A handbook of laboratory glass-blowing

by Bernard D. Bolas

Methods of Calibration.—In the case of apparatus for volumetric work, this is usually carried out by weighing, although some of the smaller subdivisions are often made by measurement. When the subdivisions are made in this way it is of importance to see that the walls of the tube or vessel to be calibrated are parallel. Great errors arise in some of the commercial apparatus from neglect of this precaution. A convenient method of testing for parallelism, in the case of a wide tube, is to close one end and to weigh in successive quantities of mercury. An observation of the length occupied by each successive quantity will indicate any change in the bore. In the case of capillary tubes, it is convenient to introduce an unweighed quantity of mercury, measure its length accurately, and then to move it along the tube in stages, either by tilting the tube or by the application of air pressure. A measurement of the length at each stage will indicate whether the bore is approximately parallel or not. Neither of these methods is to be relied on without a careful examination of the tube, as it may happen that there are local irregularities in the bore which compensate for each other, and do not, therefore, affect the volume of a given length. Obviously, the smaller the quantity of mercury with which the test is carried out and the greater the number of observations made, the less risk will there be of such an error. A liquid, such as water or alcohol, which wets the glass is not suitable for such a test, unless special precautions are taken.

When, however, a pipette or burette has to be calibrated to deliver a certain volume of water, the final calibration must be made with this liquid. Thus, the burette would first be calibrated by weighing in definite quantities of mercury of say 13.54 grammes (1 cc at 15°C.), each of the 1 cc divisions should be marked by some temporary marking. The burette is now filled with a solution of potassium bichromate and sulphuric acid and allowed to soak for some time; the bichromate is washed out and distilled water is put in. Successive quantities of water are run out of the jet, a fixed time being allowed for draining, and the weights of the quantities delivered are noted. This procedure will give the necessary data for altering the marking so that it may correspond to 1 cc delivered. Each 1 cc division is now divided into tenths by the method described below. A final verification of the markings should be made when the subdivision is completed.

Subdivision of Graduations.—Mark out the spaces to be subdivided on a sheet of paper. Take a reliable ruler on which any convenient length is divided into the desired number and place it across the lines at such an angle that the limits noted on the rule exactly bridge the gap. Now draw parallel lines through the markings.

Copying a Scale.—When a scale has been prepared on paper and it is necessary to copy that scale on the waxed-glass surface for etching, a convenient method is to employ a long wooden bar having a sharp needle passing through it at either end. The scale and object to be marked are fastened in line with one another, and the caliper bar is used from step to step. The mark is made by moving the bar through a minute portion of a circle, which provided that the bar is two or three feet in length, will not introduce any perceptible error in a scale of say a quarter of an inch in width. The arrangement is shown by Fig. 15.

Fig. 15

Graduating a Thermometer.—Assuming that the thermometer has been made of carefully selected tubing in which the bore is parallel and free from any small irregularities, we have only to fix the freezing point and boiling point. The intervening space may then be divided into 100 (if the thermometer is to be Centigrade) or 180 (if Fahrenheit). This division may be carried out by the method given under "Subdivisions of Graduations." A thermometer should not be calibrated until some weeks after making, as the glass bulb tends to contract.

Joining Glass and Metal.—It sometimes happens that one needs to make a more permanent and less flexible joint between a glass and metal tube than can be obtained by means of a rubber tube. To this end, any one of three slightly different methods may be employed. In the method of Chatelier one first coats the glass with platinum or silver, which may be done by moistening the glass with platinum chloride or silver nitrate and then heating to redness; a layer of copper is then deposited electrolytically on the treated surface of the glass, and soldering is carried out in the usual manner.

McKelvy and Taylor call attention to two other methods in the Journal of the Chemical Society for September, 1920. In one of these methods the glass is coated with platinum by covering it with a suspension of platinum chloride in oil of lavender and heating until the oil is burnt off. The metal tube is then tinned on its inner side and soldered to the prepared glass, slightly acid zinc chloride being used as a flux.

In the second method, a joint is made by means of the Kraus flux, which consists of equal weights of zinc oxide, borax, and powdered soda-glass fused together. This is coated on the inner surface of the metal tube, and the hot glass tube, which has had the end slightly flanged to give support, is inserted. Fusion of the flux is completed by heating the outside of metal tube.