<ref name=burg1905>Charles Hutchens Burgess and Alfred Holt (1905): "Some physical characters of the sodium borates, with a new and rapid method for the determination of melting points." ''Proceedings of the Royal Society of London, volume 74, pages 285–295.{{doi|10.1098/rspl.1904.0112}}</ref>
When a quantity of ordinary pure borax glass is heated for some hours at a temperature which gives it about the consistency of a thick syrup, it gradually changes to a mass of colourless crystals. The crystallisation begins at two or three points on the surface of the mass, generally around a particle of dirt, or minute fragment of imperfectly fused borax, and spreads in more or less spherulitic growths throughout the glassy portion. As the crystals grow, there appears to be a contraction of volume.
[...] the whole crystalline mass appears to be composed of dense, matted clusters of very minute needles, which are doubly refracting, and have a pearly lustre. They are about as soluble in water as ordinary borax glass, are not hygroscopic, and melt at a higher temperature than the glass, into which they are reconverted on melting, and then cooling quickly.
We find, however, that it is not borax glass alone which exhibits this phenomenon of crystallisation on reheating, but that the glasses obtained by fusing mixtures of boric anhydride and sodium carbonate, in which the ratio of the boric anhydride to the sodium carbonate is not 2:1, also exhibit it in varying degree.
Thus, all mixtures in which the ratio varies from 6:1 to 8:5, give a glass when fused, and are all capable of being changed completely into crystals on reheating, exactly like borax.
Mixtures in which the boric anhydride is present in greater proportion than 6:1, only change with difficulty on prolonged heating; and when they do, the crystals are always found to be mixed with more or less truly glassy material, till, when the ratio 40:1 is reached, it seems impossible to obtain crystals at all. So far as we can ascertain, a mixture of this composition is always a true glass in any circumstances.
Mixtures, on the other hand, in which the ratio of boric anhydride is less than 8:5, when fused, and allowed to cool, yield nearly opaque, white substances, which seem to be micro-crystalline. Nevertheless, these mixtures if heated to a high temperature, and then suddenly chilled by being poured into mercury, also yield glasses. The glasses obtained by this sudden chilling seem to be very unstable, as on gently heating by means of a Bunsen flame, they change completely, and almost instantaneously, into crystals.
There seems then to be a gradual alteration in the rapidity and completeness with which this change occurs, and as it seemed probable that the crystallisation of some borate rich in sodium was the cause of it, we proceeded to determine what was the composition of the borate richest in sodium which could be obtained by fusing boric anhydride with sodium carbonate.
When boric anhydride is fused with sodium carbonate, carbon dioxide is evolved, and the product may be regarded as boric anhydride combined with sodium oxide (Na20). Thus, by heating boric anhydride with a large excess of sodium carbonate, and determining the amount of carbonate decomposed, the greatest proportion in which boric anhydride combines with sodium oxide can be ascertained, and ought to give the composition of the richest sodium borate obtainable by fusion.
We have performed this experiment several times, and have obtained the following results :
(i) One part of B2O3 combines with 1.29 parts Na20
(ii) One „ » 1.34 »
(iii) One „ 1.32
These results are probably a little too low, as the sodium carbonate loses some carbon dioxide merely on heating alone, but this amount is negligible, and does not in any way affect the conclusion that the product obtained in this way is not sodium metaborate (NaB02), in which the ratio of boric anhydride to sodium oxide is 1:1, and that sodium orthoborate (Na8B03), in which the ratio is 1:3, cannot in any case be formed by fusion of boric anhydride and sodium carbonate.
[...] [determining the melting point of the glass and crystals] We found [that calibrating the apparatus] was a matter of some difficulty, as very many of the most ordinary salts were found to decompose to some extent on heating. Sodium chloride, sodium carbonate, potassium iodide, calcium chloride, and strontium chloride all underwent change. Of these salts sodium chloride was very little attacked, but after only a few minutes’ fusion we found it had absorbed quite an appreciable amount of oxygen, being converted into the peroxide. Potassium iodide changed in part to iodate, sodium carbonate to the peroxide, and calcium and strontium chlorides to the oxides.These changes completely prevented any accurate melting point determinations, so after repeated trials we adopted potassium nitrate, lithium chloride, and potassium chloride as standards. It is true that potassium nitrate decomposes on heating, but as the nitrite (to which it changes) has almost the same melting point, the small amount of change was found to be immaterial. The chlorides we employed seemed to be, on the whole, very stable.
[Graph of melting point as function of Na2O:B2O3 ratio shows anomalies in the melting point. Metastable forms: 450 for pure B2O3, 625 for 0.25:1, same thereafter until 1:1, when metastable ends?. Stable forms: ~900 C for 0.25:1, ~780 for 0.5:1 (pure borax), min 775 at 0.7:1, max 960 at 1.25:1, min 650 at 2.25:1, 700 at 2.75:1, about the same thereafter]
The addition of sodium oxide to boric anhydride raises the melting point continuously till a mixture of composition Na20.4B203 is reached. There seems to be no depression of the melting point at all, thereby differing from the case of ordinary solution, and indicating the probability that there is no solid phase. The curve then remains fairly level till a composition Na20.2B203 is reached, when it falls irregularly to 3Na20.4B203 The last point we could obtain on this curve was with composition Na20.B203, where the melting point practically coincided with the temperature of change from the glassy to the crystalline state of this mixture. Indeed it is a little above it, but it was possible by fairly rapid heating to melt the bead before the change had occurred.
The melting point curve for the crystals is more easily explainable than that of the glass.
We were unable to obtain any point on it between pure boric anhydride and a mixture of composition Na20.4B208. This was because the crystallisation of the small mass of substance in the beads we employed for the melting point determinations took an extremely long time, and also because the devitrification was never complete except in the neighbourhood of the mixture Na20.4B203.
From a mixture of composition Na20.4B203 the melting point falls nearly uniformly till 5Na20.8B203 is reached, at which point it begins 1 to rise rapidly. The mixture 5Na20.8B203 would then appear to be a marked eutectic point. The summit of the curve is reached with composition 5Na20.4B203, when it falls again to a mixture which may be considered to have the composition 5Na20.4B203 + 4Na2C03, and which represents the eutectic point between 5Na20.4B203 and Na2C03.
The further addition of sodium carbonate causes a gentle, almost uniform rise in the melting points.
As the summit occurs with composition 5Na20.4B203, and as this very nearly agrees with the analysis of the borate richest in 1 sodium, which can be obtained on fusion of boric anhydride and I sodium carbonate, this would seem to indicate a compound.
The results of these melting point determinations of the crystalline mixtures indicate that borax (Na20.2B203) is not a definite compound in this state, but is almost a eutectic mixture of the borate with composition 5Na20.4B203 and one of composition Na20.4B203, for the existence of which we have other evidence.
This melting point curve for the crystalline forms of these various mixtures cannot be regarded as truly representing either the solidus or liquidus. It seems probable that in this case they are situated close to each other, and that our melting points really represent temperatures close to the liquidus. The fact that a mass of crystals would apparently melt almost completely at an almost constant temperature certainly indicates that there cannot be any very great difference between the liquidus and solidus, and, as in the cases studied by Heycock and Neville, that the actually determined melting points lie probably very near to the liquidus.
The following table gives the melting points for the various glasses and crystals:—-
[...]
We also tried extraction with hot methyl alcohol which is a solvent for boric anhydride, for, if the crystals had been due to a separation of a borate from its solution in boric anhydride, this should have left the compound pure. As a matter of fact we found that the methyl alcohol decomposed both the crystals and glass, giving methyl borate, boracic acid, and sodium methylate. Both glass and crystals were decomposed at the same rate.
The results of our analyses are as follows:—
(i) Mixture of Composition about Na20.2B203.—This mixture crysÂtallised completely in a few hours, giving a hard, white, completely crystalline mass. When about two-thirds had crystallised the glass and crystals were separated, and gave on analysis :—Glass = 309 per cent. Na20 ; crystals = 31‘0 per cent. Na20.
(ii) Mixture of Composition about 3Na20.8B203.—This crystallised completely, and had the same characters as (i).On analysis obtained :—Glass = 25.86 per cent. Na20 ; crystals = 25'81 per cent. Na20.
(iii) Mixture of Composition about Na20.4B203.—This crystallised completely, and had the same characters as (i).On analysis obtained :—Glass = 18.70 per cent. Na20 ; crystals = 18.67 per cent. Na20.
(iv) Mixture of Composition about Na20.6B203.—This crystallised very nearly completely, but seemed to contain small translucent glassÂlike fragments.On analysis obtained :—Glass = 11.5 per cent. Na20 ; crystals = 13.7 per cent. Na20.
(v) Mixture of Composition about Na20.8B203.—This did not crystalÂlise completely. After very prolonged heating it nearly all changed to a crystalline mass with an almost waxy lustre, but which was not really wholly crystalline. There was always in addition a portion of the mass which remained a glass.On analysis obtained :—Glass = 1006 per cent. Na20; crystals = 11.90 per cent. Na20.
(vi) Mixture of Composition about Na20.12B203.—Only about half would crystallise, and this took a very long time.Qn analysis obtained :*—Glass = 2.1 per cent. Na20 ; crystals = 12.2 per cent. Na20.
(vii) Mixture of Composition about Na20.16B203.—After heating this mixture for several days only a very small amount crystallised On analysis obtained:—Glass = 4.18 per cent. Na20; crystals = 10.02 per cent. Na20.:
(viii) Mixture of Composition about Na2O.40B2O3.—This could not he crystallised at all.
From these numbers it appears that with mixtures between Na20.4B203 and Na20.2B203 the crystals and glass have absolutely identical composition, but with mixtures containing more boric anhydride than Na20.4B203, only a part (decreasing in quantity as the Amount of boric anhydride present increases) crystallises, and this tart has approximately the composition Na20.6B203. As the crystals of the mixture of this composition contain more sodium than the glass, it seems probable that, could one obtain the srystals really pure and not entangling any glass, the composition night approach that of Na20.4B203.
The nearness in composition of the crystals obtained from mixtures varying from Na20.16B203 to Na20.6B203 makes it seem fairly certain shat it is the same compound which separates out in all of them, and that the differences in the analyses are due to the crystals being mixed with more or less glass which it was impossible to remove.
In experiÂment (vi) the crystals and glass were particularly well separated, and She result is shown by the great difference in their analyses. It seemed so curious that with mixtures of composition Na20.2B203 to Na20.4B203 the glass and crystals should give similar analytical results, that we thought it worth while to try fractional crystallisation of the glasses, to see if we could detect any differences between the different crystalline portions. The analyses of these various portions gave the following results :—
(i) For a mixture of composition about Na20.2B203 Per cent. Na20 .
First portion ................. glass =29.7crystals = 29.3
Second portion ............. glass = 29.9crystals = 29.2
Third portion ................. glass = 30.3crystals = 30.5
Fourth portion ........... glass =30.2crystals = 30.1
(ii) For a mixture of composition about Na20.4B203 Per cent. Na20.
First portion .......!...... glass = 18.7crystals = 18.6
Second portion ............. glass = 18.6crystals = 18.4
We also fractionally crystallised borax itself five times, when the final crystalline portion was found to contain 30.8 per cent. Na20 theory for Na2B407 requiring 30.7 per cent. Na20. These differences are negligible, and the crystals that separate out from the glass have exactly the same composition from the beginning.
These results show at once that this crystallisation is not due to the separation of any single compound as we had previously supposed, but that it is more probably due to the formation of mixed crystals or a solid solution from a superfused liquid.
All the various crystalline fractions obtained from any one glass of composition Na20.4B203 to Na20.2B203 were found to melt at identical temperatures, as also did the residual glasses. Further, the crystalline portions when melted, gave glasses which had a melting point identical with that of the previous residual glass, and these residual glasses could, in their turn, be changed to crystals with similar melting points to those which had originally been separated. This shows most conclusively the identity of the crystals and glass in chemical composition.
We have also determined in the case of several mixtures the temperatures at which this crystallisation takes place. The curve indicating these temperatures is shown in fig. 2 by the line joining AA. A bead of the glass was put on the platinum wire of our melting point apparatus, and the temperature raised extremely slowly, whilst the bead was watched with a lens. In the case of those mixtures which melted before the change occurred, no weight was attached to the bead.
The crystallisation appeared to start at a fairly well defined temperature, and, if it was maintained, would gradually spread through the bead. It is a little hard to say exactly what is the meaning of this curve AA, fig. 2, and all that seems certain is that it gives approximately the temperatures at which the rate of crystallisation assumes a sufficient velocity to visibly change the state of the substance.
From a consideration of the melting point curves, together with the analytical and other observations we have described, it is possible to form some idea as to the nature of these glasses and the crystals into which they are wholly or partially transformable. The glass must be regarded as a superfused, and, therefore, metastableform of the crystals, behaving in several respects as if it were a liquidof enormous viscosity. Now, on considering the behaviour of the glasses ranging in composition from pure boric anhydride to Na20.6B203, it will be noticed that the supposition of the existence of a borate of about the composition Na20.5B203, which is dissolved in boric anhydride, will explain the observed facts. A mixture of composition Na2O.40B2O3 cannot be crystallised at all, and one of composition Na20.16B203 only partially frystallises, hence some point between these mixtures will give the Maximum solubility of this borate in boric anhydride.
On further Increasing the amount of this borate present, the glass becomes supersaturated and superfused liquid, the amount of crystallisation measuring the supersaturation. I Somewhere between Na20.6B203 and Na20.4B203 no free boric mhydride remains in the solution, and the whole mass will crystallise lor the first time.i On further addition of sodium, another borate of composition near 5Na20.4B203 begins to be formed, and we have shown that in any mixture between these two compounds, both crystals and glass have identical compositions. We are here probably dealing with a case of solid solution, and the glass is merely its superfused form.
The character of the curve would indicate that there are two types of crystals, one rich in B203, the other rich in Na20, and the lowest point of the curve is the eutectic point formed by mixtures of these two. That the crest of the curve does not occur with the composition Na20.B203 can be most conveniently explained by the fact that solid solutions cannot be treated as pure substances, and so a slight shifting of the maximum point is to be expected. The further depression of the melting point curve probably indicates the eutectic point between these crystals, rich in Na20, and sodium carbonate.
In conclusion we wish to express our thanks to Professor H. B. Dixon and Mr. D. L. Chapman, for the kindly interest they have taken in our work.
Bugess and Holt, Journ. Chem. Soc. Proc.,’ 1903, p. 221.; W. Guertler, Zeit. Anorg. Chem.,’ vol. 40, 2, pp. 225 and 268 ; yoI. 40, 3, p. 337.
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