1432 BOUSFIELD AND LOWRY: THE PURIFIOATION AND CLXI1.-The PuriJication ccnd Properties of Acetic Acid. By WILLIAM ROBERT BOUSFIELD, M.A., K.C., and THOMAS MARTIN LOWRY, D.Sc. ON account of its convenient melting point, boiling point, and other physical properties, acetic acid has been used extensively as a material for physico-chemical investigations, such as those of Oudemans on the density of solutions (Zeitsch. Chem., 1866, 2, lSO), of Ramsay and Young on vapour pressures (Phil. Trans., 1884, 175, 469) and on vapour densities (Trans., 1886, 49, 790), and of de Visser on the influence of pressure on melting point (Rec. trav. chim., 1893, 12, 101). It has frequently been assumed that a pure acid can be obtained merely by freezing the Liquid and pouring away the unfrozen mother liquor from which the crystals have separated. In recent years this assumption has been proved to be false, and it has become a matter of some importance to obtain a more efficient method of purifying the acid, and to redetermine several of its physical constants by measuring the properties of the purified material. It is, indeed, remarkable that in spite of its extensive use in cryoscopy, doubt should still exist as to the true freezing point of t h e acid; still greater uncertainty is found in the cam of other physical properties, and the conductivity data appear to be entirely untrustworthy. Downloaded by McGill University on 26 August 2012 Published on 01 January 1911 on http://pubs.rsc.org | doi:10.1039/CT9119901432 View Online / Journal Homepage / Table of Contents for this issue
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1432 BOUSFIELD AND LOWRY: THE PURIFIOATION AND
CLXI1.-The PuriJication ccnd Properties of Acetic Acid.
By WILLIAM ROBERT BOUSFIELD, M.A., K.C., and THOMAS MARTIN LOWRY, D.Sc.
ON account of its convenient melting point, boiling point, and other physical properties, acetic acid has been used extensively as a material for physico-chemical investigations, such as those of Oudemans on the density of solutions (Zeitsch. Chem., 1866, 2, lSO), of Ramsay and Young on vapour pressures (Phil. Trans., 1884, 175, 469) and on vapour densities (Trans., 1886, 49, 790), and of de Visser on the influence of pressure on melting point (Rec . trav. chim., 1893, 12, 101).
It has frequently been assumed that a pure acid can be obtained merely by freezing the Liquid and pouring away the unfrozen mother liquor from which the crystals have separated. In recent years this assumption has been proved to be false, and it has become a matter of some importance to obtain a more efficient method of purifying the acid, and to redetermine several of its physical constants by measuring the properties of the purified material. It is, indeed, remarkable that in spite of its extensive use in cryoscopy, doubt should still exist as to the true freezing point of the acid; still greater uncertainty is found in the cam of other physical properties, and the conductivity data appear to be entirely untrustworthy.
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Maximum Acetic Acid as a Standa-rd of Electrolytic Conductivity.
Our attention was first directed to the problem nearly ten yeaxs ago, when an attempt wits made to utilise the acid as a standard of electrolytic conductivity. The electrolytes usually employed for calibration include not only solutions made up to a measured con- centration, such as 21 per cent. N-, N/10-, N/50- , and N/100- solutions of potassium chloride, but also Saturated solutions, such as thme of s d t and gypsum, and solutions having a maximum conductivity, such as those of sulphuric acid, magnesium sulphate, and acetic acid. The saturated’solutions cannot be regarded it9
very satisfactmy, on account of their slowness in recovering from changes of concentration consequent upon exposure to variable atmospheric temperatures ; but solutions of maximurn conductivity are very convenient to use, as their conductivity is not appreciably affected by small errors in concentration. Two of these solutions,
are in constant use, but the third, Acetic acid ..................... 16 per cent. q 8 = O 0016 mho.,
proposed by Kohlrausch in 1876, has been abandoned as untrust- worthy, in spite of the great convenience that would attend its use in the calibration of vessels of small resistance capacity.
The reason for this abandonment was at once clear when we attempted to use tho acid for calibration, taking for its maximum conductivity the value K , ~ = 0*001619 given in Kohlrausch and Holborn’s “ Leitvermogen der Elektrolyte,” p. 155. In the case of one particular vessel, the values obtained for the resistance capacity were as follows:
KCI 21 per cent. 1.969. KCl N/10 1.966. NaCl (saturated) 1.9.71. KC1 N 1.971. KC1 N/100 1.969. Acetic acid 1.944.
The last observation, unless due to some unsuspected error, showed that the conductivity of the acid was considerably higher than had previously been supposed, namely 1640 gemmho, instead of 1619. A second sample of acid which had been frozen and drained behaved in the same way, giving a maximum conductivity of 1645 gemniho. As these values differed so seriously from thme hitherto accepted as correct, a systematic investigation was under- taken in order to ascertain the source of the discrepancy, as well as the h u e value for the maximum conductivity of the acid. In this investigation we had the advantage of using a far more delicate -test of the purity of the acid than those which had been employed
previously, since the course of the purification could be followed with ease and certainty by measuring the maximum conductivity of tlie different samples.
Fmctional Distillation of A c e t i c Acid. Fractional distillation showed at once the presence of substances
of high boiling point and low conductivity and of low boiling point and high conductivity. An attempt was therefore made to frac- tionate a pure acid from a large sazr.ple of about 10 litres. The first distillation gave eight fractions, ranging in maximum con- ductivity from 1833 to 1655 gemmho. When redistilled with a 12-pear still-head, tho first portion ( K 1833) gave fractions ranging in maximum conductivity from 2030 to 1658 gemmho, whilst the last portion ( K 165b5) gave fractions ranging from 1714 to 1560 gemmho. The last fraction ( K 1560), when redistilled, gave portions of maximum conductivity 1644, 1624, 1600, and 1500, leaving behind a residue of about 10 grams having the disagreeable odour of the higher fatty acids.
Fractional distillation had thus resolved the acid into portions ranging in maximum conductivity from 1500 to over 2000 gemmho, but a t no poiut was there any evidence of a steady value being reached, every portion examined being found to be resolvable even after repeated distillation into fractions of different conductivity.
Purijication by Freezing. Whilst the fractional distillation of the acid was in progress,
attempts were mado to separate a pure acid by freezing. Two of the intermediate fractions were selected and allowed to crys- tallise; the mother liquor mas poured off from the crystals, and both were diluted to 16 per cent. in order to measure their maximum conductivity. The maximum conductivity values obtained were as follows :
Mixture of fractions from 1625-1650. Frozen out ............ 1636 Unfrozen ............ 1636.
Mixture of fract,ions from 1650-1675. Frozen out ........... 1652 Unfrozen ............ 1652.
The conductivity of the acid was therefore not affected by frac- tional freezing, and this method could not be used to prepare a pure acid of definite conductivity. A preliminary freezing is, however, of considerable value in reducing the quantity of impurities to be dealt with by the more efficient methods of purification which are described below.
and perhaps other acids of higher molecular weight. On account of its high conductivity, formic acid would be by far the most dangerous impurity, and attempts were therefore made to get rid of it by chemical methods. The low-boiling fraction of maximum conductivity 1987 was distilled from potassium dichromate with the idea of removing the formic acid by oxidation, but the distillate still gave the high value 1923, and was evidently far from pure. A second attempt was made to remove the formic acid by distilling from sodium, in the hope that the stronger acid might thus be retained; this method was also a failure, the distillate having a maximum conductivity 1957. Success was finally achieved by the use of potassium permanganate, which reduced the maximum conductivity of a sample from 1753 to 1644 by a single distillation.
Further tests showed that by the use of potassium permanganate an wid of definite properties could be obtained without difficulty from any of the crude commercial samples. In the table which follows, C refers to a commercial &id which had been frozen out and drained in order to remove a part of the water, as well as a certain proportion of impurities of high and low boiling points. The acids R and B were derived from the fractional distillation described above; it should be noted that the original acid was an excep- tionally bad sample and had not been subjected to any preliminary purification by freezing; the portions A and 13 were almost the extreme fractions, and contained in a concentrated form the impurities of high- and of low-boiling point present in the 10 litres of acid used for the fractionation; they therefore provided a very extreme test of the methods which we have devised for purifying the acid. The course of the purification is shown in the following table :
a. B. c'. Initial maximum conductivity 1650 1780 1646
Acid distilled from permanganate with 12-pear still-head : First portion ..................... 1646 1808" 1640 Becond portion .................. 1643 1640 1639 Third portion.. ................... 1623 1638 1640 Fourth portion ................. - - 1639
* More permanganate added after this portion had been collected.
First portion ..................... *1637 (120 c.c.) 1640 (200 c.c.)
Third portion ..................... 1638 (750 c.c.) 1639 (750 c.c.) Fourth portion .................. *1630 (80 C.C. to dryness)
First portion ..................... 1640 (600 c.c.) - Second portion ................... 1640 (500 c.c.) - Third portion .................. 1639 (500 c.c.) -
Redistilled from a few crystals of permanganate:
Second portion ................. 1641 (1000 c.c.) 1640 (750 c.c.)
Again distilled :
Rejected on subsequent distillation.
It will be seen from the above table that a single distillation from permanganate with a 12-pear still-head was sufficient to produce an acid of definite propertim, except in the ca98 of a sample in which impurities of higher boiling point had been accumulated deliberately. Even here the quantity of high-boiling impuritim was only suf3cient to lower the conductivity of the acid by one or two parts in a thousand, but three distillations were required before the final fraction of acid was found to give a normal conductivity maximum. In the case of the acid in which impurities of lower boiling point had been accumulated, the only modification required was to add a rather larger quantity of per- mangamate in order to secure their complete oxidation.
It should be noticed that the v a l u s given above for the maximum conductivity are only relative; the true value of the- maximum conductivity is discussed in a later section.
The Freezing Point of Acetic Acid. The acid, purified it9 dmribed above, still contains water, which
can be removed by freezing and draining. This operation was carried out on a relatively large scale in order to avoid any lowering of the freezing point consequent on the hygroscopic character of the acid. Two Winchester quarts of the purified acid were used, and when sufficiently reduced in bulk were poured into a single bottle and again frozen repeatedly; the quantity remaining for the final determination of the freezing point-was not less than 1500 C.C. A device of some value, especially in the later stt ip of the fractionation, consists in allowing a small lump of the solid acid to remain unmelted as a nucleus for the further freezing of the acid; glaciation then proceeds smoothly over the sides of the
bottle, the thickness of the coating increasing steadily until only a small cone of liquid acid remains in the centre; when the p r e portion of water is smdl, the frozen acid is perfectly transparent and compact, and the quantity of water mechanically retained is very much smaller than when freezing is started by shaking the bottle or by introducing a crystal into the cold acid. The freezing point of the liquid acid can be determined at any stage of the freezing by breaking through the crust of acid and lowering a thermometer into the central cone of liquid.
The following table shows the results of successive freezings of the purified acid: Initial F.P. ... 16.31" 16.45 16-55 16.58 16'59 16.595 16.60 Final F.P. ...... 15.98 16-07 - 16'49 16-49 16'54 16'66
The quantity of acid remaining unfrozen when the "final freezing point" wm taken varied from 100 to 200 C.C. in the case of the first five freezings. For the last two freezings the volume of unfrozen acid was only about 20 C.C. from an initial volume of 1500 C.C. I n the last freezing the difference between the initial and final freezing points was only 0.04O.
Since each 0.1 per cent. of water lowers the freezing point by 0'2014O (de Visser, Zoc. ci t . , p. 118), the amount of water remaining in our acid would be 0.02 per cent. on 20 c.c., or 0*0003 per cent. on the total bulk of 1500 c.c.; tho corresponding error in tihe freezing point would be about 0*0005°. The thermometer used was standardised from one which had been calibrated a t the Reichsanstalt, and more recently with very great care a t the National Physical Laboratory, the readings of which could be relied on within 0*005O; we consider that our determination of the freezing point of acetic acid is correct wit.hin the same limits at 16.60 +_ 0-005O.
In this view we are confirmed by the fact that our value is in agreement with the figure 16-5965O (dt/dp=0*02435) given by de Visser (Zoc. ci t . ) in his investigation of the influence of pressure on freezing point. In them experiments I0 kilos. of acid were frozen fractionally during a, period of seven months, until the difference of freezing point between the acid and mother liquor did not exceed 0'0004° when only 1/80 of the acid remained unfrozen. These experiments on the freezing point of acetic acid are obviously by far the most accurate that have been made: the lower values given by almoet all other observers are to be attributed to imperfect purification ; occasional higher values, for example, 16'7O or 17O, appear to be only rough approximations.
The boiling point of the purified acid wits determined with the help of a thermometer graduated in tenths of a degree on an open scale from 75O to 125O. The thermometer had been standardised recently at the National Physical Laboratory, the corrections being given to 0'01O at intervals of so. I n order to reduce the correction for exposed stem, the flask containing the acid was provided with a still-head 40 cm. long, and the thermometer was arranged so that the mercury thread rose only just, above the level of the cork. Two hundred C.C. of acid wereused, and it was found that three-quarbrs of this quantity distilled between 118*10° and 118.12O under 768.2 mm. pressure, after applying a correct,ion of -0'05O for thermometer error and + 0'04O for exposed stem. The barometer- height reduced to Oo and 45O latitude was 766.0 mm., and the correc- tion to be applied to the boiling point may be taken as 0'038O per mm. (Ramsay and Young, Trans., 1886, 49, 806, give 0'032O; Schmidt, Zeitsch. physiknl. Chem., 1891, 7 , 433, gives 0.043O; Kahl- bmm, Zei t sch . physikal. Chent. , 1898, 26, 577, gives 0'038O). The corrected boiling point of the acid may therefore be taken as 118*1l0 $_0*02O under 766 mm. (corr.) and 117.88 + 0 * 0 5 O under 760 mm. (corr.) pressure. The substantial accuracy of these observa- tions and of the corrections applied in reducing the boiling point is shown by the fact that two preliminary observations under 750 mm. pressure, in which the stem correction was nearly half a degree, gave values for the corrected boiling point which were within 0'02O of that recorded above.
Our value for the boiling point is considerably below those hitherto accepted, practically all of which have been above 118O. I n particular it may be noted that Ramsay and Young (Zoc. c i t . ) give the value 118,5O, Schmidt (Zoc. c i t . ) gives the value 119'2O, and Kahlbaum (Zoc. c i t . ) the value 118-6O. The conclusion may there- fore be drawn that the last traces of impurity which are removed by the methods we have adopted are substances which have the normal effect of raising the boiling point, as well as lowering the freezing point, of the acid. It should be noticed, however, that the boiling point soon falls below the normal value if the acid is dlowed to absorb water; a low boiling point is therefore not a trustworthy indication of the purity of the acid.
J J J, .................. lr,o)150 Ramsay (I 886) ............... 20"/4" de Visser (1893) ............... 16'6"/4" Jones (1894) .................. 16-5"/4"
with those of other observers,
d. d 18O/4O. Diff. 1.0553 1.0516 +0*0001 1 *0568 1 *0530 -t- 0.0015 1'05704 1.0532 t 0*0017 1'0491 1.0516 +0'0001
1-0534 1,0516 f0'0001 1.05315 1'05143 - O.OOOG5
It will be seen that our value is, as in the case of the freezing point, in very close agreement with that of de Visser; the difference, amounting only t o 0*00005, is perhaps due to the increased accuracy which we have secured by the use of a large pyknometer of special design, and not to any difference in the quality of the acid. Three observers using acetic acid purified in the ordinary way have given the density as 1.0516, a value that exceeds ours by one unit in the fourth decimal place, and must be attributed to the same impurities which gave rise to the lower freezing points, 16'4O to 16.5O, recorded by these authors. The chemical methods of purification adopted by Perkin have raised his values for the density by more than 0.001, and must have given an impure product, containing, possibly, appreciable quantities of acetic anhydride.
The Mazimum Conductivity of Acetic Acid.
I n order to determine 6he maximum conductivity of the acid, a vessel was calibrated by means of standard solutions of potassium chloride, using the values for the conductivity given by Eohlrausch and Holborn (" Leitvermogen der Elektrolyte," p. 204), namely :
These solutions were prepared by diluting to a known volume weighed quantities of a normal solution prepared by dissolving 74.59 grams of potassium chloride to 1044.92 grams of solution, according to the directions given by Kohlrausch, and not accord- ing to the more recent values for the equivalent of the salt. The
salt had been crystallised several times from conductivity water until it showed no trace of sodium in the flame test. The water used had a conductivity of 1 . 5 ~ 10-6, which was added to the above values.
The vnlnes deduced for the capacity of the vessel were: Ftoiii hL’1 N/SO. C=0*2466, From KCl N/lOO. C =0*2466,,.
This gave for the maximum conductivity of the acid the figure:
the temperature-coefficient being deduced from a series of observa tions in the neighbourhood of 1 8 O .
I n determining the maximum conductivity, the Eohlrausch “ wliccl-bri4ce ” ~ 7 2 5 u s d , with an alternating current and telephone. The conductivity vessel contained about 40 C.C. Ten C.C. of acid were diluted in a beaker with about 40 C.C. of water from a burette; the conductivity was measured, and the observations were repeated after each of a series of successive dilutions. Thus, in the following serics of observatioiis, 5 iiiiiiiiiiiiiri bridge-mading and a maximum Watcr :~cided ...... .. 44 4 6 4h 50 1,1 52 54 57 c.c Hridg.e-re;ldiug ......... 601.9 601.6 601.1 601.0 601.0 601.0 601.1 601.5.
conductivity were reached when 10 C.C. of acetic acid were diluted with 51 C.C. of water; the bridge-reading 601.0, after making the necessary corrections, gives the value for the maximurn conductivity set out above.
Practical Methods of Purification. Having found a method by which acetic acid of the highest
degree of purity may be prepared, and having established a reliable value for the maximum conductivity of the acid, it was of interest to determine what amount of purification is needed to produce an acid which can be relied on to give a correct value for this maximum. After testing a number of acids we are able t o state that a commercial acid, when purified by freezing and pouring off the liquid portion in order to reduce the amount of water and of oxidisable material, can be relied on to give a correct value if distilled slowly from 2 per cent. of its weight of permanganate, using a 12-pear still-head to retain acids of higher boiling point. I n practice the acid can be evaporated to quite a small bulk, but if a standard acid is required it is convenient to reject the first 35 per cent., which contains an excessive proportion of water, and also the last 25 per cent., which may contain homologous acids. The following tests on three of Kahlbaum’s acids may be regarded as typical of the observations made.
A. Xaldbaum’s Acid, “ 100 per cent.”-About one-third of the
acid was melted and poured off; this gave a maximum conductivity 1643 gemmho, and is therefore nearly pure, but cannot be used as a standard without purification. B. KahLlbaum’s Acid (‘ Free from Higher Homologues.”-The
original sample, after pouring away about one-quarter from the partly frozen acid, also gave a maximum conductivity 1643 gemmlio. The last 150 C.C. from the distillation of 1500 C.C. of this acid (using a still-head and 1 per cedt. of permanganate) gave a normal maximum 1641 gemmho; the whole of the acid could therefore be used a,s a standard after distillation. C. Eahlbaum’s Acid, 99-100 per cent.-About onethird was
poured away from the partly frozen acid; 2 per cent. of permangan- ate was added, and the acid was distilled through a 12-pear still- head until only a small residue (containing manganese acetate) was left. The first and last fractions showed a normal conductivity 1641 gemmho, and the whole of the distillate was therefore available for standard acid.
From the above observations, as well as from those quoted earlier in the paper, it is clear that a pure acid can be prepared very readily with the help of potassium permanganate. The original acids usually contain oxidisable material, which raises the maximum conductivity, but this is easily removed by the methods now described. Acids of higher boiling point do not produce any marked error in the maximum conductivity unless (as in our fraction “ A ”) they axe present in altogether abnormal quantities; with a 12-pear still-head the acid can be boiled almost to dryness without spoiling the acid for use as a standard of conductivity.
Orton has recently shown that an acetic acid, which is resistant to bromine and which dots not give the Adamkiewicz reaction, can be prepared by distilling the commercial acid from phosphoric oxide-one of the two methods used by Perkin in 1884 to purify the acid from moisture. Although this method of treatment is liable to give rise to an acid of lower freezing point and higher density, we have found a normal value, 1642 gemmho, for the maximum conductivity of a specimen kindly sent to us by Professor Orton. We are also indebted to him for the information that a specimen of our acid, which had been prepared in 1902, and might therefore have deteriorated by keeping (it has been suggested that acetic acid oxidises spontaneously to glyoxylic acid), was resistant to bromine, and did not give the Adamkiewicz reaction.
ST. SW~THIXS, 130, HORSEFERRY IZOAD, HENDON, N. W. WE~TMIMTER, S.W.
