54 QUANTITATIVE AGRK.'UL'I'ritAL AXALYMX

The dilution is carried out as follows: Fill a dry IMO-w volumetric flask
exactly to the mark, add the necessary water from a burette and mix well.
This requires a flask that will hold the required added water uhorc f.h<» mark.

In case the dilution ratio has been found to be greater (ban about 1.010,
as in this example, the dilution should be accomplished in two steps. Tim
solution is first diluted, adding 3 or 4 ec of water lass than the calc-ulnfed
amount. The solution is then mixed, ^standardized and the final adjust-
ment is made with greater accuracy.

Titration.—Weigh samples of about 0.2 to 0.5 gm of the chloride sample
into 200-cc casseroles or beakers, liefer to the directions given on page
50 for weighing and recording weights. Dissolve the weighed samples in
50ccof chloride-free water, add 1 cc of potassium ehromute solution ami
titrate as in the standardization of silver nitrate .solution. Multiply tin*
number of cubic centimeters of standard solution required by its valw in
terms of chlorine, divide by the sample weight and multiply by 100, to
obtain the per cent of chlorine in the sample.

Use of a Correction Factor.— There in a too common practice
among chemists, and especially among industrial analysts, of
using standardized solutions with a correction factor instead of
diluting them to the desired concentration. In the example
illustrated above the solution would be used an a tenth-normal
solution, the factor 1.035 being used in the calculations of tit ra-
tions to correct for the over-concentration. Or if a decimal
solution were desired and if, for example, the first standardisation
showed the chlorine factor to be 0.005012 gm, instead of 0,1)05
gm, the calculations corresponding to Eq. (4) of pages 5 would
be

100 7 X 0.005 X 1.0024

- . - - __„,,_„ ..... __ ........ .5. » .,, 4
This common technical error is based upon fallacious reasoning.
In actual practice the standard solution IB generally made in
quantity for a considerable number of determinations, economy
of time resulting from lining one standardisation for all. In
such a case the solution should be diluted to the* denired con-
centration, so that simplicity of calculations may result from the
use of the milligram-equivalent (in the normal system) or of the
simple factor of one significant figure, such as 0.005 an in the cane
already considered, for the substance to be calculated. Even if
the solution is to be used for only one or two determinations the