The distance between the extreme graduations on the burette must not exceed 65 cm. The rate of outflow of burettes must be restricted by the size of the tip and for any graduated interval the time of free outflow when tested with water at 20° must not be more than three minutes nor less than the following for the respective lengths: (c) Transfer Pipettes. Pipettes for delivering a single volume are designated "transfer pipettes" by the Bureau. The suction tube of each transfer pipette must be at least 16 cm. long, and the delivery tube must not be less than 3 cm. nor more than 25 cm. long. The inside diameter of any transfer pipette at the capacity mark must not be less than 2 mm. and must not exceed the following limits: The outside diameter of the suction and delivery tubes of transfer pipettes exclusive of the tip must not be less than 5 mm. The capacity mark on transfer pipettes must not be more than 6 cm. from the bulb. Pipette tips should be made with a gradual taper from 2 cm. to 3 cm., the taper at the extreme end being slight. A sudden contraction at the orifice is not permitted and the tip must be well finished. The outlet of any transfer pipette must be of such size that the free outflow shall last not more than one minute and not less than the following for the respective sizes: (d) Measuring Pipettes. The tip of the pipette should be made with a gradual taper from 2 cm. to 3 cm., the taper at the extreme end being slight. A slight contraction at the orifice is not permitted and the tip must be well finished. The upper end of any measuring pipette must not be less than 10 cm. from the uppermost mark and the lower end not less than 4 cm. from the lowest mark. The distance between the extreme graduations on a measuring pipette must not exceed 35 cm. The rate of outflow of measuring pipettes must be restricted by the size of the tip and for any graduated interval the time of free outflow must not be more than three minutes nor less than the following for the respective lengths: (e) Cylinders. Only cylinders graduated "to contain" will be accepted for test by the Bureau. The inside diameter of cylinders must not be more than one-fifth the graduated length. Delivery Time. The delivery time marked on any instrument must be within the limits prescribed in the specifications and the error permitted in the marked delivery time is as follows: 98. Precision. - The precision which is attainable in volumetric measurements (when apparatus of standard design is used and the prescribed procedure already outlined with respect to the observance of temperature, reading the position of the meniscus, and method of filling and emptying is followed) is subjoined herewith. From an examination of the figures it will be seen that if we aim at a precision of 1 part per 1000 in our measurements, then we must employ volumes not less than 25 c.c. Further, the error of the indicated capacity of any ten consecutive subdivisions must not exceed one-fourth the capacity of the smallest subdivision. of a precision equal to that called for by the specifications of the Bureau of Standards can now be obtained, much of the apparatus on the market does not come up to these requirements, consequently it cannot be too strongly insisted upon that no piece of apparatus should be used unless it has first been calibrated either by one of the bureaus or by the analyst himself. The difficulty in regard to calibration arises not from the concept involved with respect to the liter which we have only to picture as an invariable region of space equal to one cubic decimeter, but rather from the practical question of how much water at any given temperature must be weighed out in air (when brass or other weights are used) in order that the volume of water may exactly equal one liter or the necessary number of cubic centimeters. 15 For methods of calibration where the capacities of the vessels are determined by comparing them with the capacities of standard pipettes, which are so graduated as to allow a direct reading of the volumes, see Olsen, Quantitative Analysis, loc. cit., § 13; also Morse and Blalock, Am. Chem. Jour. 16, 479 (1894). We have already shown, § 74, that the required weight of water corresponding to one liter, when the weighing is made in air with brass weights and the temperature of the water is 20°, is 997.2 grams. Suppose now that it is not practicable on account of conditions in the laboratory to work at the temperature of 20° but at some other temperature t, which, generally speaking, will be within ±10° of 20°, we still desire the capacity of the apparatus at 20° although the water is at another temperature. The procedure is to note the sum of the weights which are necessary to counterbalance the water, the water being at the constant uniform temperature t, and the weighing being made in air; this weight is then corrected to "in vacuo," and the "in vacuo" weight divided by the weight "in vacuo" of 1 c.c. of water for the temperature t as given in Table 4. The quotient of this division will be the capacity of the apparatus for the temperature t. To arrive at the capacity for 20°, we make use of the proposition established in § 92 that the change in the capacity of glass apparatus is 1 part per 10,000 per 5° change in temperature. Table 4 Weight "in Vacuo" of 1 c.c. of Water (Quoting from: Landolt-Börnstein, loc. cit., §13, p. 74; according to measurements of Thiesen, Scheel and Disselhorst. Wiss. Abh. d. Phys. Techn. Reichsant, 3, 68 (1900). 99a. Exercise No. 2. — Calibrate a 50 c.c. burette, in intervals of 5 c.c. 100. Examples. 1. What is the weight of a liter of water when weighed in air at a temperature of 25° and 760 mm. pressure, using brass weights? Ans. 996.04 g. 2. What error is introduced by using at 30° a solution of 0.1 molar HCl that has been standardized at 20°? Ans. 2.6 parts per 1000 3. In determining the capacity of a liter flask for 20° how accurately must the temperature of the water be measured in order that the calibration be accurate to 0.1 c.c.? Ans.±0.5° 4. It is required to make up one liter of an exactly tenth molar solution of hydrochloric acid. After diluting the roughly calculated volume of 12 molar HCl to one liter it was found upon titration that 40.00 c.c of the solution contained 0.2592 g. of HCl. The calculated volume of this solution was measured out, using for the purpose a 500 c.c. graduated flask and a 50 c.c. burette. This volume was diluted to the mark in a 1000 c.c. graduated flask. Assuming that the burette can be read to 0.02 c.c., and the graduated flasks to 0.2 c.c., and assuming no other errors, what is the total amount by which this solution may differ from 0.1 molar? Ans. 0.7 part per 1000 |