Page 115 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CALIBRATION OF VOLUMETRIC APPARATUS 3.16

The Lunge-Rey pipette is shown in Fig. 3.3(b). There is a small central bulb
(5-10 mL capacity) closed by two stopcocks 1 and 2; the pipette 3 below the
stopcock has a capacity of about 2 mL, and is fitted with a ground-on test-tube 4.
This pipette is of particular value for the weighing out of corrosive and fuming
liquids.

3.14 PISTON BURETTES
In piston burettes, the delivery of the liquid is controlled by movement of a
tightly fitting plunger within a graduated tube of uniform bore. They are
particularly useful when the piston is coupled to a motor drive, and in this form
serve as the basis of automatic titrators. These instruments can provide
automatic plotting of titration curves, and provision is made for a variable rate
of delivery as the end point is approached so that there is no danger of
overshooting the end point.

3.1 5 GRADUATED (MEASURING) CYLINDERS
These are graduated vessels available in capacities from 2 to 2000 mL. Since
the area of the surface of the liquid is much greater than in a graduated flask,
the accuracy is not very high. Graduated cylinders cannot therefore be employed
for work demanding even a moderate degree of accuracy. They are, however,
useful where only rough measurements are required.

3.16 CALIBRATION OF VOLUMETRIC APPARATUS
For most analytical purposes, volumetric apparatus manufactured to Class A
standard will prove to be satisfactory, but for work of the highest accuracy it
is advisable to calibrate al1 apparatus for which a recent test certificate is
unavailable. The calibration procedure involves determination of the weight of
water contained in or delivered by the particular piece of apparatus. The
temperature of the water is observed, and from the known density of water at
that temperature, the volume of water can be calculated. Tables giving density
values are usually based on weights in oacuo (Section 3.6), but the data given
in Table 3.2 are based on weighings in air with stainless-steel weights, and these
can be used to calculate the relevant volume directly from the observed weight
of water. It is suggested that the data given in the table be plotted on a graph
so that the volume of 1 gram of water at the exact temperature at which the
calibration was performed can be ascertained. Fuller tables are given in
BS 6696 (1986).

Table 3.2 Volume of 1 g of water at various temperatures

Temp. (OC) Volume (mL) Temp. (OC) Volume (mL)