The Conversion of Ammonium Thiocyanate into Thiourea and of

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11110S IU 11 THIO C TA S A T E THE C 0S T ERS IO S 0 F *I ISTO THIOI-REA A A s r ) OF THIOTREA ISTO THIO C Y-1S .IT E

BY J O H S 1V.lIIDELL

T h e course of '1 monomolecular reaction running to a n end, in one direction, is a t any instant represented by the formula

where R is the velocity coefficient, is the amount of substance a t the start, a i d I' the amount that has been changed a t the time t. If .^c = o when t ==o we arrive, by integrating between the limits zero and 2. a t the formula

TThen the reaction is reversible the form of the differential equation (and therefore of the integral equation) will be the same, the difference being in the constants. If the substance A changes only partially into the substance B, we have a t any moment (except a t the very start if the action begins with A only present) two reactions going on which finally reach a state of equilibrium ; and a t any instant

where KT and KZ are the velocity coefficients. Since after eqiiilibrium is reached, no change takes place we have a t that point


of

being the value of -1 a t equilibrium. in equation ( I1

Substituting the value

T h i s equation is of the same form a5 for the tion-reversible action, and becomes identical with it when f: = -4. T h e eqaation is integrable in the same 1v-a). as before, giving a constant which is however not the velocity coefficient, but the velocity coefficient of the reaction in one direction multiplied bj- a factor obtained by dividing the original qiiantitj- of the substance taken, by the quaiitit:- of substance which is changed a t the state of equilibrium. Since the point of equilibrium will depend upon the relative velocity coefficients, the constant obtained should be the same whether we start from one end or the other. If instead of starting with substance A, we start with substance B, the equation becomes

Rut our equation of equilibrium has already given lis t h a t

therefore

\o that the constant obtained on integration is the same as before. In a reversible reaction lve should therefore get the same constant, no matter from which end t h e reaction proceeds. KiisterI thought of experimenting on the change of ammonium

' Zeit. phys.

Cherri. 18, 161 (IS9j).


thiocyanate into tliioiirea but w i s cleterrecl from doing .so, because he coiisidered that side-reactions \voiild interfere too seriously with the cotirse of the main reaction : and he decided to work on the change of liesa-chlor-a-~eto-~-R-peiiteiie into liexa-chlor-a-keto-y-R-peiiteneand 1-ice-versa. His coiistaiit is not calculated in so siinplc a manner as tlie aboT-e, atid cloes not s l i o ~ vthe similarity bet\\-ern the rex-ersihle and noii-reversible reaction. Keister's constant is ho\veT-er iclentical with the one so calcnlatecl, and lie saw that i t should lie the same, no matter from \vliicli end he started. H e nas riot fortunate enough to arrix-e at this result : a slight impuritJ- in the y-K-pentene appearing to produce hj-clrocliloric acid \vliich hastened the reaction when starting froin that end. TValker aiid Hambly' showed that tlie change of aiiiiiioiiiiiiii cyanate into urea is not a niononiolecular reaction, and the!. conclucled froin the coiistaiit they calculated that it is bimolecular. It l~rcaiiie all the inore interesting to study the behavior of aiiinioiiiiiiii thiocyanate, and in spite of the risk of side-reactions I decided to in\-estigate the niatter. Solid ainiiioiiiiini thiocyanate heated at 93" for a couple of iiioiitlis on the average eight hours a day, \vas soiiien-hat changed. the color becoming a deep yellow, but this change though apparently great u-:is almost inappreciable on titration aiid 110 thiourea could be detected. Similarly a solution of thiocyanate heated a t tlie same temperature for about fiftJ- hours gave iio iiiclicatioii of change. approximately clecinoriiial solution in which there \vas about j cc sulpliuric acid to 2 j O cc solution kept boiling for eighty hours gave very little color to ferric aluiii and contained a large quantity of thiourea. some evaporation had taken place, although I used a reflux condenser, I did not deteriiiiiir whether the thiourea corresponded in quantity to the original thiocyanate. Xn aqueous solution in which there was about j o grains thiocyanate in IOO cc solution was kept at its boiling teinperature, 109' C, for fifty hours atid after that period showed the

' Jour. Chern. SOC.67, 746 ( 1 S 9 j ) .


328 presence of thiourea, but a t most there could not ha\-e been more than 0.2 j percent. T h e main experiments were conducted a t the temperatures 15zC-1 53" and 160~-161~. A l tthese temperatures the substance was liqnid, a i d i t was hoped that the bj.-products formed n-ould not be vcrj. considerable. Test-tubes containing in some series abont one grain, in other series two grams of the solid were placed in a n oil-bath kept stirred by a sniall motor. .Is tlie substance had to melt the time was not taken froin the minute n-hen the tubes \yere put into tlie bath, but fi-om tlie minute when the first tube was x-itlidrawii. I t was for this reason that the qnantity of substance in each of the tubes was approxiniatelj- the same, otherwise the reaction in one tube might he inore or less advanced tlian iii another, at the same time. each tube \vas taken out i t was rapic1l.i. cooled, and the analysis of its contents made later on. I had not anticipated anj- difficult>. in using the ordinar). niethod of titration for the thiocyanate. bj- iiieans of sill-er nitrate acidified with iiiti-ic acid, but concordant results were not obtained. This was doiibtless due to the forination of a certain amount of the conipound of silver nitrate and thioiirea described lij. Reynolds' and by Kiirnakon-,' which tlioiigh partially deconiposable, is somewhat stable in the presence of nitric acid. T k fore I had seen the papers of Keynolds and Knrnakow I had decided to use sulphuric acid as less unsatisfactorj- than nitric acid, but unfortunatelj-, even in that case there is mnch to be desired. T h e contents of the test-tubes were dissol\.ed i n water to the volume IOO cc, portions of this solution pipetted off, and diluted till 0.2 grain of the substance )vas contained in about 2 0 0 cc in soine cases; while in other cases about fi1.e times as much water was added. 5 cc or I O cc of a fairlj- concentrated solution of ferric aiiinioiiiiini aluni was added as indicator and aboitt I O cc of dilute sulphuric acid.

' Jour, Cherri. SOC.61,2 j O 1 1Q92). Ber. cheiii. Gea, Berlin. 24, j 9 j 6 (1S91 1


Coiiiv~~ioiz nf' A I H U L ~Thiocynnnte ~~Z~UZ

529

T_'sually i n the :uiialj-sis of an!. given sample tlie first approximation was made by using the less dilute solntion and afterwards, with tlie iiiore dilute solution, a little iiiore tlian tlie proper q n a n t i t ~ .of silver nitrate n-as riiii in and then a back titration made with aiiiiiioiiiiiiii thiocyanate solution. Since the results obtained were iiot so good as one would wish I attempted on tlie other hand to estimate the tl1.iourea. I'olliarcll saj.s t h a t thiourea can be deteriniiiecl by means of aiiimoiiiacal silver nitrate solntion xliich ,oil-es silver sulphide and cj-aiiaiiiid.' T h e reaction is represented by the equation C S ( S H , 2 )= , HIS

+ C?;.NH,

Cyanamid, n-hen treated with sill-er nitrate, forins silver cyanamid, a yellow coiiipoiiiid, but not till all the thiourea is decoiiiposed. Tlie precipitate formed b y riddiiig silver nitrate, chaiiges froiii black to yellow when the thiourea is exhausted, and in order to deteriniiie tlie exact point at wliicli this occiirs a piece of filter-paper is, frcm time to time, tiioisteiied with a test-drop of the solution, and tlieii a drop of silver nitrate added n.liicli gives to tlie paper a dark stain of silver snlpliide, or a liriglit :.ellow one of silver cyananiicl. T'olliard says that the iiietliod is slow in the presence of considerable thiocyanate but that i t can he used with siifficieiit accuracJ-. He did not howel-er anticipate its w e in a series of estiiiiations, and it is iiot ver?- satisfactory as a iiietliod of titrntion. 111 the first place silver thiocyanate is iiot very readil?. soluble in ainiiionia and when silver sulphide is precipitated ir is liable to enclose some of the thiocyanate, wliicli would prohatjlJ- he all extracted by sufficiently long digestion in the aiiiiiioniacnl solution, bnt wliicli tends to vitiate results in ail ordinar!, titration. Moreover, I found that it was difficiilt to cletenniiie wit11 exactness tlie point at which tlie stain 011 the filter-paper, clianged __~_

._______

Jour. prakt. Clirrri. [ 2 ] 9, Z S ;rS71'. IVatts' Dictionary, II-,709 ( 1Sg4) states. n-itliout gix-ing the autliorit!. that ammoniacal silver nitrate produces silver sulplii.!e I


from being dark to being piire yellon, tlie limit of accuracj. lit.iiig one or two or in some cases ex-eii three-tenths of a cubic ceiitimeter of decinormal silver nitrate solntion. Finall\- n-hen I had reached the point at wliich I decided that the stain was certainly yellow I found that 011 filtering mcl adcling silver nitrate to the filtrate, a dark cloiidiness and in some cases a slight hlack precipitate macle its appearance. however I do not knoiv an\- perfectly reliable method of titration I was forced to content myself with these two, using the first method Tvlien the sui)stance heated was aiiiiiioiiiiiiii tliiocT-anate, and tlie second n-lien the substance heated was thiourea. Tlie result as a n-hole then fell far short of perfection, but there seems to be no doubt regarding the character of the reaction. Tlie coiistaiits obtained bj- heating aiiiiiioiiiiiiii tliiocjmiate 011 the one hand, and thiourea on the other, are the same, aiid are as near that required for a monoiiiolecular reaction as the conditions of the experiment permit. I t was foiuid that when heated a t I j 2 " - I 53" the aiiiount of thiocj-anate changed into thiourea \vas 2 I . I percent aiid at 160°, 2 0 . 9 percent. 1-olhard gives the amount of thiourea obtained by lieatiiig tliiocj-anate a t temperature 1 6 0 " - - 1 ~ o ~ as 17,7 percent. It maj- be that a t the higher temperature some of the thiourea was decomposed into cj-anainid, or it ma?. he that in using large quantities some disturbing element entered into the reaction. Thiourea heated at I j 2 " - 1 5 3 " for the same time as the t1iiocj.anate contained 2 1.3percent of thiourea so that practicallj, tlie same end-point \vas reached, iio matter wliich substance \\-(is heated. In calcnlating the constant for the change of tliiocyanate into thiourea I have used = 2 1 . 2 ; aiid for tlie reverse reaction 5 = 78.8. T h e time in all cases \vas measured in ininutes. Tables I aiid I1 show two separate series heated at I j z 3 -

153".

53' T . ~ H LIE Thiocyanate f= T ~ I ~ l 1pj 2. O - I j2'.

21 2


log :-xi)- 10gi{-,I

t niin

To. __

x

0

2.0

2

3

3 8

3 4

I2

6.2

19

69

I

6

9

35 48 60 78

IO

480

6

'

1

*

28

3

~

IO

1

~

4

12.3

I

13.5

4 ---.I' 19.2

1

.... -___

17 4

0.0083j

Ij.0

0.0089

14.3 10.8 13.0

I

8.9 7.7

14.5

6.7

21.2

0.0

I

f - t,

0.00684 0.0060j 0.006 j 8 0 .00.;00

~

0.00661 o 00j87 . . . a

.Irerage, omitting the first two

0.00649

TABLE I1 Thiocyanate Temp. 152°-1j33. f=

2.0

1

3.6

4.5 5.4 7.1 7.9

1

~

0.007j j

16.7 Ij.8

0.00j64

0.00606

14. I

0,006 70

13.3

0.00j90

11.2

11.9 13.3 13.7

9.3 7.9 7.5

,

....

19.2 17.6

10.0

I

21.2

Average

0.00688 0.00684 0.006 76

o.ooj7j 0.0064j

In Table I the value of x was obtained, by taking as correct the titration in the more dilnte solution, a preliminary approsiillation having been made in a less dilute solution. T h e second titration was iiiade in a large Erlenmeyer flask with about a liter of water, and I O cc of the ferric alum indicator. -4fter a slight excess of silver nitrate had been run in. the flask was getitlj,


shaken so that the silx-er thioc: anate shonlcl be as flocculent as possible, then I O cc of dilnte sulphuric acid added. and decinormal animoniiun thiocj anate till the color of the ferric tliiocyanate appeared. I n Table I1 the value is that obtained b y taking the ax erage of the titrations that were considered the best. Sometimes fonr or five titrations were made of the same sample. Table I11 shows how far the values of x given in Table I1 differ from what n-ould have been required for a perfect constant, the x in the third column being calculated on the basis of the constant 0.0063.

TABLE I11 Thiocyanate Temp I j z 3 -I j3' t

SO.

I 2

3 4 5 6 7

1

1

. . e

5

3 31

10

1

l

5 20

8

9 IO

x calc

0

1

27 31 46 57 71

1

Y

'

2 .o

I

4.59 5.75

36 F j

5.4 7.1

1 1

I

'

fou11d

8.22 683 9.47 11.35 12.80 14.34

TIBLE

-0 35 -0.2'7

'

10.0

11.9 I33 13.7

~

.... -026 -0.09

1

79

~

Dlff

32

-0

-0.53 -0.j \ j -0 j0

'1 1

64

-0

Iv

Thiourea = j8.8

Temp. I 52'-1 53'.

log I f SO.

Z niin

I 2

I5

0

3

4

2 /

1

;;

E-x

Y .

1 ~

53 68

I

90

I

31'

41.4 45.2 51.5 54.4 56.3 6j 0

~

-

x,) 1

-

log ( i - x )

f,

....

4'7.7 3'7 4 33.6 27.3 24.4

0.00-oL)

o oojh4 0.00638 O.OOjj0

2 2 .j

13.8

~

I

Average

o 00637 O.OOj88

--

0.00622


Coiii*eir.sioiz of

z - ~ i i i i ~ i o ~ z l i iThior~~cz~znic~ iii

533

Table IT gives the results obtained by heating thiourea at tlie temperature I j.2'-153'. Thiourea is said bj- PratoriusSeidlerI to fuse at I~o', h i t if the mass is cooled tlie meltingpoint is lowered to 149~.I t v o u l d seem natural to suppose that soiiie of the tliionrea is changed into aiiiiiioiiiiiiii tliiocj-anate, thus lolveriiig tlie nielting-point, b a t one n-ould expect that melting-points iiiteriiiediate between 149" and I 70' iiiiglit lie obtained, and though I did not make a special inr-estigatioii of the matter m y experience seems to confirm that expectation. T h e tliionrea that I iised \vas not entirely freed from tliioc\-anate. T h e tubes were placed in the bath a t tlie temperatiire I jl', b u t as tlie substance did not fuse the temperature of the bath n-as raised to I 66"when the contents of tlie tubes were entirely liquefied. T h e bath was then cooled d o i m to 1 5 2 ' aiicl the first tube taken out forty minutes afxer the beginning of the experiment. this time, as \vi11 be seen i n the table, more than thirtj- percent of tlie thionrea had been changed. T h e thiourea which n-as heated at 160' was a portion of another lot, and was not so pure as tlie first, and melted more easily. TABLE

1'

Thiocyanate

Temp. 160"-161'.

0

I 2

3 4

6 ~

5 6 7

8

.T

I

....

ga

11.1

12.9

i

50

YO

13.1

7.8

16.2

2:;

14.6 IS

7

I

2.2

I

( 2 ) 21, 141

(rSSo\l.

o orj8 o.0162 0.012;

1

-%verag e

' Jour, prakt. Chern.

1,

0.01 I j

1

43

f

1j.j

I

,

-x

IQ.2

,

2.7

'7 36

E

5.4

22

29 1 i

log(t-

log(:-x, -~

t

so.

f = 20.9

o 016; 0.010; O.OIS3 O.OI4j

I

,


534

*... 0.0091 0.012; o.0128 0.0125

o.oro0 0 . 0 1 3j

Average, omitting the first

0.0123

Tables l'and VI represent the change of tliiocj-anate into thiourea and of thiourea into tliiocj-anate at temperature I 60'-

161'. T h e coilstarit x-aries still more than at tlie lower teinpernture. The 1-elocitj-of the reaction is ahout double and any ineqiialities of teinperature iii different parts of the bath ~vould ha\-e more effect. *It the same time the del-iatioiis from the constant are irregular and tlie constant is practicallj. the same whether starting from the tliiocj-anate or thiourea end. T h e coiiclusions arrived a t in the coiirse of tlie inr-estigation are mainl!. these : I. ,Imiiionimn tliiocjminte is T-ery don-ly if a t a11 cliaiiged at temperatiires below I 1 0 3 whether heated alone or in aqueous solution. II. T h e addition of sulphuric acid to the acltieo11s solutioii caiises the disappearance of the tliiocjmiate, tliiourea being formed a t the same time. 111. Heated at temperatures above 130' tlie yelocity of tlie reaction increases rapidly, cloubling in the eight degrees hetweeii I j z " aiid 160". Il'. A I tthese temperatures tlie point of eyiiilibriiiiii reached by heating aiiiiiioiiiiiiii t h i o c p i a t e and by heating thiourea is


the m i l e ; the product containing a little iiiore than 2 0 percent o f thionrea. I-.T h e constant obtained in each reaction is the same.

1-1.T h e change of aiiiiiioiiitiiii thiocJ-aiiate into thiourea is a moiioiiioiecalar one as is also the conr-erse reaction. I desire to express in!. tliaiiks to Professcr Rancroft. for SUKgestiiig this in\-estigation aiid for the interest taken in its progress. C o m e / / Clziz,ci-sirj

The Conversion of Ammonium Thiocyanate into Thiourea and of

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