I 4 QUANTITATIVE AGRICULTURAL ANALYSIS
I may be used or where no great accuracy is required. In such cases
j the sample weight may be accurately and quickly adjusted to
I the second or third decimal and the remaining uncertainty will
| be relatively insignificant. For example, if a 10-gm sample of
j soil is to be used for a nitrogen determination, an uncertainty
i of 1 mg in weighing will involve only 0.01 per cent of the total
nitrogen found. But if a 0.5-gm sample of limestone were to be
used for a determination of calcium, this same uncertainty
] would amount tO o.2 per cent.
; Temperature Systems.—In nearly all scientific work the
J Centigrade system is used exclusively for indicating tempera-
. tures and in this book all temperatures mentioned are in Centi-
grade unless otherwise designated. In some instances the special
agricultural analyst will have to use the Fahrenheit system in
| order to conform to established usage. When this is done in the
following pages, the letter "F" will follow the figures indicating
f, the temperature.
' Volumetric Analysis.—The final determination of per cent
1 by volumetric methods is not made by means of weighing a pre-
\ cipitate. The balance is generally used, as in gravimetric
I methods, for weighing the sample. The solution of the latter
is then brought into definite reaction with another solution of an
I appropriate reagent (a standard solution) until the reaction is
* exactly completed. The concentration of the standard solution
>' is accurately known as a result of a previous analysis (a stand-
ardization) and the volume required is measured accurately by
means of a graduated burette. The product of the required
volume of the standard solution and its concentration, giving
the weight of the dissolved reacting material, serves as a measure
of the determined constituent of the sample, just as the weight
of the precipitate does in gravimetric analysis, the only difference
; in principle being the use of the weight of a reacting body instead
of that of a containing body as a measure of the thing to be
; determined. With this exception the calculations will be
similar to those of gravimetric analysis, a titration serving
instead of a weighing.
; As an illustration, the determination of sodium hydroxide
in an impure sample may be cited. A weighed quantity of the
material is dissolved and titrated by a standard solution of