Hypochlorite Ion Strength[1]

8/3/2019 Hypochlorite Ion Strength[1]

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DECOMPOSITION OF SODIUM HYPOCHLORITE:THE UNCATALYZED REACTION1

ABSTRACT'The decomposition of s od iu ~n ypochlorite has been re-examined. Th e resultsshow that Foerster and Dolch's mechanism of the decomposition to chlorate andchloride is correct; they postulated a slow bimolecular reaction to chlorite andchloride, followed by a fast er reaction of chlorite with more hypochlorite. Valuesof t he rate constants of both steps are reported; they make th e activationenergies 24.8 l;cal./g~n-m olec~~leor the first step and 20.8 kcal./gm-moleculefor the second. The rates a re such t ha t a t 40" C. a solution of sodium hypo-chlorite will contain about lyo as much chlorite as hypochlorite. The rate isstrongly affected by changing ionic strengt h; a t low ionic strengths it is nearlyconstant or falls slightly; above about 0.8, the rat e rises and a t high ionicstren gths t he rise is quit e rapid. No signs of specific catal ytic effects of sodiumchloride, hydroxide, or carbonat e could be observed, and i t seems probable th at

earlier reports of this were due to variations in ionic strength. Th e decompositionto chloride and oxygen has been measured and is a unimolecular reaction, whichis possibly, but not certainly, uncatalyzed. Values of i ts rat e constant are re-ported; th ey also are much altered by changing th e ionic streng th.Although this reaction was first investigated a considerable number of

years ago, there are several matt ers connected with it th at ar e still not entirelyclear. Briefly, the position seems to be this. Th e best early work on the sub jectwas tha t of Foerster and his co-worlters, particularly Foerster and Dolch (2).They found it to be a second order reaction, and consequently deduced thatthe mechanism was:

2NaOC1+ NaClOz + NaClNaOCl + NaCIO?+NaC103 + NaC1 .Th e first step is the slower. They showed by an independent experiment t ha tthe second step was indeed f ast er, and their rate cons tants gave activationenergies of 224 and 20%kcal./gm-molecule for the first and second ste p re-spectively. In addition to the reactions given above it has long been knownthat decomposition to sodium chloride and oxygen also occurs. Foerster andDolch's rates are the total reaction rates, though they knew that the reactionto chlorate was very much the more impo rtant. The y also found th at the rateincreased as they added sodium hydroxide. La ter Giordani (3) investigated thereaction, and concluded t ha t i t was a combinatiori of a termolecular reactiongiving chlorate, and a bimolecular reaction giving oxygen. He also found amarked effect of sodium hydroxide, bu t in th e opposite direction from t ha tfou~lci y Foerster; an d, a t least from about 0.5 to 1.0 M sodium hydroxide,obtained rate co nstants proportional to (Na0H)-?. He therefore assumed t ha thypochlorous acid was essential to the reaction, and that it went in one step,as follows: OCI- + 2HOC1+ C103- + 2HC1He proposed that the reaction to oxygen was

20C1- -, C1- + 0 ,.'Manuscript received November 14, 1955.Conlribzrlion from the De par tn~ ent f Chevcistry, Univers ity of T oro~ zto, oro nto, Ontario.

465


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466 CANADIAN JOURNAL O F CHEMISTRY. VOL. 34. 1956Skrabal and Skrabal ( G ) examined the dependence of ra te on p H, and agreedwith Foerster (and Giordani) that the rate was second order at high pHPierron (5 ) examined the relative stabi lity of lithium, sodium, and po tassiumhypochlorites. He seemed t o favor Giordani's mechanism, bu t supposed th aalkali metal peroxides were formed in the alkaline solutions, and that thesepromoted the reaction. Barredo (I) examined the reaction and found it to bsecond order ; but he also found the rate t o be altered by add ition of chlorideand over a certain range the rate was proportional to the chloride concentration. He believed that this explained Giordani's results. Such a dependencon chloride concentration would of course make the reaction autocataly ticTh e present auth or (4) has examined the rate of decomposition of neutral oslightly acid hypochlorite solutions, and found th at under these conditionthe main decomposition was of hypochlorous acid molecules, by a second ordereaction. I t can be calculated from these results (combined with those in thepresent paper) th at once qu ite a small excess of sodium hydroxide is presentthe concentration of hypochlorous acid is so slow as t o make only a negligiblcontribution to the total decomposition.

I t seemed possible that these repor ted effects of sodium hydroxide a ndchloride might really be due to changes in the ionic strength. In earlier workthe ionic strength was not always kept constant from run to ru n; and it is noalways possible in re-examining the data to deduce what the ionic strengthreallj. was. Accordingly the effect of ionic strength on the ra te of reaction waexamined in the present work. Foerster and Dolch's results seemed to establisth at th e reaction was second order; however, in view of contradictory lateresults, this point was checked.

Sodium hypochlorite decomposes both to chlorate and chloride, and tooxjygen and chloride. The relation between these reactions was by no meanclear, and the oxygen evolution had been relatively little investigated. Hencin the present work the two reactions were measured, so that their kineticscould be determined separately. Since the reaction to oxygen is stronglycatalyzed by certain metallic oxides, notably copper or nickel oxides, there haalways been some doubt that there really is ally appreciable uncatalyzedevolution of oxygen. Hence i l l the present work, an attempt was made to besure that the oxygen evolution was not catalyzed ; and t o find the order of th ireaction, and t he effect on the rate of varying temp eratures or ionic streng th

M A T E R IA L S A N D E X P E R I M E N T A L M E T H O D SS o d i u m H y p o c h lo r i te

This was produced in solution by the usual method of passing chlorinthrough cold sodium hydroxide solution. The product always containedsmall excess of sodium hydroxide t o avoid hydrolysis t o hypochlorous acidTh e chief uncerta inty with these solutions was tha t they might contain traceof metallic oxide which could act as a catalyst. Such impurities could easilybe present in an y commercial sample of sodium hydroxide to a n extent t hawould be undesirable, and it would be very difficult to measure how much


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LISTER: UNCATALYZED REACTION 467was present accurately enough to allow for its catalyt ic effect. I t was foundhowever that i f nickel or copper salts were added to the hypochlorite solution,they could be removed by the following procedure. A sinall amount of calciumchloride was added, and then enough aqueous sodium carbonate t o precipitateall the calcium. The calcium carbonate was filtered off by a s intered glass disk.I t was found tha t if this procedure was repeated four or five times, the rate ofgas evolution fell rapidly to a constant low value; further precipitations didnot reduce the rate any more. Presumably this procedure removed the copperor nickel salts by coprecipitation with the calcium carbonate. The fact that afinal slow ra te was reached is some evidence tha t this was the ra te of t heuncatalyzed reaction. It was checked that calcium carbonate did not affectthe ra te ; nor did pyrex glass, since the same rate was obtained in a polyethyleneflask.So diu m Chlorit e

This was the best available commercial material, manufactured by theMathieson Alkali Works, recrystallized from water. I t contained abo ut 0.04%of chlorate, and a trace of chloride.Sodium Carbonate

Th e reagent grade of sodium carbonate, manufactured by Merck ChemicalCo., was recrystallized from water.Sod i um C h lor ide

Th e reagent grade of sodium chloride, manufactu red by the General Chem-ical Co., was recrystallized from water.A ppara t us

The solutioils were contained in a 1-liter flask fitted with (i) a mercurysealed sti rrer , (ii) a side arm, normally closed, through which samples could betaken, and (iii) a glass capillary tube leading to a water-jacketed gas burette.Th e flask was immersed in a water t hermos tat of conveiltional design, whichwas found to maintain t he temperatu re over long periods of time to f0 .1" C.,though the swing during any on-off cycle was only about 0.02" C. Th e tempera-ture was read on a thermometer graduated to 0.1" C.

The oxygen evolved was measured in a gas burette graduated in 0.1 ml. Inpractice the volume could be easily read to 0.05 ml. In reducing the observedvolumes to N.T.P. it was always assumed tha t the oxygen was saturat ed withwater vapor a t the tempera ture of the burette. Th e confining liquid wasmercury, bu t during a run a trace of water usually condensed out of the oxygen,especially in the runs a t higher tempera ture . Th e pressure of the oxygen wasadjusted to atmospheric pressure, as shown by a small mineral oil manome ter;the atmospheric pressure was noted to the nearest + mm.Analy t i cal Methods

The solutions were analyzed at various times for sodium chloride, hypo-chlorite, chlorite, chlorate, hydroxide, and carbonate. The methods were thesame as those outlined by the author in a previous paper (4).


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468 CANADI.4N JOURNAL OF CHEMISTRY. VOL. 34, 195GE X P E R I M E N T A L P R O C E D UR E

The general procedure in almost all runs was as follows. A quan tit y of tstock hypochlorite solution was poured into the reaction flask, the amouadded being weighed to the nearest 0.1 gm. Sodium chloride, sodium carb onaor water (as required) were also weighed o ut and added. The final volume walways close to 900 ml. When appreciable additions were made to the stohypochlorite solution, a sample was pipetted out a nd its density determinefrom this the volume of solution could be found, so th at the oxygen evolutiper liter of solution could be calculated.

Th e flask was then brough t to the right temperature in the thermostat, aa t intervals samples were taken for analysis. Between samples t he gas evlution was followed by means of the gas burette. I t was found tha t th e gevolution was usually slower a t first but built up to a practically const ant raI t is believed that this is due to the building up of a certain degree of supsaturation in the solution, and th at eventually the ste ady rat e of gas evolutiequals the true reaction rate; it is always the steady rate that is reportedthe following da ta .

Th e only exceptions to this procedure were in two runs intended to discowhether t he pyrex flask catalyzed t he reaction. I n one of these the flask wfilled with pyrex beads, and in the other a polyethylene flask was used. Theruns are not reported in detail, bu t it was found that they gave the same raas in the ordinary pyrex flask. I t was also checked that calcium carbonawhich might be present in traces from the purification process, did not affthe rate. In a few runs sodium chlorite solutions were used, but the geneprocedure was the same.

E X P E R I M E N T A L R E SU L TSTh e results for the decompositioil of the stock hypochlorite solution various temperatures are given in Table I . Table I1 gives various rates of g

evolution. In this table are given data on some runs which were done wfiltered hypochlorite solution, but which had not had the complete calciucarbonate trea tment. These gas rates are high relative to later runs, so thrate constants for oxygen evolution have little significance; however the dare needed for calculations on the rate s of reaction leading t o chlorate. I tbelieved tha t the ra te constant s of the reaction to chlorate obtained f rom theruns are reliable, since (i) it was found that the purification process did naffect the ra te of chlorate production, bu t only th at of oxygen, and (ii) it wfound (to anticipa te results which will be reported late r) th at ca talyst s suas copper oxide did not affect the rat e of chlo rate formation. These ru ns also of interest because they show that the rate of g as formation is proportionto the hypochlorite concentration, at least in these solutions; and they thrsome light on the activation energy of the (presumably) uncatalyzed reactito oxygen. Table I11 shows the effect of ionic strength on the r at e constantchlorate, and Table IV shows the effect of ionic streng th on oxygen evolutioTab le V gives results on t he effect of sodium carbon ate, and sodium hydroxiTab le VI gives da ta on the chlorite-hypochlorite reaction.


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LISTER: UNCATALYZED REACTIONTABLE I

-,I emp ., 'rime, (CIO-), (ClOa-), (CIOa-), (NaCI),Run " C. min. M M M M Remarks1 40 0 1.556 0.007 0.044 1.80 Ionic strength138 1.548 0.008 - 3.79289 1.545 0.0075 0.047 (NaOH) = 0.32 d l581 1.532 0.008 0.051 (NaaCOa) = 0.02 Ad1244 1.506 0.0075 0.059 1.832 50 0 1.497 0.0095 0.061 1.84 As in run 1171 1.476 0.010 0.067279 1.457 0.013 0.070521 1.433 0.012 0.079 1.893 60 0 1.356 0.011 0.105 1.94 As in r ~ ~ n112 1.316 0.015 0.112250 1.278 0.010 0.128380 1.237 0.011 0.138 2.024 40 0 1.201 0.006 0.152 2.05 As in run 1193 1.192 0.008 0.154409 1.187 0.009 0.1541386 1.164 0.007 0.162 2.085 60 0 1.134 0.007 0.174 2.09 As in run 1358 1.045 0.009 0.199

6 50 0 1.481 0.009 0.178 1.84 As in run 1347 1.434 0.007 0.088658 1.397 0.010 0.103 1.907 50 0 1.320 - - 1.96 As in run I

0 0.973 0.007 0.015 2.48 Ionic strength82 0.961 3.63210 0.942 (xaOH) = 0.042 M302 0.929 (NazCOa) =0.036 M

TABLE I1Gas rate,Run Temp., Mean (CIO-), ml. N.T.P./mi n. per KO, 1in.-'

" C. crm-niol./l. 1. of soln.


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470 CANADIAN JOURNAL OF CHEMISTRY. VOL. 34, 1956TABLE I11

--Temp., Ionic Tim e, (CIO-1, (ClOz-), (Clan-), (NaCI),

R u n O C. strength mln. M A f M M I i e ~ l ~ a r k s

As i l l r u n I)

(NaOH) = 0.28(NapC03) = 0.02 M(NaOH) = 0.29 iM(Na2COI) = 0.05 M

(NaOH) = 0.32 M(NazC03) = 0.02 M(NaOH) = 0.29 M(Na2COa) = 0.02 M(NaOH) = 0.18 M(NarCOn) = 0.03 iM

(NaOH) = 0.23 11f(NalC03) = 0.01 M

(NaOH) = 0.28 M(NarC03) = 0.03M

(NaOH) = 0.19 M(Na?C03) = 0.02 M(NaOH) = 0.06 M(NazCOn) = 0.02 M

(NaOH) = 0.11 M(Na?C03)=0.01 M(NaOH) = 0.05 iM(Na?COa)= 0.00 M

(NaOH) = 0.28 iM(Na?COn)= 0.03 M


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LISTER: UXCATALYZED REACTIOXTABLE I\!

DATAON RATES OF EVOLUT ION OF OXYGENGas rate,R L ~ y p . , Ionic M ea n (C10-), ml. N .T .P ./min.C. stre ngth gm-mol./l. per I. of soln.

TABLE \ '

RLII I Temp., Time , (CIO-), (C102--), (CIOs-), Remarks" C. min. ilI llf ilf

0.070 Ionic strengt h 4.57(NaOI-I) = 0.28 A4(NaL03) = 0.51 A l0.086 Ionic str engt h 4.25(NaOH) = 0.07 A[0.133 (Na?COa) = 0.265 d l0.118 Ionic stre ngth 3.70(NaOH) = 0.023 iM(Sa?CO,?)= 0.019 M0.161 Ionic strengt h 3.73(NaOH) = 0.275 d l(Ka?COa)= 0.04 M

Ionic strength 1.75(NaOH) = 0.06 M(Xa?CO,) = 0.02 MAs n r u n 22


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C4KADI4N JOURNAL O F CHEMISTRY. VOL 34 . 195GT A B L E V I

DATA N CHLORITE-HYPOCHLORITE REACTIONIonic strength 3.79Run Temp. , Time, (C10-), (ClOs-), (C102-)," C. mln . M M JM

D I S CU S S IO N O F R E S U L T SI t is apparen t from these results tha t the main reaction in the decompositioii

of sodium hypochlorite is the oiie to chlorate, and only a small part goes tooxygen. Consequeiitly the over-all kinetics are those of the reaction to chlorate,and the results of Tab le I fit the equation of a second order reaction: this canbe seen from the long run, run 7, or by comparing runs 1 and 4, or 3 and 5 .Th e possibility of catalysis by chloride, or effects of sodium hydroxide, will beconsidered later when th e rate constants have been evaluated. Table I1 showsthat for these runs the reaction to oxygen is first order, though owing to therelatively small amounts of gas evolved the rates are not very accurate . Anycomparisons of this sort mus t be made between runs a t the same ionic strength ;fortunately the decomposition of hypochlorite does not change the ionicstrength during a run.

Th e mechanism of the reaction is evidently th at proposed by Foerster, with


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LISTER: UNCATALYZED REACTION 47 3the addition of the reaction to oxygen. Let the r ate constan ts of the variousreactions be kl, k2, and ko as follows:

2NaOCI + NaCI + NaC102 k INaOCI + NaC102 +NaCl + NaCI03 k2NaOCI +NaCl + $02 ko

and let the concentratioils be: (C10-) = x, and (ClOz-) = y. Then the rateequations are (assuming no effect of chloride) :

These equations are difficult to solve rigorously, but various simplifyingassumptions can be made appropr iate to t he conditions of the reaction. I nparticular th e chlorite concentration is low and changes very little during a

run; so approximately dy/dt = 0. Equation [i] then becomes:

whose solution is most useful in the form:[iii] l l b- - +-(e" 1- )x xo kowhere x = x~ at t = 0; and b = +kl+ko/xo.

T o evaluate kl it seemed simplest to plot l/ x against t, and to determine theslope of the best line through the points. Calling this slope s, we get:

l/x-l/xo bS = = -(ekO'-1) = b(1+$kot+&02t2+ .. ).t kot

As kot is always small (never above 0.04), to a good approximation:s = b(l+$kot)and

ko was determined a s explained in the next paragraph, and kl was then fouildfrom equation [iv]. I t will be seen that kl could also be obtained from th echlorate concentrations, and this was occasionally done, bu t t he hypochloriteailalyses are probably the more accurate.

For the determination of ko, let v be the qu anti ty of oxygen evolved a t time t,in units of gm-atoms of oxygen per liter of solution. Then :

Putting in the value of x from equation [iii], integrating, and since v = 0 a tt = 0, we get:


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474 C A N A D I A N J O U R N A L O F CHEMISTRY. VOL. 34. 1956T h i s is a w k w a r d t o a p p l y , b u t s u p p o s e w e t a k e a s a n a p p r o x i m a t e v a l ue of

i .e . t he ave rage r a t e o f gas evo l u ti on d i v i ded b y t h e m ean hypoch l or i te conct r a t i on du r i ng t he m eas u r i ng pe r i od ; t hen f rom equ a t i ons [ii i] an d [v ], epand ing in pow ers of t u p t o t 2 :

T h e t e r m in t2 g i ves t he e r ro r i n t roduced by t h i s app rox i m a t i on fo r k o . Us itypical figures for 60" C., th i s t erm is a b o u t 2.5X10-* t2 . The gas co l l ec t ipe r i od never r an above 400 m i n ., when t he e r ro r is a b o u t 0.4%. H e n c e t happ rox i m at i on is suff ic ien t ly accu rate . T he va lues of k1 a n d k o given below af rom equa t i ons [ iv ] an d [vi ]. Th e eva l ua t i on o f k2 wil l be d i scussed in a l asect ion .

T he r e s u lt s in T ab l e I1 g i ve va l ues of ko whi ch a re app rec i ab l y h i ghe r t hthose obta ine d for the m os t carefu l ly puri f i ed so lu t ions . Never theless theres u lt s a r e of i n t e res t a s s howi ng t h a t t he gas r a t e i s p ropo r t i ona l t o t h e hypch l o r it e concen t ra t i on ( a t l eas t fo r t hese s o l u t i ons ) ; an d t h e da t a a r e needin evaluat ion s of k l . T h e m e an k o va l ues fo r each run a re :

R u n 1 4 7 2 3 5Temp. 40 40 50 50 60 60 " C.k o 2.28 2.48 6.95 6.81 19.45 19.4 XlO-%in.-l

T h e a v e r a ge k o a t each t em pera t u re w as u sed i n ob t a in i ng k l for these runI t is i n t e res t i ng t o no t e t h a t a l t hough a t r ace of i m p ur i t y m us t be p res et hes e runs g i ve an ap par en t ac t i va t i on ene rgy of a bo u t 21 kcal . /gm-mowhich is cons i de rab l y h i gher t h an for an y of t he ca t a l yzed r eac t ions t h a t weexam i ned .

T h e effect of ionic s t re ngth can be obta ined f rom T able s I11 an d IV. F rot he gas r a t e s in Tab l e IV , we ge t :I i u n 9 10 14 15 16 21 17 18Iorlic str eng th 5.83 4.84 4.71 3.69 2.76 1.75 1.74 0.59ko (60 C.) 25.1 10.2 17.6 9.4 5.7 4.2 4.7 5.0 X1O-i min

k o var ies cons iderably wi th ionic s t reng th . As wil l be seen in the n ext sect ioa paral le l change is al so found for k l . Th e r e s u lt s in run s 21 , 22 , a n d 23 enabk o t o b e f o u nd a t d i ff e re n t t e m p e r a t u r e s b u t t h e s a m e i on ic s t r e n g t h . Tresul t s are :

Temp. 60 70 75 " C.k o 4.2 0.2 16 XlO-%ir~.-l

T h e s e m a k e t h e a c t iv a t i on e n e r g y a b o u t 20 I t cal. /gm-mol . Ra th er surpr is ingt h i s is lower than f rom runs 1 t o 7, b u t poss ibly th e d if ference is due t o expem en t a l e r ro r , fo r t he ac t ua l r a t e s a r e ve ry s m a l l . Howe ver t he va l ue s eem sb e a b o u t 20 to 21 l ical. I t is a l s o su rp r is i ng t ha t t he ac t i va t i on ene rgy s eem sbe l ower t han for t he r eac t ion t o ch l o rat e . Th i s m a li e s i t doub t fu l t ha t whave an u i i ca ta l yzed r eac t i on ; t he ca t a l y s t is no t py rex g l a s s , no r coba


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LISTER: UNCXThLYZED REACTION 475nicltel, or copper (which give lower activation energies). However it is im-possible to be sure that t races of some unltilown catalyst are not present, an dthere must remain this element of dou bt in the inte rpretat ion of t he results.

Th e runs in Table I give the kl values in Table VI I. I t will be seen, forinstance by comparison of the rat e coilstants of runs 3, 5, and 11, th at the

TABLE VI I(NaCI), i\lIiu11 Temp., k r , Ionic

" C . min.-'(gm-mol./l.)-l Initial Final strength

7 5n 3.62

purification did not affect kl; to anticipate results reported in another paper,addition of cata lyst s such as copper also did not affect kl. Table VII also givesthe sodium chloride concentrations during the runs. All these runs exceptrun 8 were a t an ionic strength of 3.79; in run 8 it was 3.63. Hence it is possibleto discover whether sodium chloride has a specific catalytic effect, apart fromits contribution t o the ionic str eng th. Although the range of concentrationsmight perhaps have been profitably extended, these runs definitely show thatsodium chloride has no effect on k1, provided the ionic strength remains un-changed. In particular , run 8 gives a value of k1 perhaps slightly low for thisionic st rength , although the sodium chloride concentration is some 25% higherthan in run 3. Th e long runs 5 and 7 show no sign of autocatalys is, althoughthe sodium chloride increased 10-1570 during them. Further evidence insupport of this contention is provided by the runs with sodium carbonatepresent. At an ionic strength of 3.79 the mean ra te cons tan ts are:

Temp. 40 50 60 " C.k I 1.02 3.61 11.4 XlOP min.-'(grn-rnol./l.)-I

These give a good linear plot of log k1 against 1/ T, an d make the ac tiva tionenergy 24.8 (to the nearest 0.2) kcal./gm-mol. Foerster's constants a t 25' an d90' C. give a value close to 26 kcal., but his result a t 50' C. does not fall on ast raight line with these values in a plot of log kl against 1 /T .

Tur nin g now to the effect of ionic strength on kl, we get from Table I11 fo rk1 a t 60' C.:

Run 9 10 14 19 11 15 8 16 12Ionic strength 5.83 4.84 4.71 4.16 3.79 3.69 3.63 2.76 2.73k 1 17.4 16.75 15.65 12.1 11.4 11.2 10.5 7.7 7.8Ru n 20 17 2 1 18 13Ionic strength 2.61 1.74 1.75 0 .59 0.515k1 7.6 5.5 5. 4 3.85 4.3 X10-6 min.-'(grn-mol./l.)-I


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476 CANADIAN J OURNAL OF CHEMISTRY. VOL 34 , 1956These values give somewhat the same type of tr end th at might be expectfor the ac tivi ty coefficients of sodium hypochlor ite over this range of ionstrengths. Th e trend in ko s somewhat, though not very, similar.

Run 24 makes k l = 1.98X10-5 mill.-'(gm-mol./l.)-I a t 50' C. and an ionstrength of 2.61. Thi s is proportionally a somewhat larger drop than a t GO0 Cbu t not very much. Run s 25 and 26 (Table V) give the effect of adding sodiucarbonate; they make kl at 50' C.:

R u n 25 26Ionic strength 4.57 4.25k I 4.25 3.95 X10-j n1in.-'(gm-mol./l.)-I(NaCI) initial 1.58 1.99

Relative t o the more carefully measured rate a t an ionic streng th of 3.79 theruns show an increase il l k 1 which is nearly what might be expected purefrom the increase in ionic strength. The increase is perhaps a little less thain the runs when sodium chloride was added : of course specific effects are bounto occur, and one would not expect two different salts to give exactly th e samchange in kl or the same change in ionic strength . Thi s is especially so whethe tw o salts, as here, are not of the same ionic type. Nevertheless the machanges in k l do seem to be attr ibutable to the changes in ionic strengt h.

Runs 21, 22, and 23 enable us t o evaluate kl at three temperatures at aother ionic strength (1.75) :

R u n 21 22 23Temp. 60 70 75 " C.k l 5.4 16.0 24.0 X10-5 min.-'(gm-rnol./1.)-~

The figures at the higher temperatures are rather rough; they give a modeately linear plot of log k l against 1/T, with a slope corresponding to an actvation energy of abou t 24 kcal. The experimental error is too large to sawhether there is any real difference between the activation energies at ionstrengths of 3.79 and 1.75.

Table V gives the data at low sodium hydroxide concentrations. The buof the runs in th is paper were done in the presence of 0.32 M sodium hydroxidSome of the runs a t low sodium hydroxide concentrations have already beeconsidered, and found to fall illto line with the other runs (e.g. runs 8 and 21The k lvalues for various runs which provide evidence on the effect of sodiuhydroxide are as follows:R u n (mean) 27 8 21 17 (mean) 28Temp. 60 60 60 60 60 50 50 " CIonic strength 3.70 3.70 3.63 1.75 1.74 3.79 3.73(NaOH) 0.32 0.023 0.042 0.057 0.19 0.32 0.275 i l lk I 11.4 10.9 10.5 5.42 5.48 3.60 3.48

These results show that sodium hydroxide has a negligible effect apart fromits contribution to the ionic strength. A t very low sodium hydroxide concetrations hydrolysis to hypochlorous acid and its decomposition become impor tan t; but t he rate of this reaction has been measured (4), and it call bcalculated t ha t the sodium hydroxide would have to be close to 0.001 M befothis reaction made much difference to the total rate.


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LISTER: U N C A T A L Y Z I S D K P S A C T I O N 477Finally the results in Table VI give information on the hypochlorite-chlorite

reaction. Firstly runs 29 and 30 show th at t he decomposition of chlorite byitself is entirely negligible. From the remaining runs k:! was evaluated as fol-1 0 ~ ~ s .s one molecule of chlorite removes one of hypochlor ite, and since thi sreactio~l s much faster th an the decomposition of hypochlorite (kl ), thenapproximately :

cEx/dt = dy/dt = -k?xywhere, as before, x is hypochlorite and y is chlorite. Th e solutioil of thisequation is:

where c = xO-yo. 111 reactioils of this sort k2 is ilorinally obtained froill thi sequation, but in the present case a small correction has to be applied for theclecomposition of the hypochlorite, which is slow but not negligible in coin-parison. I t therefore seemed simpler t o ta ke a n approx imate value of kz definedby k2(approx.) = corrected rate/(meau ?c) (mean y) .By "correctecl rate" is meant dx/dt between any two successive readings minusthe par t of the rate du e to decompositioil of hypochlorite, which call be calcu-lated from kl. If the values of x and y from equation [vii] are sub stituted inthis expression, and the exponentials expanded in powers of t, we get :[viii] k2(approx.) = k?[l- (.vi'+xoyo+y$) (kz%2/12)+ . . . ] .This gives the error in ks in this approximation: in the ru ns reported it roseto abo ut 1%, b ut the error could then be allowed for. To examine the validityof our methocl of allowing for kl ant1 ko, we call sub tra ct eq uat ions [i ] and [ii]to get: dy/dt -d.t-/dt = Qkl.xz+kox .Substituting the value of .2: froin [vii], integrating, and expanding in powersof t , it is found th at the fractioilal error in k2 so introduced is:

Fractional error = (Gkly0+3k1xo- ko)3klxo+2ko .xoyokat.Th e size of this erro r was checked for runs 31 to 34, and il l no case was above0.3y0. Consequer~tly t was assumed t o be adequate to obtain k:! simply b ytaking the observed slope of x (or y) over each interval between analyses,correctiilg for the decompositioil of hypochlorite i f the slope of x is used, an ddividing the slope by the mean x times the mean y over the interval. A smallcorrection was applied in accordance with equatioil [viii]. Th e mean values ofk:! for each run were:

Run 31 32 33 34Temp. 50 50 45 40 " C.k 2 2.75 2.77 1.64 0.97 X 1 0 - 3 ~~~i n . - l ( g m- mo l . / l . ) - 1


14/14

478 CANADIAN JOURNAL OF CHEMISTRY. VOL 34 . 1956These results give an activation energy of 20.8 kcal. In all these runs t he istrength was 3.79, and the sodiurn hydroxide co~lcentrationwas 0.32Foerster's results made the activation energy 203 lccal., in good agreemHowever his constant a t 50 C. is about 8.3X but the difference madue to a different ionic strength. These values of kz mean th at a decomposodium hypochlorite solution will contain about 1/100 as many chlorite as hypochlorite ions a t 40' C.; this was roughly found. If t he constant s k lk p are written a s AecEIRT,he A factors are fairly similar:

T o surllmarize these conclusions, it is believed th at these results show (i) the mechanism of Foerster e t a l . via chlorite is well established; (ii)first stage to chlorite has an activation energy of 24.8 kcal., and is slower tthe secoild stage, which has an act ivat ion energy of 20.8 kcal.; (iii) the' rare strongly influenced by the ionic strength; (iv) added sodium hydroxcarbonate, or chloride changes the rate by changing the ionic strength, apa rt from this they exert no specific catalytic effect; and (v) there is a s itaneous unimolecular decomposition to oxygen, which is possibly, butcertainly, uncatalyzed.

REFERENCES1. BARREDO,. M. G. Anales fis. y quim. (Madrid),37: 220. 1941.2. FOERSTER,. and DOLCH,. Z. Elektrochem. 23: 137. 1917.3. GIORDANI,. Gazz. chim. ital. 54: 844. 1924.4. LISTER, . W. Can. J . Chem. 30: 879. 1952.5. PIERRON,. Bul l . soc. chim. France, 10: 445. 1943.6. SKRABAL,1. and SKRABAL,. Monatsh. 71: 251. 1940.

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Hypochlorite Ion Strength[1]

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