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Fill the burette to a point above the zero-point and draw off the water until the meniscus is just below that mark. It is then ready for calibration. Chromic acid in sulphuric acid is usually found to be the best cleansing agent, but the mixture must be warm and concentrated.
The rider should also be compared with a 5 or 10-milligram weight. In an instructional laboratory appreciable errors should be reported to the instructor in charge for his consideration. When the highest accuracy is desired, the weights may be calibrated and corrections applied. A calibration procedure is described in a paper by T.W. Richards, !J. Am. Chem.
There is a possibility that a written symbology did at one time exist, for just that purpose. If so, it has probably survived as a ritualistic form when an officer is appointed to a post, let's say, he may get a formal paper that says so. They may use symbols to signify rank and so on. They certainly must have a symbology for the calibration of scientific instruments.
The bore of burettes is subject to accidental variations, and since the graduations are applied by machine without regard to such variations of bore, local errors result. The process of testing these instruments is called !calibration!. It is usually accomplished by comparing the actual weight of water contained in the instrument with its apparent volume.
The latter is the usual course for small corrections, and it is convenient to calculate the correction corresponding to each cubic centimeter and to record it in the form of a table or calibration card, or to plot a curve representing the values.
Draw off the liquid from 10 cc. to about 20 cc. into the same flask without emptying it; weigh, and at the expiration of three minutes take the reading, and so on throughout the length of the burette. When it is completed, refill the burette and check the first calibration. The differences in readings represent the apparent volumes, the differences in weights the true volumes.
In all work, whether of calibration, standardization, or analysis, the temperature of the liquids employed must be taken into account, and if the temperature of these liquids varies more than 3° or 4° from the standard temperature chosen for the laboratory, corrections must be applied for errors due to expansion or contraction, since volumes of a liquid measured at different times are comparable only under like conditions as to temperature.
The latter cannot exceed 0.01 cc. in accuracy, which corresponds to 0.01 gram. The student should clearly understand that !all other weighings!, except those for calibration, should be made accurately to 0.0001 gram, unless special directions are given to the contrary.
If the titration has occupied more than the three minutes required for draining the sides of the burette, the final reading may be taken immediately and recorded in the notebook. Refill the burettes and repeat the titration. From the records of calibration already obtained, correct the burette readings and make corrections for temperature, if necessary.
It had been intended to set up recording magnetic instruments at the base, and to take a continuous series of records throughout the whole period of residence there, absolute measurements of the earth's horizontal magnetic force, of the dip and declination being taken at frequent intervals for purposes of calibration.
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