THE SYSTEM SODIUM DISILICATE-SODIUM FLUORIDE HAROLD SIMMONS BOOTH, BERNARD A. STARRS,' BAHNSEN2
AND
MONROE JOHN
Morley Chemical Laboratory, Western Reserve Universitu, Cleveland, Ohio Received July 86, 1933
The ternary system NazO.SiOz.NaF has been shown to be of fundamental importance in the composition of porcelain (vitreous) enamels. Two of the binary systems contained in the above ternary system have been published, namely, sodium oxide-silica (1) and sodium metasilicatesodium fluoride (2). This paper describes the study of the system sodium disilicate-sodium fluoride. This is of particular interest to enamelists in view of the suggested use of sodium silicates as a raw material (3)) especially since the disilicate contains a high per cent of silica (66.0 per cent) and has a low melting point (874°C.). The preliminary study (2) of this system showed that difficulties would be encountered owing to the very small heat arrests and the sluggishness of the disilicate in attaining equilibrium. However, by using a special recording potentiometer and supplementing this continuous record with differential readings taken a t least every 30 seconds, the breaks could be readily detected. I n this way we were able to obtain the data without using the tedious and somewhat less accurate quenching methods. EXPERIMENTAL PROCEDURE
Chemicals Some of the silica was obtained from large clear quartz crystals. The foreign matter was scraped off and the crystals boiled in aqua regia. After being thoroughly washed they were placed in a large platinum dish, heated, then suddenly chilled by a stream of cold distilled water. This shattering process was continued until the pieces became small. They were then ground to a powder in an agate mortar. The other pure silica was from commercial powdered quartz. After a water wash to remove any lint it was boiled three times with hydrochloric acid and twice with aqua regia, followed by a thorough water washing. 1 Holder of the Cushman Fellowship for Fundamental Research in Vitreous Enamels, 1930-1931. * Holder of the Cushman Fellowship, 1931-1934. 1103
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H. S. BOOTH, B. A. STARRS AND M . J. BAHNSEN
Both of these samples were then dried in an electric oven a t 120°C. On treatment with hydrofluoric acid they gave, respectively, 0.03 and 0.04 per cent residue. The sodium oxide content was obtained by using C.P. sodium carbonate monohydrate, which by conversion to the sulfate was shown to be 99.90 per cent pure. Merck’s C.P. sodium fluoride was recrystallized twice from water, dried in platinum in an electric oven, and by conversion to the sulfate was found t o be 99.00 per cent sodium fluoride. This sample contained 0.80 per cent water. The sodium disilicate was made by fusing the calculated amounts of sodium carbonate monohydrate and silica in covered platinum crucibles, in a small electric furnace. Since the high viscosity of the disilicate prevented it from being poured, the outside of the crucible was chilled with a stream of cold water. This caused the melt to crack enough so tha.t it could be removed. The material was then ground to a powder in an agate mortar. This was re-fused and re-powdered until it became homogeneous.
Apparatus The furnace (2) was a vertical resistance furnace, having two windings, an inner one of platinum-20 per cent rhodium alloy wire and the outer of Chrome1 resistance wire, 110 volts A.C. was used on both coils, the current being controlled by two water-cooled rheostats. The thermocouple was a standard platinum and platinum-10 per cent rhodium couple. A differential junction surrounded by a platinum-covered nickel “neutral body” of approximately the same heat capacity as the platinum crucible and melt was suspended about 6 cm. above the melting point junction. The melting point crucible was a heavy cylindrical, platinum crucible, 5 cm. high and 1.6 cm. in diameter. The thermocouples and supports for holding the crucible were in a single unit which could be raised and removed from the furnace. The heating curves were obtained with a specially designed Leeds and Northrup “Micromax” recording’ potentiometer. This recorder balances every second and has a paper speed of sixteen inches an hour. The temperature can be read to f 0.5”C.from 500°C. to 1200°C. The differential temperature readings were made with a ‘(studenttype” potentiometer and a “wall type” galvanometer. At every reading of the differential temperature a mark was made on the recorder paper by means of an electrically controlled pen, In t8hisway the differential curve could readily be plotted on the recorder sheet.
SYSTEM SODIUM DISILICATE-SODIUM
FLUORIDE
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Preparation and melting points of mixtures The various mixtures were prepared by weighing out theoretical amounts of the disilicate and sodium fluoride, mixing thoroughly, fusing slowly in a small electric furnace, then chilling rapidly, as mentioned previously. After being ground to a powder each was re-fused and again powdered in order to obtain a homogeneous mixture. T o obtain a melting point, this powder was introduced into the crucible, the thermocouple inserted, and the entire unit placed in the furnace. It was rapidly heated to above the melting point of the mixture, and allowed t o cool slowly, so that crystallization could take place. The furnace was then heated again, this time slowly, and both the melt and the differential temperatures were recorded. After the melting point was reached the furnace was again cooled slowly. Only heating curves were taken, as the mixture always undercooled before crystallizing. I n every case a t least three checks were obtained on both the solidus and liquidus points. It is difficult to obtain sharp breaks by the usual method of thermal analysis in the sodium disilicate region. However, by carefully controlling the rate of heating of the furnace and taking a continuous melting point record and simultaneously taking the differential readings, the breaks in the heating curve could be easily detected and duplicated, The thermocouple was checked before and after every melting point. The standards used were sodium chloride (800.4"C.) (4) and sodium metasilicate (1089°C.)
.
(1). I n a number of cases the melts, after being used for a melt,ing point determination, were powdered and weighed; definite quantities of either the disilicate or sodium fluoride were then added. The melting points of such mixtures were identical with those of the mixtures of like composition prepared by mixing only the disilicate and sodium fluoride. This indicates that the melt does not change in composition on being fused. RESULTS
The data obtained are given in table 1 and plotted in figure 1. The system is a simple V eutectic type, no compounds being formed. The eutectic composition is 39.5 f 0.1 mole per cent of sodium fluoride and the eutectic temperature is 797°C. f 1°C. The melting point of the disilicate was found to be 874"C., agreeing with the value obtained by Kracek (1). The solidus disilicate relations have been shown by Kracek (1) to be very complex. At present the solidus relations for the system sodium disilicate-sodium fluoride below the eutectic temperature have not been studied sufficiently to enable one to draw any definite conclusions.
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H. 8. BOOTH, B . A. STARRS AND M. J. BAHNSEN
TABLE 1 The system sodium disilicate-eodium .fluoride POINT NO
MOLE PER CENT BODIUM BLUORIDEl
WEIGHT PER CENT SODIUM FLUORIDE
TEMPERATURE8
Liquidus degrees C.
1 2 3 4
5 6 7 8 9 10 11 12 13 14
0.00 10.00 20.00 32.52 39.50 42.10 47.04 53.00 59.11 70.00 81.14 90.97 96.06 100.00
0.00 2.50 5.45 10.00 13.09 14.36 17.00 20.64 25.00 34.98 49.80 69.91 84.90 100,00
874 861 850 825 797 817 870 915 946 973 985 988 991 995
Solidus deQTee8
799 796 797 797 799 798 799 796 797 802 793 798
FIG.1 . THESYSTEM SODIUM DISILICATE-SODIUM FLUORIDE
C.
SYSTEM SODIUM DISILICATE-SODIUM
FLUORIDE
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SUMMARY
The system, sodium disilicate-sodium fluoride, is a one eutectic type and the eutectic occurs at a composition 39.5 f0.1 mole per cent of sodium fluoride and a t a temperature of 797°C. f l.O°C. REFERENCES (1) (2) (3) (4)
KRACEK, F. C.: J. Phys. Chem. 34, 1583 (1930). BOOTHAND STARRS: J. Phys. Chem. 36, 3553 (1931). MANSON, M. E. : J. Am. Ceram. SOC.14, 490 (1931). International Critical Tables, Vol. 1, p. 54. McGraw-Hill Book Co., New York (1926).
