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XII.—On the application of liquid diffusion to produce decompositions

 

作者: Thomas Graham,  

 

期刊: Quarterly Journal of the Chemical Society of London  (RSC Available online 1851)
卷期: Volume 3, issue 1  

页码: 60-67

 

ISSN:1743-6893

 

年代: 1851

 

DOI:10.1039/QJ8510300060

 

出版商: RSC

 

数据来源: RSC

 

摘要:

MR. GRAHAM ON DECOMPOSITION XII.-On the application of Liquid Difusion to produce Decom-positions. By THOMAS F.R.S. F.C.S. GRAHAM The experiments to be described in the sequel of this paper were conducted in the same manner as those contained in a late com- munication to the Royal Society.* A set of phials of nearly equal capacity were made use of all cast in the same mould and further adjusted by grinding to a uniform size of aper-ture. The dimensions for a phial (see Fig.) were 3.8 inches in height with a neck 0.5 inch in depth; aperture 1.25 inch in diameter and capacity to base of the neck 2080 grains of water or between 4 and 5 ounces. For each phial a plain glass jar was also provided,4 inches in diameter and 7 inches in depth. The me- ~ thod of observing the diffusion of a salt will be best explained by an example.For my present object it was necessary to observe the spontaneous diffusion into pure water of several salts already dissolved in 100 times their weight of water. The phial was filled with such a solution of sal-ammoniac for instance to the base of the neck or more correctly to a distance of exactly 0.5 inch from the ground surface of the lip. The neck of the phial was then filled up with distilled water a light float being placed on the surface of the solution and care taken to avoid agitation after the phial had been placed within the jar. The latter was then filled up with distilled water so as to cover the open phial to the depth of an inch which required about twenty ounces of water.The saline liquid in the ‘‘solution-phial” is thus allowed to communicate freely with the water of the “water-jar.” The water of the latter forms an atmospliere into which salts spread or diffuse escaping from the solution-phial with different degrees of velocity. The phial and jar together form the “diffusion-cell.” The diffusion is interrupted by placing a small plate of ground glass upon the mouth of the phial and raising the latter out of the jar. The amount of salt diffused or the ‘‘diffusion-product,” is learned by evaporating the water of the jar to dryness or with this and the following chlorides by precipitating with nitrate of silver. The diffusion was always allowed to proceed for a period of exactly seven days unless another time is expressly mentioned.The experiments were conducted in a vault of which the temperature did not vary more than one degree on either side 50° F. * On the Diffusion of Liquids. Read December 21 1849. BY LIQUID DIFFUSION. The one per cent solution of sal-ammoniac in the solution-phial would contain 20.8 grains of salt. Of this quantity of salt 3.49 grs. were found to diffuse out into the water-jar in the time mentioned in one experiment and 3-86 grains in another experiment. The mean of these two diffusion-products is 3.42 grs. which is therefore the quantity of hydrochlorate of ammonia diffused out of the solution phial containing a I per cent solution of that salt in a period of seven days. The diffusion of a 1 per cent solution of chloride of sodium in similar circumstances gave in eight cells the following products 3-02 2.83 2.86 2-68,2.74 2.70 2.80 and 2.94 grs.of which the mean is 2.85 grs. These results for the chlorides of ammonium and sodium approach to the theoretical ratio of 1.4142 to 1.7320 that is of the square root of 2to the square root of 3. Anhydrous chloride of calcium gave 2.01 and 2.04 grs. in two experiments ; mean 2.02 grs. Anhydrous chloride of magnesium gave 2-15and 1-90grs; mean 2.03 grs. These two earthy chlorides appear therefore to be equally diffusible. I may place along with these results the diffusion- products which the alkaline hydrates and a few other salts would have afforded in similar circumstances the latter numbers being deduced from experiments detailed in the paper on the diffysion of liquids already referred to.SALT DIFFUSED FROM A 1 PER CENT SOLUTION IN EQUAL TIMES Hydrate of potash . 4-84! grs. > soda . . 4.03 ), 7 Chloride of ammonium . 3.42 , , , potassium. . 3.42 ,, ... . 2.85 ,, , , sodium . . 2.35 ,, Sulphate of soda. . 2.02 ,, Chloride of calcium . . , , magnesium 2.03 , Sulphate of lime. . . 1.21 , 1.21 ,, , , magnesia . The alkaline hydrates have the highest diffusibility being twice as diffusive as the sulphates of the same bases and four times as diffusive as the sulphates of magnesia and lime. Tkie salts of potash and ammonia of the same acid have an equal diffusibility which is greater than the diffusibility of the corresponding salts of soda.Now it has been shown that when two salts are dissolved together MR. GRAHAM ON DECOMPOSlTION in the solution-phial they diffuse independently each salt maintaining its own rate of diffusion. Hence the possibility of separating salts to a certain extent by diffusion in a manner analogous to the separation of substances of unequal volatility by distillation. Further decompositions may be effected by diffusion such as the decomposition of alum the sulphate of potash being separated from the sulphate of alumina from the higher diffusibility of the former substance. It appeared probable also that in a mixture of several salts the acids and bases would have a tendency to arrange themselves so as to form the most diffusive compounds when an opportunity for diffusion was presented.This would be analogous to the sublimation of carbonate of ammonia when carbonate of lime and hydrochlo- rate of ammonia are heated together. As the order of affinity is often determined in mixed salts by volatility or insolubility according to the canons of Berthollet so it may be regulated and determined in a similar sense by diffusibility. The application which I have particularly in view is the possible decomposition of the sulphates of potash and soda and of the chlorides ofpotassium and sodium by means of lime when the affinity of that base for an acid is aided by the high diffusibility of the hydrate of potash or of soda. 1. A solution was made of 1 part of sulphate of potash in 100 parts of lime-water with which six phials were filled and placed to diffuse in jars containing lime-water instead of water simply for the usual period of seven days.To obtain the salts diffused the fluid of the jars was treated with an excess of bicarbonate of ammonia and evaporated twice to dryness. The filtered liquid contained only sulphate and carbonate of potash without a trace of lime. The r>ro.xlortions of these salts found indicated a diffusion-product from .I.& two phials of Hydrate of potash . 1.08 grs. 23-69 Sulphate of potash . 3.48 , 76.31 c___ 4.56 , 100*00 The diffusion-product of the remaining four cells was similar Hydrate of potash . 2.19 grs. 21.66 Sulphate of potash . 7.94 , 78.34 10.13 , 100.00 More than a fifth part of the diffused salt appears thus to be hydrate of potash and a considerable decomposition of the sulphate of potash has therefore taken place.BY LIQUID DIFFUSION. 2. Sulphate of soda dissolved in lime-water was diffused into lime-water in a precisely similar series of experiments. The diffusion-product obtained in a set of four cells appeared to consist of Hydrate of soda . . 0.90 grs. 11-45 Sulphate of soda. . . 6.87 , 88.55 7-77 , 100~00 Of another set of four cells the diffusion-product was repre-sented by Hydrate of soda . . 093 grs. 13.22 Sulphate of soda. . 6-10 , 86.78 c__ 7.03 , 100*00 The hydrate of soda formed and diffused is very sensible amount- ing on an average to about 12 per cent of the whole diffused salt.It may be remarked that although sulphate of soda may be justly supposed to require a less powerful affinity to decompose it than sulphate of potash still the latter salt appears to yield to the action of the lime to a greater extent than the former in these experiments ; the weight of hydrate of potash diffused being fully double that of the hydrate of soda. The result in question may be confidently referred to the superior diffusibility of the hydrate of potash and establishes beyond doubt the nature of the agency by which the decomposition in both cases is principally if not wholly effected. The low diffusibility of the sulphate of lime (one fourth of that of hydrate of potash) retains a large proportion of that salt behind in the solution-phial where indeed it was deposited in crystals.It is proper to remark that a similar deposition of sulphate of lime took place in lime-water containing so much as 1 per cent of sulphate of potash or sulphate of soda in the course of two or three days in a close vessel quite irrespective of diffusion. One of the solution- phials was found to contain so much as 2.04 grains of hydrate of potash formed in consequence of this deposition of sulphate of lime without any diffusion in the course of seven days. The same decom- position of sulphate of potash with deposition of sulphate of lime was observed by Scheele and afterwards referred by Berthollet to the insolubility of the latter salt which enables the affinity of lime for sulphuric acid to prevail over that of potash for the same acid.MR. GRAHAM ON DECOMPOSITION 3. Similar solutions of 1per cent of chlorides of potassium and sodium in lime-water were diffused into lime-water. Eight cells of chloride of potassium gave 25-51grains of diffused salt containing only 0.04 grain of hydrate of potash. Eight cells of chloride of sodium gave 20-77 grains of diffused salt containing no more than 0.08 grain of hydrate of soda. The decomposition of the alkaline chlorides is so small as to be barely sensible not exceeding in the most favourable case more than &th part of the salt diffused. Lime therefore appears in- capable although aided by diffusion to decompose the chlorides of potassium and sodium to a sensible extent.4. Solutions in lime-water were diffused of 0.25 and 0.5 per cent of the alkaline sulphates not with the view of increasing the product of alkali but for the purpose of observing the diffusion where no deposition of sulphate of lime is possible owing to the dilute con- dition of the solutions. The experiments however come to be more liable to derangement from currents produced by small changes of temperature and other accidental causes of dispersion where the solution in the phial differs so little in density from the water of the jar. One set of four cells containing the quarter per cent solution of sulphate of potassa in lime-water gave 0.321 gr. of hydrate of potash Another similar set gave 0.614 gr. of hydrate of potash.The hydrate of soda diffused from four cells of the quarter per cent solution of sulphate of soda was 0,260gr. The diffusion was always into lime qwat er. The half per cent solution of sulphate of potash in lime-water gave 0 62 gr. hydrate of potash in two cells ; or twice as much alkali as the quarter per cent solution. The diffusion-product was alto-gether 2.60 grs. so that 23.86 per cent of the salt diffused con-sisted of hydrate of potash. No sulphate of lime crystallized or was deposited in the phial or jar in any of these experi-ments so that the decomposition of the sulphate of potash by lime cannot be referred in any degree to the insolubility of sulphate of lime but must be ascribed entirely to the high diffusibility of hydrate of potash.5. Carbonate of lime dissolved in carbonic acid water or a satu- rated solution of bicarbonate of lime was now applied to form a 1 per cent solution of sulphates of potash and soda. These solutions were diffused from the phials into pure water as the liquid atmos- phere of the jars. Decomposition of the alkaline sulphate always took place and without any visible deposit of sulphate of lime but BY LIQUID DIFFUSION. to a less extent than in the preceding experiments with hydrate of lime. The proportion of potash salts diffused in two pairs of cells was 4-86 and 5.84 grs. ; of which 0.26 and 0.30 gr. was carbonate of potash or 5.35 and 6.2 per cent of carbonate of potash. The proportion of soda-salts diffused in two pairs of cells was 3.40 and 3-78 grs, of which 0.26 and 0.29 gr.was carbonate of soda; or 7.65 and 7.67 per cent of carbonate of soda. The excess of carbonate of soda diffused over the carbonate of potash in these experiments is probably accidental. The experiments on the decomposition of an alkaline sulphate by means of carbonate of lime aided by diffusion are chiefly interesting as they illustrate a decomposition which may occur among the salts of the soil and with the formation of an alkaline carbonate from a reaction between carbonate of lime and an alkaline sulphate although the solutions may be too dilute to admit of any separation of sulphate of lime in the solid state. But the decomposition of the chlorides of potassium and sodium is a more important problem than that of the sulphates of potash and soda.The direct diffusion of these chlorides with hydrate of lime appears to be inadequate to produce this effect for no more than a trace of fixed alkali was obtained in the experiments already described. It was further observed that a saturated solution of the chlorides of potassium and sodium in lime-water did not afford the smallest appreciable quantity of alkali by diffusion. Bicarbonate of lime had no greater effect upon the alkaline chlorides. But the conjoint action of lime-water and the sulphate of lime upon these chlorides gave better results. 6. Lime-water and solution of sulphate of lime both saturated solutions were mixed together in equal volumes and the liquid was employed to dissolve 1 per cent of chloride of sodium.The phials charged with this solution were allowed to diffuse into pure water. After separating the hydrate of lime from the water of the jars the latter exhibited only the faintest possible alkaline reaction due to soda. The proportion of alkali was too minute to be appre- ciated by the alkalimetrical method although a quantity so small as 0.01 gr. could be determined. To enable the hydrate and sul- phate of lime to act upon the chloride of sodium it was found necessary first to heat the solution before diffusion. 7. The solution of sulphate of lime with an addition of 2 per cent of chloride of sodium was kept at the boiling point for half an hour No deposition of sulphate of lime occurred then or after the liquid cooled.Two or three days afterwards this solution was VOL. 111.-NO IX. F MR. GRQHAM ON DECOMPOSITION mixed with an equal volume of lime-water and diffused into pure water for the short period of three and a half days. The liquid of the jars was evaporated to dryness as usual with an excess of pure carbonate of ammonia to precipitate the salts of lime. The hydrate of soda diffused in three cells amounted to 0.234gr. and the sulphuric acid to 0.209 gr. Allowing the sulphuric acid to have diffused as sulphate of soda and putting out of consideration the undeeompoaed chloride of sodium which was also diffused we have a diffusion-product of Hydrate of soda . . 0234gr. Sulphate of soda . . 0.371 , 0.605 gr. These quantities are necessarily small from the form of the experi- ment but could easily be increased by enlarging the diffusing surface which is at present limited to the area of the aperture of the solution phial.They are amply sufficient however to establish the fact that the united affinities of hydrate and sulphate of lime are sufficient to decompose chloride of sodium when aided by diffusion of the hydrate of soda formed. Of chloride of potassium we have reason to believe that the decomposition would be more considerable in similar circum- stances. Two phials of the same liquid as in the last experiment were diffused into water for the longer period of seven days and eighteen hours. The whole lime found afterwards in the water-jar and pre- cipitated by the addition of carbonate of ammonia was represented by 1.01 grain of carbonate of lime.The soluble salts filtered from the last amounted to 6-14! grains and contained carbonate of soda equivalent to 0.686 gr. of hydrate of soda and sulphuric acid equiva- lent to 0.373 gr. of sulphate of soda. It is difficult to decide in what form the lime reached the water-jar but this earth was probably diffused out of the solution-phial partly as hydrate of lime partly as sulphate of lime but principally as chloride of calcium. The last two salts would destroy a portion of free alkali in the evapo- ration but the hydrate of soda obtained even after this deduction amounted to 10.52 per cent of the whole salts diffused. With a smaller quantity of chloride of sodium than 2 per cent in the original mixture the alkali although not increased in absolute quantity might no doubt come to form a conside2ably larger proportion of the diffusion-product.The experiments also throw a curious light upon the condition of BY LTQUID DIFFUSTON. mixed salts. It follows from the absence of hydrate of soda in the diffusion product of the first experiments that cold solutions of sulphate of lime and chloride of sodium may be mixed without decomposition or without any sensible formation of sulphate of soda. But on heating this change is induced and it is permanent ;sulphate of soda is formed and continues to exist in the cold solution for it is the decomposition of that salt alone by hydrate of lime which appears to afford the diffused hydrate of soda.More than one con- dition of equilibrium is therefore possible for mixed solutions of sulphate of lime and chloride of sodium. It would be interesting to submit such a mixture to a diffusion experiment after being kept for different periods The effects of time and temperature are so often convertible that we might anticipate a gradual formation of sulphate of soda. If such be the case we have an agency in the soil by which the alkaline carbonates required by plants may be formed from the chlorides of potassium and sodium as well as from the sulphates of potash and soda; for the sulphate of lime generally present will convert those chlorides into sulphates. The mode in which the soil of the earth is moistened by rain is peculiarly favourable to separations by diffusion.The soluble salts of the soil may be supposed to be carried down together to a certain depth by the first portion of rain which falls while they find after- wards an atmosphere of nearly pure water in the moisture which falls last and occupies the surface stratum of the soil Diffusion of the salts upwards into this water with its separations and decompo- sitions must necessarily ensue. The salts of potash and ammonia which are most required for vegetation possess the highest diffusi- bility and will rise first. The pre-eminent diffusibility of the alkaline hydrates may also be called into action in the soil by hydrate of lime particularly as quick-lime is applied for a top-dressing to grass lands.

 

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