Sodium tetraborate flux

ABSTRACT A rapid and precise X-ray fluorescence method has been developed for the multielement analysis of gypsum and gypsum products. Gypsum specimens are calcined at IOOO°C and then fused with sodium tetraborate flux into flat and transparent disks. The choice of a suitable flux system for the specimen preparation is critical because of a rapid decomposition of anhydrite. CaSO,. in lithium ba fluxes at temperatures above 95O C. This decomposition causes not only visible imperfections in the didi surface but also alters considerably the concentrations of the major elements, calcium and sulfur. The procedure used for a fast setup of ten element analysis of gypsum on the Philips PW-1400 spectrometer utilizing synthetic standards and off-line calculated alpha coefficients is presented. Calibrations carried out with chemically analyzed specimens and their mixtures are compared lo those performed with synthetic standards prepared by blending pure chemicals and anhydrite into the flux. 

Sodium tetraborate (Spectroflux 200 , Johnson and Matthey Co.) was tried with much greater success. Disks produced by fusion of 0.5000 g of calcined gypsum and 6.00 g of sodium tetraborate (Table 1) produced absolutely clear and transparent disks with perfect surfaces. The use of a higher specimen to flux ratio was dictated by the lower solubility of anhydrite in molten borax. 

The advantages of using sodium tetraborate for the fusions outweigh the loss of sodium as an analyzed element. Times required for the fusion and swirling of the flux on flame (Table 1) were quite short. The use of lithium fluoride and lithium bromide was eliminated completely. The disks prepared from sodium tetraborate release easily from the molds each time, without any sign of sticking, cracking, or crystallization. 

After reaching the upper temperature set limit, the temperature was held constant (isothermal hold) for several hours. The weight-loss curves (TG) are shown in Fig. 1. The weight losses recorded on lithium and sodium fluxes alone (Spec-troflux 100 and 200) caused by thermal decomposition above i000°C were negligible. Similar results were obtained with mixtures of anhydrite and sodium tetraborate. The latter showed a weight loss of less than 0.1% when heated at lOOO C for 1 h. 

The gum was suspected to make a major contrlhution to the flux decline. Experiments using a solution containing only a gum, a guar gum similar to that used in the process, confirmed this hypothesis. An additional experiment in which the guar gum was transformed into a gel by the addition of sodium tetraborate at pH 8 was performed. The flux declined further when the gel was formed and then increased when the pH was lowered to 6 to redissolve the gum. 

Borax is valuable as a flux - that is, a substance that will react with difficultly fusible substances to form compounds that fuse readily. The action of borax as a flux in soldering and welding depends upon the reaction of sodium tetraborate with metallic oxides on the surface of the metal. Metaborates that are formed are easily melted, and the metallic oxides are... 

A fourth method involves the dilution of the sample into a matrix so that the major constituents are the same for both standards and samples. Fusion fluxes often used for this purpose include borax, boric acid, lithium tetraborate and sodium tetraborate (Plowman, 1971 Matsumura et al., 1973 Chandola and Mobile, 1976). Dilution into a dry powder mixture is also effective. Lytle and Heady (1959) recommended LiCOj as a suitable diluent. For both the thin sample and the dilution techniques, detectability is sacrificed for improved accuracy. Consequently, these methods are mainly applied to rare earth determinations at the major and minor constituent levels. 

The fluxes used with both silver and gold jewelry brazes are standard formulations, nsed also in industrial brazing. The most common fluxes used with gold brazing alloys are based on sodium tetraborate (Na2B407 IOH2O), commonly known as borax, which is fluid above 760°C, applied in the form of a paste. 

Probably the most common fluxes are sodium carbonate (Na2C03), lithium tetraborate (Li2B407), and lithium metaborate (LiB02). Fluxes maybe used by themselves or in combination with other compounds, such as oxidizing agents (nitrates, chlorates, and peroxides). Applications include silicates and silica-based samples and metal oxides. 

Inorganic substances that do not dissolve in acid can usually be dissolved by a hot, molten inorganic flux, examples of which are lithium tetraborate ( 26407) and sodium hydroxide (NaOH). Mix the finely powdered unknown with a mass of solid flux that is 2 to 20 times the mass of the unknown. Fuse (melt) the mixture in a platinum-gold alloy crucible at 300° to 1 200°C in a furnace or over a burner. When the sample is homogeneous, carefully pour the molten flux into a beaker containing 10 wt% aqueous HNO to dissolve the product. 

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