shown by tho clat'a of Table IV. On the hasis of the Ilkovic equation we find that t8he avcrage diffusion coefficient of the reducible
TABLE IV CONSTANCY OF DIFFUSION CURRENT WITH pH CHLORANILIC ACID id,
PH
mm.
PH
4.32 3.80 3.48 3.30 2.88
106 107
2.44 1.82 1.70 1.23 0.91
109
107 109
FOR
10-8 A!
id,
mm.
110 110 106 106 110 Av. 1 0 8 f l . t i
species HzQ, HQ- and Q-- of chloranilic acid is 1.04 X set.-'. Acknowledgment.-The work reported here was part of a project carried out under a contract between the U. S. Atomic Energy Commission and the University of Oregon. J. W. was the beneficiary of a Research Fellowship, for which he wishes to express his gratitude.
IiIGIL TEMPERATURE IIEAT CONTENTS OF CALCIUM ORTHOSILICATE BYJ. P. COUGHLIN A N D C. J. O'BRIEN Con1d)ulion from the Miiwrnls Thnrniod!jnamics Expcrimenl Station, Region I I , Burcau of Mincs, United Stales Department of the Interior, Berkeley, Cal. Received December 20, 1066
High tciiii)cr:rturc Iicat uontctit mcnsurcinctits wcrc made involving four cryst:Llliiic varicties of calcium orthosilicato. Tlie entropies attd hcnt)s of the p 4 a', a' 4 a , nnd y -+ a' transformations were evaluated. A table of smooth v n l u c ~of hcnt content nnd entropy incrcmcnts nbovc 208.15'K. and algebraic representations of the heat contents arc included, for use in thmnociynainic cnlcril:itions.
Introduction.- -There arc four cryst8nlliiie v:rricties of calcium orthosilicate (CazSi04) according to the work of Brcdigl and Tromel and R.Zoller.2 The desigiiatioris a, a', p and y, uscd by those aiithors, are adoptccl 1ict.c. Two varicties arc common a t room tcmpcraturc, p and y. The 0-form is ( I ) h1. A . Brcilig. Y . A m . Ceram. Roc., 3 3 , 188 (I!l50). (2) G . Troincl and H . Miillor, Vorlsch. Alineral., 2 8 , 80 (1!240).
thermodynamically unstable throughout its tomperature range of existcnce. It transforms at 970°K. into the a'-variety which it,sclf is a metastable modificattion at8this temperature. The yvariety is thr: "dustjing" form sotiictiirirs o1)scrvocl i n the manufacture of Portland cement( and during cooling of high lime-co~it~:iiiiingslags. I t is tho tlie~inodyiiamictlllystahlc form to about 1120°K.
