T H E H Y D R O G E S ELECTRODE I S T H E STUDY O F T H E RATE OF SAPOKIFICATIOS O F OILS A S D F I T S BY AQUEOL-S ALKALI
B Y J . W. X C B A I S , 1%.S. HO
. 4 S D MURIEL T H O R B U R X
The only available mcasurements of the rate of saponification of triglycerides or commercial oils antl fats by aqueous alkali are those of Sorris antl hIcBain’ antl of H . I,angton.* Sleasuremcnts of this type are, however, all open to the serious criticism that vihen the reaction is allowctl to proceed from beginning to end, numerous changes of statc occur. Thus, inspection of the diagrams of phase rule equilibria of soaps noiv availahle bhoivs that six different sets of phases may be present a t different stages of the reaction. S o t only so but the concentrations of all the substances are continually changing including any emulsifiers as well. Hence, there are insufficient data available upon which t o judge whether it might be intlustrially preferable t o carry out saponification in “crutcher;” nvith concentratetl alkali before putting the resulting soap into the soap pan. Previous work shoTvvetI that the rate oi reaction is alinost entirely dependent on the rate of movement of alkali to antl of the products of reaction from the surface of the droplets of oil, and it intlicnatetl the factor- which are involved and even some of their effects. However for m y real atlvance it was necessary o devise a new method such as is tlescrilwl in the pre-ent Investigation. I t has been found possible to vary one factor lit a tinw keeping all the others constantSand thus to isolate antl tleterminc the effect of cach factor. Each of the reagents escept the hylrositle is kept in suflic’ently large excess t o be appreciably constant throughout any one espe1,iment thus keeping all the physical. as well as chemical factors. constant. I t is found t,hat the hydrogen electrode is :ictual:y able to folloIv the rate of disappearance of the dilute hydroxide. This olieys a monomolecular law in so far as hydroxyl ion is concerned ant1 the result< arc > urprisingly reproducible. We have thus a rapid and quantitative method for measuring the rate of saponification under n:l contlitions. l h c quantitative information here obtainetl suhstantiates the conclusions inrlicatetl in the previous communication. I t is now a simple task t o extend thenc results so ns t o obtain representative data for a large number of pure triglycerides as well as commercial materials. The results here recorded refer chiefly t o coconut oil together with a few data for trilaurin and tripalmitin. J. C h m Sac.. 121, 1362 ( 1 9 2 2 ) . Trciilj hns cnllcrl o u r ;itt,,ntion t o papers by himself fJ. C F m . phys., 16, I i j j (1918). 1i-c. Trnv. ckiin ’ 13231. However, they do not mr:isurem(ant of rati‘, but onl\ nnJ mathemstical formulations which in p k t do not find c>onfirmntionin our ex;? * J. Oil and Colour Chrm. Assocn., 5 , 4 1 !192zl. Compare Lxsh Miller: Tr:inn. Roy. 5oc. Canada, 13’ 2, q j (190%; Harcourt nnd Esson: J. Chem. S o i . . 20, 460 :1866).
contain an\- artual
J. W. MCBAIS, H . S . HOWES A S D MURIEL THORBURN
132
Experh e n tal Chemicals used. Standard solutions of sodium hydroxide were made by dissolving sodium drippings, free from carbon dioxide, in boiled out distilled water. The solutions M ere standardieed by titration against standard hydrochloric acid (made by the method of Hulett and Bonner’) and kept in flasks fitted with vaselined stopFers. The oils used were of two kinds-pure and commercial. The pure triglycerides, trilaurin and tripalmitin, were supplied by Kahlbaum. The commercial coconut oil used was obtained through the kindness of Chris. Thomas and Bros. Ltd, and had been carefully neutralised by them. On keeping for many months however, it tended to become very slightly rancid and was therefore again neutralised before use by heating with aqueous sodium hydroxide, and was subsequently washed free from alkali with boiling distilled water. Sodium laurate and sodium palmitate were obtained f:om Kahlbaum. Coconut oil was saponified with alcoholic sodium hydroxide, and soap formed was decomposed with sulphuric acid and the fatty acids thoroughly washed with boiling water and dried a t 100’. From them coconut oil soap was prepared by the method of Bunbury and Martin.2 Method of measurement of rate of reactaon. The hydrogen electrode was coupled with a decinormal calomel electrode
Pt Hz Sapon”.
3 eatd. XC1 ’ 3 satd. KC1 1 o.IX” KC1
’
mixture 90°
satd. with
I HgzC12I H g ~
(solid)
I
90’ I
As the potential of t h e calomel electrode remains constant throughout, the rate of change of E. M. F. of this cell is nearly proportional to the rate of saponification as is shown below, under the heading “Calculation of Results.” A p p a r a t u s employed. Saponification was carried out in a Pyrex, three-necked, round-bottomed flask of goo cc. capacity (see Fig. I ) . -5 stirrer passed through the middle neck. I t consisted of a small silver propeller (made from a silver disc) bound on the end of a glass rod with silver wire, and was driven at a constant speed of 2400 r. p. m. by a small A. C. induct.ion motor. The speed was frequently checked by a tachometer. The other two necks were utilised for the hydrogen electrode and the connection t o the calomel electrode respectively. The reaction flask was about three-quarters immersed in a thermostat at 91’ ( i 0 . 1 0 ) the surface of which was covered with melted paraffin to prevent evaporation. The hydrogen electrode was a slightly modified form of the Hildebrand type (see Fig. Ia). It, was made by welding platinum wire to a piece of platJ. Am. Chem. SOC.,31, 390 (1909) J. Chem. Soc., 105, 417 (1914).
RATE O F S A P O N I F I C A T I O S O F OILS AND FATS
I33
inum foil about I cm. square and sealing the wire into a narrow-bore glass tube. This tube was txo-thirds filled with mercury and connection made by an amalgamated copper wire dipping into the mercury. The outer glass tube was made of glass tubing of about 3” bore. It was closed a t the top by a rubber stopper through which passed the inside tube. h small side-tube, sealed at right angles to the large outer tube ju t below the rubber stopper admitted hydrogen. Three circular holes about I em. in d ameter a t the bottom of the large outer tube provided the hydrogen exit and enabled the reaction mixture
FIG.I Diagram of apparatus used
to have free access to the platinum foil (see Fig. la). The electrode was cleaned by electrolysing it as anode in 3 5 hydrochloric acid solution, and then platinieed by making it alternately anode and cathode with another platinum electrode in a solution of platinic chloride. An E. M , F. of four volts was used and the deposit of platinum black obtained was very fine and thin. Occluded chlorine was then removed by immersing the electrode in a solut,ion of acidified ferrous sulphate and it was finally well washed with d’stilled water. The electrode did not FIG.I a require replating: it was cleaned in hot, acid dichromate Section of hydrogen solution after each experiment and when not in use was kept in electrode. distilled water. The hydrogen supply mas kept constant by means of a screw clip on a rubber tube which connected with a screwvalve of a cylinder of hydrogen. The hydrogen was allowed to pass through a safety flask and a flow indicator, and was saturated with water vapour by passing through a tube containing 0 . IS, sodium hydroxide at, 90’ (i. e. a solution of the same concentration as the alkali initially present in the reaction flask) from which it passed through a spray trap to the hydrogen electrode and thence bubbled through the reaction mixture. X small tube for the outlet of hydrogen passed through the cork of the second side neck of the reaction flask, which also carried the capillary siphon filled with half-saturated potassium chloride that formed the connection between the reaction mixture and an ordinary calomel electrode. The end of the capillary that dip1:ed into the flask was drawn out to a fine
J. W. MCBAIS, H. S . HOWES A S D YURIEL T H O R B U R S
I34
upturned point in order to avoid undue mixing of the potassium chloride with the reaction mixture. To lessen vibrat'on of the capillary, an extension consisting of an ordinary piece of glass tubing was fitted to the neck carrying the capillary siphon (see Fig. I ) . A pyknometer calibrated to contain the required w i g h t of c,3X\;.- sodium hydroxide saturated with hydrogen at room temperature mas used for measurement of the sodium hydroxide added to the reaction mixture. For measurement of E. 11. F. the ordinary compensation method of Poggendorf was used except that it was found more convenient to use a voltmeter instead of the usual standard cell. This n-as a Weston D. C. model and read to 0.001 volt. A cnpillarj- electrometer of the Lippmann type was used in obtaining the balance (see Fig. 113). Experimenlul procedure. In each experiment in this investigation the effect of the concentration of oii and of soap was kept constant by initially adding both these constituents in large excess of the concentration of alkali. The rate during any given experiment was then proportional to the concentration of one reactant only (the alkali); it was then found to lie a niononiolecular reaction. Each experiment was carried out on a basis of j o grams of water. Two thirds of the water mas adtled with the soap and one third with the sodium hydroxitle. All concentrations are expressed in n-eight normalities (the number of gram mols associated with 1000 grams of water). Before every experiment the hydrogen electrode was tested in c. IK,,. sodium hydroxide at room temperature as follows:-
I
Pt, H2 o . I S ,S a O H '
1+
satd. KCl
I
0.1s" IiCl
!
Hg2C12 Hg satd. with solid HgLL All at room temperature If, after bubbling hydrogen through for about ten minutes an E. M. F. of about 1.083 volts (theory 1.089) was obtained the electrode was considered to he in order. The requisite amounts of soap and oil were then weighed into the reaction flask and stirred at 90' for half an hour, hydrogen being bubbled t,hrough the XThole time t o ensure saturation. hfeanwhile the hydrogen electrode was allowed to attain equilibrium in a c.IS,solution of sodium hydroxide at 90'. The potassium chlor.'tle siphon was then inserted and the required weight of saturated standard sodium hydroxide heated by placing the pyknometer containing i t in the thermostat for about five minutes. It was then poured into the hot mixture of soap and oil, the hydrogen electrode inserted and E. M. F. readings taken every one or two minutes according t o the speed of the reaction. Calculation o j resulfs. Since the progress of the reaction was followed by means of a hydrogen electrode, its E. M. F. is directly proportional to the logarithm of the concentration of hydroxyl ions or inversely proportional to the logarithm of hydrogen
RATE O F SAPONIFICATION O F OILS A S D FATS
I3 5
ions, the two being connected through the constant ionic product a t the constant temperature. S o w for a simple monomolecular reaction the rate constant k should be equal to I t times the logarithm of the radio of the initial concentrat on of the hydroxyl ions to that at time t. Hence, since the logarithm of the concentration of hydroxyl ion occurs both in the expression for rate and in the value of the E. 11.F., the rate constant is proportional t o I;t’ times the change in E. 11.F. up to time t. Therefore if the E. >I. F. is plotted against time a straight line should result for a simple monomolecular reaction by the concentration of the hydroxyl ion. Actually however, go\-ernetl solelv .~ althoiigh the rate is monomolecular the reaction is incomplete and reversible OWing to equilibrium being obtained when the hydrol3-sis-alkalinity is of the order of I , 1000. Therefore, although the chonge in E. 11. F. is at first proportional to the time, the E. 11.F.must ultimately tend to a constant value independent of time corresponding to the hydrolysisFIG.i b alkalinity. This is illustrated by a t,ypical Potentiometer circuit. graph in Figure z and we will now proceed AB = meter bridge wire. to describe a very s:mple tievice by which C = sliding contact. all our results can he integratcd lvithout D = 2-volt accumulator. calculation, merely by correcting each E = capillary electrometer. point on the curve by amounts shown in Table I below, If the rate is really proportional to the concentration of hytlros\.l ion this must result in a straight line 3s is shonm in Fig. 2 ant1 the lope of this straight line is the rate constant.
Let t be the eqii lihriurn concentration of OH ions. >’ ., 0 >, ’ ’ a ‘ ’ ” initial s ’ ’ ” amount of OH ions reacting in a time t . k 1 ir ’ . the vrlocit?; constant of the forward reaction. J k? ’ ( * ’ ,. ‘’ ” ’’ back T h o rate o h s r w d is tls tlt = k, (a - s) - k 2 s , I
”
‘1.1IC11
l’rittinp the constant ( : I = n and i a - s) = ( 0 H ’ ) t 111 ( ( 0 f I ’ ) t- $) = In n - ik, = k,)t as k, is small comp:iretl with k, kl k 2 = kl (appros.) i.e. In (tOEI’), - $1 = In n - k,t = const. - k,t
+
Therefore In - l ) plotted against time shoultl givc n straight line to infinity ifathe reaction behaves as an incomplete or reversil~lcmonoinolrcular one.
J. W. MCBAIS, H. S. HOWES A S D MURIEL THORBURS
136
In ((OH’)t - 5 ) expanded 5 = In (OH’)t - -
-
Xow if (OH’)t =
-5
0.24255
IO,
(OH‘)t In ((OH’)t
42 ___
43 - Z(OH’)*t 3(OH‘l3t = In(OH’)t - 1 / 1 0 = ln(OH’)$ - 0.1053 = loglo(OH’)t - 0 . 2 4 2 j j
I,!ZOO
is thus the vertical error on a log (OH’) : t graph where t-(OH’) =
E
1.15
IO
I IO
1.05 IO0
0.95
050
p An
3
O I
70
>O
40
50
60
70
FIG.z Typical tirne/E. 11.F. graph with 2 S , coconut oil soap, SaOH at 90” showing correction for hydrolysis.
I
S,. coconut oil and
0 . 1 STY
It is now necessary to express this vertical error in terms of E. M. F.
+
(barometric correction - (correction to normal H? electrode from calomel electrode) E (COIr.; = 0.000198j T log I/(”) = 0.0001895 . 363 . log I;(”) = 0 . 0 7 2 0 6 log I,/(”) But (H’) times (OH’) = K, Therefore E(,,,,.) = 0.07206 log (OH‘)/K, = 0.07206 (log(OH’) - log K , ) Let Et he the hydrolysis E. M. F. Then Et = 0.07206 log 4 - 0.07206 log Kw and E (obs.: = 0.07206 (log(OH’)t - log K,! - E = 0 . 0 7 2 0 6 (log(OH’)t - log 5) therefore E
B
(COrr.)
=
E(”bs.)
RATE OF SAPONIFICATIOS OF OILS AXD FATS
I37
Correction to E (ob..) is therefore 72.06 times correction to log (0H’)t where (&.> is in millivolts. The following table was drawn up for different values of
E
E 20 IO
8 5 3
(obs.)
-E
(millivolts) E 93.75 72.06 6j.07
TABLE I
Correction to log(0H’)t o.oj2
50.35
0.242jj 0.307 0.j I 4
34.38
0.993
Vertical correction (millivolts) 3.747 17.48 22. I2
37.04 71.55
FIG.3 Dependence of the rate constant (k) upon the concentrations of oil, soa and S a C l respectively. The standard experiment contained I S , oil, I S, soapbut no S a & . Sormalities in excess of this are measured to the right and deficiences to the left.
To apply the vertical correction a point. is found on the graph where the E. AI. F. is greater than the final equiIibrium E. 31. F. by an amount corresponding to a number in column 2 . The corresponding number in column 4 is then the amount by which this point has to be lowered verticallj- to put it in its corrected position. The slope of the straight line so obtained is the rate constant of the reaction expressed in electrical units. I t is seen from Figure 2 t h a t a st,raight line do2s result from applying this correction and its slope; namely, 19.2 j is the rate constant, in millivolts per minute. Experimental Results Each experiment was carried out a t least in duplicate and the results under favourable conditions usually agreed within a few per cent. In the case of some of the later experiments where salting out of t,he soap took place such close agreement was not possible owing to the thick consistency of the reaction mixture which rendered efficient stirring impossible.
J. W. MCBAIN. H. S . HOWES AND MURIEL THORBURS
138
The f i s t series of experiments was carried out to test the effect of varying the amount of oil in the reaction mixture. I n every case the oil was in sufficient excess for its influence to be constant during any one experiment. The results presented ,'n Table I1 and Fig. 3 show that the rate constant is greatly dependent upon the first small portion of oil and that it becomes only slightly greater as the amount of oil is furthei increaheci. This is very different from the expectations of Treub (loc. cit.), but these actual esperiments leave no doubt with regard to this result.
TABLE I1 (see Fig. 3 ) Comparison of thc rate of saponification of coconut oil when the concentrations of soap (made from coconut oil) and initial sodium hydroxide are kept constant but the volume of oil is varied. Soap I . ON, ,I
,, ,,
Initial concentration of SaOH 0.
IXw
,,
1,
,>
Oil
2s,
0
0.5 I
.0
2 . 0
(millivolt li minute units) j . j8
5.53 6.0 8.0
In the second series of esperiments the effect of increase of concentration of soap was similar y tested. It will be seen from the results in Table I11 and Fig. 3 that t,he increase in the rate constant is more rapid under these conditions than under any others studied.
TABLE I11 (see Fig. 3) Comparison of the rate of saponification of coconut oil when the concentrations of oil and initial sodium hydroxide are kept constant but the concentration of coconut oil soap is varied. Soap
Initial concentration of KaOH
Oil
(rnilliT,olt k minute units)
o.2xw
0 . Ili,
1 . OK,
0.Tj
0.41 I .o 2.0*
,>
1
,I
,)
>,
>>
I.
13
6.0 19.2j
*This mixture was so viscous that difficulty \\-as experienced x i t h stirring.
Table IV gives t,he results of experiments to find the effect of adding glycerine to the initial reaction mixture. -4large escess of glycerine lowers the rate constant. The effect however is small in comparison with the proportion of glycerine added. Presumably glycerine slightly diminishes the emulsifying power of soap hecause t,he latter is less colloidal in presence of glycerine.
I39
RATE O F SAPOSIFICATIOS O F OILS A S D FATS
TABLE IV Comparison of the rate of saponification of coconut oil when the concentrations of oil, coconut oil soap, and the initial concentration of sotlimi hytlroside are kept constant but the concentration glycerine is varied. soap I.
(milliyolt k minute units)
Arine 0.
IS,.
j s
-
I .os,.
>,
I
6.0 4.7
.os,
Table \- gives the results of a comparison of the rnte of saponificntion of coconut oil with that of trilaurin. Trilanrin is the chief constituent of coconut oil antl, as would therefore be esgected, the rate constants arc very comparable.
TABLE Y Comparison of the rates of saponification of coconut oil, trilanrin, tripalmitin and tristearin Oil soap
Coconut * Tr:laurin* Tripalmitin* Tristearin
Initial concentration of SaOH
IS,.
Oil
(millivolt k minute units)
I . os,,-
0.
I .8S,-
1.0
0. I
I
I .0
0.I
I .0
.o
0.I
I .o
1.0
0 .I
I .0
IO
I .o
0.
I
1.0
Ij.3i
I
.8
6.i** 7 . 2
6t I0.j
6t
*These results %re due to Miss Harrington. **Interpolated. tThese results are due to .\. V. Pitter.
It is seen from Pitter's results (comprising twenty esperimentsi that the rnte of reaction rapidly increases in passing from trilnmin upwards in the homologous series, being douliletl when tristearin is reached. This however does not necessarily mean that higher oils and fats are intrinsically more r:ipidly attacked but must lie largely clue t o the simultaneous change in the soap present as emulsifying agent. I n other words the highest so:ilis are the best soaps a t high temI:erat,ures. A coinprehensix-e series of esperimenta will he carried out in which the soap is kept constant antl only the oil or fat varied. I n order t o test if the size of the platinum electrode affected the results, an esperiment was carried out with I.oS,coconut oil, I.oS, coconut oil soap and o.~?;, sodium hydroxide using an electrode roughly .;or; larger in area than that used in a11 other esperiments. -1rate constant of j . 7 was obtained insteatl of 6.0 (stantlard esperiment), I n all the foregoing experiments two phases only were present during the whole of the reaction, aqueous phase and oil phase. these were maintained in intimate contact by t,he rapid stirring. In the following esperiments salts was added a t the commencement of the reaction, rendering the physical state of the mixture much more complex, and in many experiments the viscosity
140
J. W. MCBAIN, H. S. HOWES A S D MURIEL THORBURN
was increased to such an extent that stirring was impossible. The results are collected in Table VI and VI1 and are graphed in Fig. 3 . The physical state of the reaction mixture and the phases present are described in the last column of each table. The general form of the curve appears to be the same in the case of coconut oil and that of the very different oil, pure tripalmitin, but further experiments are required to confirm the exact shape of the curve.
TABLE VI* (see Fig. 3) Comparison of the rate of saponification of tripalmitin when the concentrations of oil, sodium palmitate and the initial concentration of sodium hydroxide are kept constant but sodium chloride is added in increasing amounts. Initial concentration of Concn. of k Appearance of reSoap
SaOH
I.ON,
O.IS,
I1
I,
,, ,,
lj
,,
Oil
SaCl
action mixture
-
I.OK,
,,
0,
,,
I
I,
IO. j
sKn.
33.3
.03
Ij
.8
Homogeneous soap, high viscosity. Heterogeneous soap.
39.5
1.25
,,
lj
*These results are due to Miss Harrington.
TABLE VI1 (see Fig. 3) Comparison of the rate of saponificat,ion of coconut oil when the ooncentrations of oil (1.09,) s m p (I.oS,") and the init,ial cmcentration of sodium hydroxide (o.IN,") are kept constant but sodium chloride is added in increasing amounts. Concn. of k Appearance of reaction mixture Phases NaCl
I . ON,
6.0 11.1
1.5
14.5*
2.0 2.5
3.0
14.8 14.8 I4
3.5 4.o*+
(12)
5.0
(18) 16.2
6 . 7 (satd.)
(18)
Homogeneous soap solution. Homogeneous soap solution oil in granules f,
I,
12
>,
,
,I
,I
,,
>)
,t
,,
ff
Heterogeneous soap, oil in granules.
,,
1,
,, ,,
It
,,
I,
f,
))
I,
,,
1,
I)
I,
,f
,,
Nigre only
,, ,,
I, :t
9,
),
)>
11
Neat soap &- lye Curd and lye
,,
,,
I,
f)
1,
11
*When x.oSW glycerine was present the rate was 12.4 instead of 14.5 thus confirming the result shown in Table IV above. *'It was found difficult to obtain reproducible results between concentrations of g 0 . S ~ and s.oNw,sodium chloride, probably owing t o the very complicated physical state of the react'ion mixture.
RATE OF SAPONIFICATION OF OILS AND FATS
141
Discussion of Results The results given above are quire sufficient to illustrate that a quick and reliable method has been established for the comparative measurement of rate of saponification in heterogeneous reactions. Further, definite information has been gained with regard t o the nature of the reaction. It is evident from the results of Table I1 that doubling the amount of oil present does not double the rate constant as might have been expected. Thus we can conclude in accordance with the direct observations of Norris and McBain (loc. cit.) that a given amount of soap has a limited emulsifying action, since the surface of the oil evidently is not proportionately increased when the volume of oil is doubled. It was found that only a portion of the oil was emulsified, the remainder settling out immediately on standing. This conclusion is supported by the results of Table I11 which show the effect of increasing the amount of initial soap. The rate constant is rapidly increased up t o a concentration of 2Nwsoap beyond which concentration the reaction mixture becomes so viscous that the exFerimenta1 method breaks down. The only possible explanation of this increase in rate of reaction is that the oil is more perfectly emulsified when more initial soap is present. This is in accordance with observations of surface tension against oil which is indefinitely lowered as the concentration of soap is increased. When we consider the results of Tables VI and VI1 from experiments where salt was added a t the commencement of the reaction, the conclusions to he drawn are not quite so certain. It is well-known that soap boilers add solid sodium chloride to their reaction mixture during saponification in order t o ‘‘salt out” the soap thus rendering the whole mass much less viscous and more manageable, Norris and McBain (loc. cit.) found that the presence of salt sometimes increased the rate of saponification and at other times decreased it according to the kind of oil used, the concentration of salt added and the physical state of the reaction mixture. However, no conclusions could be reached definite enough to enable them t o predict the rate under any given set of initial conditions, owing firstly to their inadequate method of experimental procedure to which previous reference has been made, and secondly t o the very limited amount of data then available with regard t o the conditions governing the equilibria in the complicated ternary system soap: electrolyte: vater. Since 1 9 2 2 however our knowledge in this direction has been greatly increased. Phase rule diagrams for the systems sodium palmitate: sodium chloride: water,’ potassium oleate: potassium chloride: water,* potassium laurate: potassium chloride: water,$ have been mapped out, and further, IT.J. Elford, working on a semi-technical scale has proved conclusively that the equilibria occurring in a mixture of commerical oils with electrolytes, are exactly similar to those found in the systems of pure chemicals mentioned above.4 RlcBain and Langdon: J. Chem. SOC., 127, 852-870 (1925). McBain and Elford: J. Chem. SOC., 1926, 421. a McBain and Field: J. Phys. Chem., 30, 1j45 (1926). 4 See Jerome Alexander’s “Colloid Chemistry,” 1’01. I, Chap. V, by 3. W. McBain.
J. W. MCBAIN, H. S. HOWES A S D MURIEL T H O R B U R S
142
In the light of this new knowledge it was determined to attempt, to apply our new method of investigation of saponification to the systems resulting from the addition of sodium chloride to the mixture of oil, soap and aqueous alkali. From Fig. I of McBain and Langdon's paper (loc. cit.) we know that the addition of salt in increasing quantity to a homogeneous 0 . jN, solution of sodium palmitate a t goo produces no change in the homogeneity of the solution until a concentrat'ion of about I.oN,. sodium chloride is reached. The solution then divides spontaneously into two (or possibly three) liquid layers (neat soap and nigre. or neat soap, nigre and lye). A further small increase in the concentration of salt suffices to convert this into two liquid layers, neat soap and lye. Miss Harrington when saponifying pure tripalmitin in presence of sodium palmitate and sodium chloride in this laboratory (see Table VI), chose her concentrations of salt so that the first and the third types of equilibria were represented. Her experiments were carried out in duplicate and the general form of the curve is that shown in Fig. 3 . With the soap in homogeneous solution the rate constant is greatly increased by addition of salt. When sufficient salt has been added to produce neat soap: lye k is halved, but rapidly regains a higher value even within the neat soap: lye region. The curve with coconut oil and coconut soap (Fig. 3) is the results of a more extensive series of experiments carried out by one of us (M. T.). Here again by utilising the unpublished diagram of K. J. Elford for coconut oil soap and sodium chloride it was possible to choose concentrations of salt which produced equilibria corresponding to those esisting in the solutions of sodium palmitate: sodium chloride: water studied. With this oil also the rate constant increases while the soap and salt are in homogeneous solution anti drops as soon as liquid layers are formed. Further addition of salt again increases the rate constant to above its previous maximum value and the high rate is maintained xvhen the soap is grained out and the sodium chloride is added up to saturation. The general conclusions to be drawn from the rcsnlts so far obtained are reserved for a subsequent communication dealing with esperiments in more concentrated solution. Summary
A rapid and convenient method has been developed for the measurei. ment of rate of saponification of oils by aqueous alkali wherehj- the influence of each factor can be isolated and measured independently. d very simple graphical method suffices to integrate the results. The rate is in all cases proportional to the concentration of hydroxyl 2. ions as measured by the hydrogen electrode. 3 . The first small amount of oil greatly enhances the rate, probably because it is all emulsificd, whereas further additions have a relatively slight, effect. 4.
The rate is more t,han proportional to the amount of soap in solution.
RATE OF SAPONIFICATION OF OILS A N D FATS
I43
5 , The effect of salt depends upon the physical state of the system but the fastest rates have been observed in presence of concentrations of sodium chloride approaching saturation. 6 . Addition of glycerine slightly diminishes the rate. 7 . Under comparable conditions tripalmitin is more rapidly saponified than coconut oil, in agreement with 3 and 4 above. Department of Physical Chemistry, T h e University Bristo2, EnglaAd. October 16, 1926.
