THE ALKALI CARBONATE FUSION IN QUALITATIVE ANALYSIS JACOB CORNOG AND HERSCHEL HOPSON, UNIVERSITY 08 IOWA, IOWA CITY, IOWA Historical Historically, the use of alkali carbonates in metallurgical and ceramic processes is very old. Mineralogists were among the earlier ones to use the alkali carbonates in the laboratory for disintegrating minerals, in a way analogous to the present practice in making sodium carbonate fusions. From H. Rose,' i t may be learned that in 1765 Marggraff used the alkali carbonate disintegration of minerals in the "dry way"; and that during the earlier part of the nineteenth century, workers who disintegrated minerals by alkali carbonates in the dry way were troubled by contamination of the sample through disintegration of the crucible. To avoid such contamination the refractory sample was disintegrated in the "wet way," that is, by repeated extraction of the sample with-a water solution of the alkali carbonate. It may be inferred, though Rose does not so state, that disintegration in the dry way came to be preferred when platinum crucibles became available. H. Rose2studied the decomposition of barium sulfate by fusion with alkali carbonates from what now would be termed the viewpoint of mass action. He reports as follows: 1. That when barium sulfate and potassium carbonate were fused together in a 1:l ratio, the barium sulfate was only partly decomposed, but tbat a 1:6 ratio of potassium carbonate or a 1:8 ratio of sodium carbonate caused complete decomposition of the barium sulfate. 2. That the addition of potassitlm sulfate to fused mixtures of potassium carbonate and barium sulfate hinderedrthe decomposition of the barium sulfate. When the ratio of potassium sulfate to potassium carbonate was 3:l or greater, no barium sulfate was decomposed by the fusion. S ~ r i n g Parker,4 ,~ and Tammens found tbat partial decomposition ensued when mixtures of powered barium sulfate and alkali carbonate were subjected to great pressures a t temperatures below the melting point of the mixed dry salts. Niggli6 reports that a mixture of 46 mole per cent of potassium carbonate and 54 mole per cent of sodium carbonate, constitute a eutectic mixture having a melting point 712'. Since the melting point of sodium carbonate is 85Z0, and that of potassium carbonate is S9l0, the eutectic mixture, because of its lower melting point, would seem a more convenient flux than either sodium carbonate or potassium carbonate alone. However, Scott7 raises the objection that silicates are less
' Pogg. Ann., 94,481 (1855). Ibid., 95, 96 (1855). Bull. 5'06. Chim. (2) 44,166 (IS%), and (2) 46,299 (1886). J . Chem.Soc., 113,397 (1918). ". anorg. Chem., 149,21 (1925). IIbid., 106, 126 (1919). "Standard Methods of Chemical Analysis," 4th ed., D. Van Nostrand Ca., New
York City, p. 444. 618
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apt to be disintegrated in a flux having a 712" melting point than in a flux having an 852' melting point, though he does not give any reason why the low-melting flux could not be heated to the higher temperature. In concluding this historical review, i t may be noted that until recent times the gas flame was used to cause fusions and that temperatures were gaged by the incandescence of the cruable. Later workers used the more accurately controlled electrical heating devices. Experimental In the present report the following items relating to the alkali carbonate fusion have received experimental study, particularly with reference to their use in elementary qualitative analysis. A . The crucible. R. The quantity of flux. C. Getting the fusion mixture to melt. I. The eutectic flux. II. The flower pot furnace. D. Removing the melt from the crucible. Apparatus and Materials.-An electrical resistance furnace was used for heating where closely regulated temperatures were employed. Temperatures were measured by a platinum, platinum-iridium thermocouple, and a millvoltmeter and are accurate to 3'., The reagents used were of the "analyzed variety as obtained from commercial sources. A . The Crucible. Typical fusions were ma& in crucibles of platinum, nickel, iron, and porcelain. The platinum and nickel crncibles had diameters of 4 cm. and depths of 4'/, cm. The iron crucibles were hemispherical, having a diameter of 4 cm. Barium sulfate was the refractory material used in the typical fusions. The flux used was the eutectic mixture of sodium carbonate and potassium carbonate described later in this publication. The quantity of flux used corresponded to six moles of the alkali carbonate to one mole of the barium sulfate. The fusions were made in unshielded crucibles with a Bunsen burner used as the source of heat. In each case the melt was digested in water, the water-insoluble portion was isolated by filtration, dissolved in G A4 hydrochloric acid, precipitated and weighed as barium sulfate. The recovery of barium sulfate is indicated in Table I. TABLE I BARIUM SULFATE RECOVERED AFTER FUSION Using platinum aucible Using nickel crucible Using porcelain crucible Using iron crucible
99.75 99.47 98.G8 97.25
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These data indicate that any one of these four kinds of crucibles may be used in qualitative analysis if, in each case, consideration is given to the kind of contamination that results. The iron crucibles merit special mention. They were hemispheres of sheet iron costing less than one cent each. They were the so-called "domes of silence" which furniture dealers sometimes use to cap chair legs. During fusion these iron crucibles flaked badly. The flakes appeared later in the water-insoluble portion of the melt. These flakes are not appreciably dissolved by water or by 6 M hydrochloric acid and may be removed by filtration. These iron crucibles will not ordinarily withstand more than one fusion but their cheapness commends their use where iron contamination is permissible. Porcelain crucibles showed less disintegration than might have been expected, but because of the nncertainty of their composition the nature of the contamination they give is also uncertain. Where available, platinum crucibles are most satisfactory. B. The Quantity of Flux and Other Factors. In these experiments barium sulfate was fused during 30 minutes a t 850' with sodium carbonate. The quantity of flux used varied from one-half to four moles of sodium carbonate to one mole of barium sulfate. In each case the melt was analyzed for each of the four substances represented in the following equation. Bas04
+ Na2COs--f BaCOa + Na2S04
(1)
The data thus obtained w a r r q t the following observations: 1. The percentage of barium sulfate decomposed increased progressively as the quantity of flux was increased. This relation is indicated in the following summary.
TABLE I1 Initial molar ratio BaSO.: Na2C0. D/o Bas04 decomposed
I:% 1:1 1:2 1:s 1:4 16 35 78 92 99
2. Values computed for the equilibrium constant of the reaction represented by equation (1) vary widely when the initial ratios of barium sulfate and sodium carbonate are varied. This variation may he due to any one or all of the following causes; equilibrium might not have been reached during fusion, the reaction may he heterogeneous, an indeterminate shift in equilibrium might have occurred while soaking the melt. 3. When successive runs were made using .the same initial ratio of barium sulfate to flux, discordant results were obtained when the duration of soaking was prolonged beyond 30 to 60 minutes. This tendency to reversibility during soaking decreases as the initial quantity of flux is increased. In the present study no experiments were made to determine the influence exerted by the length of time the fusion mixture was in the molten
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state. Park& studied the rate of transformation of sodium carbonate when fused with barium sulfate and found that after the mixture had been in the fused state for three minutes no further change was noted up to thirty minutes. After the contents of the crucible have been completely melted for a few minutes there is little point in continuing the fusion for a long time unless the refractory material being reduced is in rather large pieces. C. Getting tlte Fusion Mixture to Melt. When the fusion mixture does not melt after a few minutes heating, i t follows that the melting point of the mixture is above the temperature of the heat applied. Since the melting point of sodium carbonate is 852' and that of potassinm carbonate is 891' i t may be supposed that a temperature of approximately 900' is suitable for use in alkali carbonate fusions as ordinkily conducted. With adequate gas pressure the necessary temperature may be obtained with special burners of the blast or Fisher type. But in many laboratories such burners are not available. In the following paragraphs two expedients are suggested to meet the difficulty. I. The eutectic $ux. The use of mixtures of sodium carbonate and potassium carbonate as a flux rather than either one alone is an old practice. As reported by Niggli6 a mixture of 54 moles of sodium carbonate with 46 moles of potassium carbonate forms a eutectic having a As redetermined in the present study the eumelting point of 712'. tectic mixture is composed of 59 moles of sodium carbonate to 41 moles of potassium carbonate with a melting point dose to that reported by Niggli. Fusion mixtures using the eutectic flux are easily melted by the heat of a Bunsen burner. No shielding is necessary. To put to the proof Scott's' objection, the eutectic flux was used in a number of fusions that were made in unshielded platinum crucibles that were heated for 30 minutes by an ordinary Bunsen burner. Samples of silica, barium sulfate, and granite, as representative refractory materials, were decomposed in this manner. After each fusion the residue not dissolved by water or by dilute hydrochloric acid was weighed. The weights of the original samples and of the residues are shown in Table 111. Material
Granite Granite Silica
TABLE 111 Wt. of samole Wt. of residue 0.9849 0.9524 1.000
0.0024 0.0024 0.0000
Per cent of sample undecomnosed
0.24 0.25 0.00
These data indicate that the eutectic flux is suitable for routine fusions in qualitative analysis. Loc. cit., p. 396.
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II. The jbwer Fat furnace. To make a flower pot furnace, enlarge, if necessary, the gas inlet a t the bottom of a Bunsen burner till a flame ten or more inches high may be obtained. Also knock the bottoms from two clay assay crucibles. Clay flower pots will serve. Assemble the apparatus as indicated in Figure 1. It is essential that the opening into the lower clay crucible be a t sufficientdistance above the burner to allow an abundant supply of air to enter the combustion cavity with the flame. During the operation of the flower pot furnace, a slight roar suggestive of a gas muffle may be heard. .The authors have had no difficulty in fusing sodium carbonate in this furnace. D. Removal of the Melt from the Crucible. Both economy of time and the undesirable effects of long soaking of the melt in water indicate the desirability of quickly removing the melt from the crucible. The following expedients, none of them new, have been tried. If the crucible used is of platinum and is free from dents and deformities, the melt may ordinarily be removed by allowing the crucible to cool to room temperature and inverting the crucible. The melt, having the greater coefficient of contraction, will often crack completely loose from the cru-
-
-
I
0
\
cible. The expedient of inserting a platinum wire in the liquid melt and later using it as a handle when the melt has solidified may be used. A variation of this practice is to heat the crucible enough to loosen the melt, then use the handle to lilt the melt. The removal of the melt is more difficult when the crucible is rough or deformed. If the crucible is tipped in an oblique position so that the mass solidifies in contact with only one side and part of the bottom of the
FIGURE1
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crucible, a few sharp raps usually suffice to dislodge the melt. If undesirable contamination does not occur, a very good method for handling the melt is to pour the hot liquid melt out on a cold slab, just as one would form a batter cake. Whenever the melt is removed from the crucible without water soaking, the solid cake should be pulverized before placing it in water. This is for the purpose of reducing the time of soaking. For routine use in qualitative analysis the practice of pouring the hot melt on a cold slab is commended. In cases where it is impractical or undesirable to remove the melt by any other method than water soaking, the time necessary for soaking will be reduced if the surface of the solid melt in contact with the water is increased. Hence allowing the melt to solidify while the crucible is in an inclined position, or rolling the crucible just before tKe melt hardens s'o as to spread the melt over much of the interior surface of the crucible, or any other expedient that will increase the free surface of the melt when it congeals will lessen the time necessary for soaking. Conclusions The data described in the text of this paper support the following conclusions regarding the use of the alkali carbonate fusion in qualitative analysis. 1. Crucibles of platinum, nickel, iron, or porcelain may be used for making alkali carbonate fusions if heed is given to the kind of contamination that results in each case. c 2. When barium sulfate and sodium carbonate are fused together the quantity of barium sulfate decomposed increases as the molar ratio of the flux used increases. When the ratio of four moles of sodium carbonate to one of barium sulfate is reached, the barium sulfate is completely decomposed. The decomposition reaction is somewhat reversible when the melt is soaked in water more than 30 to 60 minutes. 3. The composition and melting point of a eutectic mixture of sodium carbonate and potassium carbonate have been redetermined. The eutectic mixture is composed of 59 moles of sodium carbonate to 41 moles of potassium carbonate and has a melting point of 710' * 1.5'. 4. The eutectic mixture has been found to be a satisfactory flux for decomposing silica, barium sulfate, and granite, a t temperatures obtained by the use of a Bunsen burner. 5. The flower pot furnace has been described. By its use sodium carbonate may be fused without resort to the use of the blast or other special burners. 6. Methods for removing the melt from the crucible are described. The practice of pouring the hot liquid melt out of the crucible onto a cold slab is commended.
