Open Tube Test

Glass tubing of hard glass is used in making what are known as open tube test. The tubing g should be cut into approximately 8- inch lengths and have an internal diameter of ¼ inch. An open tube is used ordinarily for making g oxidation test. A small amount of the mineral to be tested is commonly powdered and placed in the tube at a point about one-third of its length from one end. A narrow strip of paper folded into a shallow trough will serve as a boat to introduce the powder into the tube. The tube is the inclined at as sharp an angle as possible, with the mineral lying nearer the lower end. The tube is then held over a Bunsen burner flame in such a way that the flame plays on the upper part o f the tube. This serves to concert the inclined tube into a chimney, up which a current of air flows. After a moment the tube is shifted so that the flame heats it at a point just above the mineral, or in some cases the flame may be directly beneath the mineral. The mineral is being heated under these conditions in a steady current of air, and it will be oxidized if such a reaction is possible. Various oxides may come off as gases and either escape at the end of the tube or be condensed as sublimates upon its walls. The following table gives a list of those elements which yield characteristic reactions when heated in open tubes;

  

Element   Description of Test Sulfur. Sulfur dioxide, SO2, comes out of upper end of tube as a gas with a pungent and irritating odor. If a moistened strip of blue litmus paper is placed at the upper end of the tube, it becomes red, due to the acid reaction caused by the sulfurous acid. Arsenic. Arsenious oxide, As2O3, condenses at a considerable distance above the heated portion as a volatile coating of small colorless octahedral crystals Antimony. Antimonious oxide, Sb2O3, deposits as a volatile white ring closer to the heated portion of the tube than the arsenious oxide. Antimony sulphides yield also a dense nonvolatile white sublimate of antimonate of antimony, Sb2O4, which collects along the bottom of the tube. Molybdenum Molybdenum trioxide, M0O3, collects near the heated portion as a network of pale yellow to white crystals. Mercury. Collects in minute gray globules which can be rubbed together.

Other reactions may be obtained from some of the above elements if the mineral is heated too rapidly or without the establishment of a strong current of air flowing through the tube.