432 XXXVIII.-On the Heat Devetoped in the Combination of Acids and Bases. (Second Memoir.) By THOMAS ANDREWS,M.D. F.R.S. Hon. F.R.S.E. Vice-President of Queen's College Belfast. [From the Transactions of the Royal Society of Edinburgh Session 1869-70.1 INa paper communicated to the Royal Irish Academy in 1841 I gave an account of a large number of experiments on the heat disengaged when acida and bases taken in the state of dilute solution enter into conibination and when bases in- soluble in water are dissolved in dilute acids. The following general conclusions or laws were deduced from those experi- ments :-Law 1.-The heat developed in the union of acids and bases is determined by the base and not by the acid the same base producing when combined with an equivalent of different acids nearly the same quantity of heat ; but different bases different quantities.-Law2.-When a neutral is converted into an acid salt by combining with one or more atoms of acid no change of tem-perature occurs. . Law 3.-When a neutral is converted into a basic salt by combining with an additional proportion of base the combina- tion is accompanied with evolution of heat." Three years later I laid before the Royal Society of London the results of an experimental investigation of the heat deve- loped when oiie base is substituted for another in chenzical compounds. The law deduced from this inquiry is implicitly involved in the foregoing of which it may indeed be regarded as a necemary consequence. It was enunciated in the following terms :-Law 4.-When one base displaces another from any of its neutral combinatioiis the heat evolved or abstracted is always the same whatever the acid element may be provided the bases are the same.? Finally the law of metallic substitutions first announced in * Transactions of the Royal Irish Academy vol.xix p. 228. t Philosophical Transactions for 1844 p. 21. ANDREWS ON THE HEAT DEVELOPED ETC. the “ Philosophical Magazine ” for August 1844 was thus stated in a paper published in the ‘‘Philosophical Transactions ” for 18425. Law 5.-Whelz an equivalent of one and the same metal replaces another in a solution of any of its salts of the same order the heat developed is always the same; but a change in either of the metals produces a different development of heat.In 1845 a paper appeared by Graham on the heat disengaged in combinations the second part of which refers to the heat produced when hydrate of potash is neiitralised by different acids.* The results arrived at by this distinguished chemist exhibit a close agreement with those contained in my first com- munication to the Royal Irish Academy. The concluding part of the elaborate memoir of MM. Favre and Silbermann on the heat disengaged in chemical actions is chiefly devoted to the same subject. A large number of experiments are described which are nearly a repetition of those I had previously published. Their results bear a general resemblance to those given by myself in 1841 but they widely differ in the details.The authors of this able memoir fidly recognize the accuracy of my fourth law which anserts the equality of thermal effect when one base is substituted for another. “31. Andrews,” they observe ‘‘avait en effet dtabli que quelque soit l’acide d‘un sel la quantit6 de chaleur d8gagde par la substitution d’une base B une autre pour former un nou-veau sel est la meme lorsque l’on considkre Ies deux m&mes l)ases.”t In a preceding paragraph of the same memoir the authors object to what they conceive to be my first law and state that it is not in accordance with the results of their investigations. As the question is one of some importance I may perhaps be permitted to quote the passage in the original language.“Ses conclusions savoir que la chaleur dkgagbe par l’kquivalent &une meme base combinde aux divers acides est la m&nie ne s’accordent pas avec les rksultats de nos recherches et ne nous paraissent pas pouvoir &re admises.” No doubt through inad- vertence MM. Favre and Silbermann have here given an inaccurate statement of my first law. It did not declare that * Hemoirs of the Chemical Society vol. ii p 61. t Annsles de Chimie et de Physique 3eme eerie xxxvii p. 49’7 (1853). 212 43-2 ANDREWS ON THE HEAT DEVELOPED IN TRE precisely the same amount of heat is disengaged by all the acids in combiiiiiig with the same base but that the heat is deter- mined by the base &' the same base producing when combined with an equivalent of' different acids nearly the same quantity of heat." A comparison of the results of MM.Favre and Silbermann with those in my original memoir will show that I had fully recognized and described the deviations from the other acids exhibited on the one hand in excess by sulphuric acid and on the other in deficiency by tartaric citric and auccinic acids. "If we refer,'' I remarked in the oiiginal memoir of 1841 c6 to the first second and fourth tables as being the most extensive from the large number of soluble compounds formed by potash soda and ammonia it will be observed that sulphuric acid developes from OO.8 to nearly 1' more than the mean heat given by the other acids; while tartaric citric and succinic acids fall from OO.4 to OO.55 short of the same.A minute investigation of the influence of the disturbing sources of heat will no doubt discover the causes of these discrepsncie s. The high numbers for sulphuric acid are probably connecte d with that acid's well known property of developing much heat when combining with successive atoms of water. All the other acids develope nearly the same amount of heat in combining with the same base the greatest divergences from the mean quantity being in the case of potash + 0'024 arid -0'013; in that of soda + 0'026 and -OO.14; and in that of ammonia + 0'-17 and -OO.05. These differences are almost within the limits of the errors of experiment."" But although there is a superficial agreement between my original results and those of MM.Favre and Silbermann they will be found when examined closely to differ widely in detail and on points of great importance. I had found that oxalic acid disengages almost exactly the same amount of heat in combining with the Roluble bases as hydrochloric nitric and inany other mineral acids and this observation I have always regarded as one of the ma.in foundations of Law 1. MM. Favre and Silbermann on the contrary have inferred from their experiments that "the following organic acids-oxalic formic valeric and citric-disengage sensibly the same quantity of heat but it is less (plus faible) than that given by the foregoing mineral acids "-among which they enumerate the nitric alld * Transactions of the Royal Irish Aeademy vol. xix p. 240. COMBINATION OF ACIDS AND BASES.435; hydrochloric According to my experiments no distinction of this kind can be admitted between acids derived from the mineral and organic kingdom inasmuch as odic acid devclopes at least m much heat in combining with the bases as hydro-chloric nitric and several other strong mineral acids. The experiments to be described iu this paper were made iwne years ago but their publication has been deferred from accidental circumstances I have however recently repeated a few of the more important of them with a slightly modified form of apparatus. The solutions were taken in so dilute a state that the heat disengaged never exceeded 3O.5 C. A standard solution of sulphuric acid was prepared and carefully analysed by precipitating a given weight with a soluble salt of barium and weighing the sulphate of barium.The strength of the alkaline solutions was adjusted with great care by means of this standard acid. The same solution of each alkali was employed in all the experiments and the quantity used in each experiment was determined by careful weighing. The acid solution was of such a strength that after being mixed with the alkali an excess of two or three per cent. of acid was present. The alkaline solution was contained in a light glass vessel in which a large platinum crucible holding the acid was carefully floated. By giving a rapid rotation by means of a light stirrer to the acid solution in the platinum crucible a perfect equilibrium of temperature was soon established between the two liquids.The initial temperature of the solutions was usually about 1O.5 below that of the air and the final temperature of the mixture about 1O.5 above it. The corrections for the heating and cooling action of the surrounding medium were determined with great care. The mechanical process of adding the acid to the alkaline solution produced 110 change of temperature and as the heat disengaged in the combination raised the liquid almost instantly to the maximum temperature the whole cor- rection required was for cooling. The first temperature was read one minute after the addition of the acid to the alkaline solution the mixture being stirred during the whole of that time. If 6 represents the correction and E the excess of tem-perature above the air in centigrade degrees the value of 6 will be given by the following expression :-6 = e x 0°*012.As a proof of the accuracy of the method of mixture adopted 436 ANDREWS ON THE HEAT DEVELOPED IN THE in this inquiry I may mention that being desirous to know whether the dilute acids employed in these experiments pro- duced any change of temperature when mixed with water I made the experiment with nitric acid by the method just de- scribed substituting water for tlie alkaline solution with the unexpected result of a fall of OO.01 On varying the conditions of the observation so as to obtain a larger effect it wita ascer- tained not only that a diminution of temperature had actually occurred but that the observed fall represented approximately its true amount.When hydrochloric acid of equivalent strength was diluted to the same extent an elevation of temperature of OO.05 was produced. The accuracy of experiments of this kind where the whole thermal effect observed amounts only to 2' or 3O depends greatly on the thermometer employed. Unless its indications are perfectly trustworthy in every part of the scale the labour of the inquirer will only end in disappointment. I have there- fore taken every precaution to secure this important object. The tube of the thermometer was calibrated and divided with care according to an arbitrary scale by means of a dividing instrument contrived for the purpose and provided with a short screw of great accuracy made by Trough t on and Simms.The divisions etched fine17 on tlie glass corresponded to about OO-05 C. and the readings could be made with certainty to less than Oo-01. The division of the scale corresponding to O" was determined from time to time in the usual way; and another point about 30°C. was fixed by comparison with four other thermometers similarly constructed whose scales extended born the freezing to the boiling-point of water. The readings of these four instruments when reduced to degrees rarely differed fi-om each other within the limits to which they could be read or 0'-02. The reservoir of the thermometer used in these experiments was 75 niillimetres long and when immersed in the liquid occupied nearly its entire depth. As some uncertainty always exists with regard to the thermal equivalent of glass vessels I made two sets of comparative experiments-one with a thickly-varnished copper vessel and the other with a vessel of platinum.The mean result of these experiments coincided almost exactly with the result obtained when the glass vessel was employed. The weight of the glass vessel which contained the alkaline COMBINATION OF ACIDS AND BASES. solution was 58 grammes and corresponded thermally to 11.4 grammes of the solutions formed. The thermal equivalent of the reservoir of the thermometer and of the stirrer was 0.9 grammes. The alkalhe solution weighed 160 grammes and contained the equivalent of 1.738 gramme of SO,. The acid solution weighed 42-5 grammes.Hence the entire thermal value of the apparatus in terms of the solution formed WdS-Solution ................ 202.5 Glass vessel .............. 11.4 Thermometer and stirrer .. 0.9 214-8 grammes. A correction (additive) of d5was made to the direct reading8 for the mercury in the stem of the thermometer. The results are given to thousandths of a degree but thirs apparent minuteness is due to the reduction of the indications of the arbitrary scale to degrees. The following table gives the mean results of the new experi- ments the acids being arranged in the order of their thermal action- Acid. Potash. Soda. Ammonia. Sulphuric acid ...... Oxalic acid. ......... 3O.378 3'0058 3O.353 3O.040 2O.976 2O.648 Hydrochloric acid. ... 3O.021 2"*982 2O-623 Nitric acid ..........2O.993 2'0929 2°*5S6 Acetic acid.. ........ 2O.852 2O.832 2'0492 Tartaric acid ........ 2O.732 2O.710 2'-376 It is interesting to observe how closely the results in the three vertical columns agree relatively with one another. The acids follow in the Same order under each base and even the differences in the amount of heat disengaged by the several acids in combining with the different bases approximate in many cases closely to one another. Thus the heat given out when sulphuric acid combines with potash exceeds that given out when oxalic acid combines with the same base by Oom32O,the corresponding dzerences in the case of soda and ammonia being O0-313and OO.328. If in like manner we compare the differences between the heat disengaged by the acetic and tar- taric acids we fdl upon the numbers OO1120 0'9122 and OomllC;.438 ANDREWS ON THE HEAT DEVELOPED IN THE Even in the case of oxalic hydrochloric and nitric acids which disengage 80 nearly the game amount of heat the same order is observed with the three bases It must be particularly re- marked that the oxalic acid disengages from Oa*022 to 0°*058 more heat in combining with these bases than the hydrochloric acid and from OO.065 to 0"*111 more than the nitric acid. The conclusion of MM. Favre and Silbermann that the organic acids (oxalic formic acetic &c.) disengage sensibly less heat than the mineral acids is thus entirely disproved; and the original results recorded in my work of 1841 according to which oxalic acid disengages at least as much heat as nitric phos- ph oric arsenic hydrochloric hydriodic boracic and other mineral acids (with the exception of the sulphtuic acid) are fully confirmed.Tartaric citric and succinic acids it is true (as was also shown in the Same work) give out about &th less heat than the average of the other acids but acetic and formic acids fall scarcely &th below the mean and oxalic acid is always above it. These results in all their main features are fully corroborated by the experiments recorded in this paper which were performed with a more perfect apparatus and a more exact thermometer than I had at niy command in my earlier investigations. A reference to the Same paper will show that while acids differing so widely from one another as oxalic phosphoric arsenic nitric hydrochloric and boracic acids scarcely present any sensible difference in the quantities of heat which they disengage in combining with the bases; and while of the other acids examined sulphuric acid (and probably also sulphurous acid) presents an extreme devia-tion of about +th above the mean and the tartaric acid group a deviation of about &th below it; the bases on the contrary (and the subsequent researches of Favre and Silbermann have confirmed this result) differ altogether in thermal power fi-omone another.Thus equivalents of the oxides of magnesium and of silver give out 4O.1 and 1O.8 of heat respectively in com-bining with nitric acid the former oxide having therefore 2.3 times the thermal power of the latter.Yet as is well known both theBe bases fully satrwate the acid and the resulting solu-tions are even neutral tQtest-paper. For Obese reasons I have 110 doubt whatever that the first law as enunciated in 1841 is the expression of a true physical law and that in the combina- tion of acids and bases in preaence of water the heat dirjengaged COMBINATION OF ACIDS AXU BASEb. is determined by the base and not by the ad It is true that in this as in similar physical inquiries experimental results cannot immediately be obtained free from complication or dis-turbing influences. The same remark applies to the experi- mental proof of the great law discovered by Dulong and Petit which connects the specific heats and atomic weights of the elementary bodies and also to that of the reniadrable reLLtions discovered by K opp between the composition and boiling points of many organic liquids.We have already seen an illustration of one of these disturbing influences in the fact that dilute nitric acid when mixed with water gives a slight fdl of teni-peratnre hydrocldoric acid a rise ; and the clifferences of specific heat in the solutious formed will to a sniall extent modify the results. But the cause of the higher thermal power of sulphuric acid I have not been able to discover and fnture researches niuat ciecid e whether it depends upon some clisturb-ing cause or (which is less probable) upon its possessing an exceptionally high thermal power.One condition is however essential or Law 1will not apply. The acid and base must be capable of combining when brought into contact and of forming a stable compound. In the paper so often referred to I showed that hydrocyanic acid and potash which fail to fulfil this con- dition do not disengage the normal amount of heat when mixed; and the same observation will doubtless be found to apply to a large number of metallic oxides which form unstable compounds with and imperfectly neutralize the bases. As regards the experimental proofs of the other laws even those of the fourth law the truth of which is admitted by MM. Favre and Silbermann they are only approximative and here also we meet occasionally with peculiar and unexpected results.Thus a slight fall of temperature occurs as Hess showed long ago in the conversion of the neutral sulphate of potash into the acid salt ; and I found as indeed might have been expected from their alkaline reaction that in the con- version of the ordinary phosphates and arseniates into super-salts a disengagement of heat occurs amounting to about one-seventh of that disengaged in the formation of the salts them- selves. In other cases results at first view startling and ap- parently anomalous will be found to be strictly in accordance with the general principles already laid clown. In the forma- tion of double salts there is no cliaengagernent of heat-a prin-213 440 ANDREW3 ON THE HEAT DEVELOPED IN THE ciple announced in 1841 and wliicli ought perhaps to be enun-ciated as a distinct law although it is implicitly involved in Law 2.Again if tribasic phosphoric acid or arsenic acid is added in fractional portions to ii solution of potash till the sub-salts are formed the heat disengaged on each addition of acid corresponds to the amount of acid added; but after this point has been reached the disengagemcut of heat follows a diEereiit law. Pyrophosphoric acid on the other hand behaves in the same way as nitric and most other acids when added in successive portions to solutions of potash or soda; equal incre- ments of heat being evolved for equal additions of acid till tli 3 pyrophosphate of potabh or soda is formed.* APPENDIX. In the following tables I have given the results described iii this communication and those of 1841 in a form which admits of comparison with one another and with those of MM.Favre and Silbermann. I have also added a few deterniinatioiis recently made by M. Tho sen of C0penhagen.t It will be seen that the original experiments of 1841 exhibit on the whole a $air agreement with those now communicated to the Society. From the small scale on which they were performed (the whole weight of the solutions after mixture being less than 30 grammes) the imperfect form of the apparatus and the uncertainty of the thermometric indications I have indeed been surprised to find them so near the truth. The results of MM Favre and Silbermann do not exhibit the precision which might have been expected fi-om the high character of those experimentalists and from the accur<tcy of other parts of their great work.The mercurial calorimeter employed by them ap- pears to have been little adapted to its purpose; but after making due allowance for its imperfections I am at a loss to account for the serious errors into which they have fallen M. Thomsen’s experiments have evidently been made with cue and his results agree comparatively with my own ;but the absolute amount of heat obtained by him falls far short of what I have found. It is indeed much easier to obtain results rela-* Transactions of the Rojal Irish Academy vol. xix,pp. 245-248. The observa- tions of Graham confirm the statement that no heat is evolved in the formation of any double salt. Memoirs of the Chemical Society vol.i p. 83. t Poggendorff’s Annalen cxxxviii p. 78. COMBINATION OF ACIDS AND BASES. 44 1 tively than absolutely correct. The numbers given in thk paper will I believe be found rarely to differ relatively more than &th from the truth but they may hereafter require a small correction in respect to their absolute value. That cor-rection can however be scarcely more than &th of the whole amount and I have little doubt that the number for example given by Thomsen to express the heat disengaged in the com-bination of soda with nitric acid will prove to be as far below the true number as that given by MM. Favre and Silbermann is above it. TABLE L-Potash. Acid. ‘ Andrews 1841. Favre and Sil b erman n. Andrews 1870.Sulphuric ............ Nitric ................ 16330 15076 16083 15510 16701 14800 Hydrochloric .......... Oxalic ................ 14634 14771 15656 14156 14940 15124 Acetic.. .............. 14257 139’73 13805 Tartaric .............. 13612 13425 13508 ~ ~~ ~ TABLE11.-Soda. ~ Acid. Andrew s Favre and A nd re w 8 Thomse n. 1541. Silbermann. 1870. Sulphuric .......... 164 83 15810 16580 15689 Nitric .............. 14288 13283 14480 13617 Hydrochloric ........ 14926 15128 14744 13740 Oxalic .............. 14796 13752 15032 .. Acetic.. ............ 14046 13600 14000 .. Tartaric ............ 13135 13651 13400 .. TABLE111.-Ammonia. Acid. Andrew s Favre and Andrew s, 1841. Si1 b erman4. 1870. Sulphuric ............ 14136 14690 14710 Nitric ............... 12440 136?6 12683 Hydrochloric ......... 12410 13536 129b4 Oxalic,. .............. 12684 13088 Acetic ................ 12195 12&9 12316 Tartaric .............. 11400 11744