Bauhaus-Universität Weimar

Handbook for the Physiological Laboratory. Text
Burdon-Sanderson, John Scott E. Klein Michael Foster T. Lauder Brunton
Rules for Reading Burettes and other Graduated Instruments used in Volu- 
metrical Analysis.—When liquid is contained in a narrow tube, its surface 
is higher at the edges where it touches the glass than elsewhere ; and if we 
examine the curved surface by transmitted light, it seems to be formed of 
several zones or bands, the lowest of which is dark (fig. 350). To avoid 
errors and uncertainty, the under border of the dark zone is always regarded 
as indicating the level at which the liquid stands. In reading, the eye 
must of course be exactly level with the surface, otherwise the reading 
will be either too high or too low. The under surface of the liquid is more 
easily seen if a card, with its under half blackened, while its upper half 
remains white, be held behind the liquid, so that the division between the 
black and white parts is about one-eighth of an inch below its surface. 
The lower surface of the liquid then seems to be bounded by a sharp black 
line (Sutton). Burettes may be read very easily and with great accuracy by 
using Erdmann’s float (fig. 351). This is an elongated glass bulb, weighted with 
mercury at its lower end, so that it floats upright. Its diameter being a very 
little less than the calibre of the burette which contains it, it moves freely, 
but at the same time steadily, up and down. A horizontal mark round its 
middle is taken as indicating the height at which the liquid stands, the 
absolute height being disregarded. 
Litmus Solution.—The solution used in the neutralization of albuminous 
liquids is prepared by dissolving a little litmus in distilled water, decanting 
the liquid from the sediment and diluting it as required. For determina¬ 
tions of the strength of acid, the litmus solution is made by putting 10 
grammes of solid litmus into half a litre of distilled water, letting it stand 
for a few hours in a warm place, decanting the clear fluid, adding a few 
drops of dilute nitric acid so as to produce a violet colour, and preserving 
it in an open bottle with a narrow neck. If the colour should at any time 
partially disappear, it may be restored by exposing the liquid to the air in 
an open bottle (Sutton). 
Volumetric Solution of Soda.—Fill a burette with solution of soda, and 
cautiously drop this into 6*3 grammes of purified oxalic acid in crystals, 
quite dry but not effloresced, dissolved in about 70 c. c. of distilled water, 
until the acid is exactly neutralized, as indicated by litmus. Note the 
number of grain measures (n) of soda solution used, and having then 
introduced 900 cub. cent, of it into a graduated jar, augment this quantity 
by the addition of water until it becomes-^^ — cub. cent. If, for exam- 
pie, n = 93, the 900 cub. cent, should be augmented to 
= 967’7 
cub. cent. 100 cub. cent, contain ^th of an equivalent in grammes 
(4 grammes) of hydrate of soda, and will neutralize T‘öth of an equivalent 
in grammes of an acid. 
Soda solution for estimating the acidity of gastric juice is made by 
diluting 100 c. c. of the above solution to the bulk of a litre. 
218. Polariscope.—There are several organic substances whose solu¬ 
tions possess the power of circumpolarization, i.e., of rotating to one side 
or another the plane of polarization of a ray of polarized light passing 
through them. Some of them, such as glucose, cane-sugar, and tartaric 
acid, turn it to the right hand, while others, such as albumin, uncrystal- 
lizable sugar, and oil of turpentine, turn it to the left. As the amount of 
rotation increases in proportion to the concentration of the solution and 
the thickness of the stratum through which the ray passes, it is easy to 
ascertain the quantity of a substance held in solution by simply observing 
the extent to which a ray is rotated in passing through a stratum of 
a definite thickness. The apparatus used for this purpose is shown in fig. 
353. It consists of a stand in which are placed two Nicol’s prisms, a and b<


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