TM 5-6630-215-12
(3) As an aid, the following sample dilution table 2-1 shows the
amount of water sample taken, the amount demineralized water used
to bring the volume up to 25 ml,
and the multiplication factor.
Table 2-1. Sample Dilutions
Demineralized Water Used
Multiplication
Water sample
to bring the volume to 25 ml
factor
25.0 ml
0.0 ml
1
12.5 ml
12.5 ml
2
*10.0 ml
15.0 ml
2.5
* 5.0 ml
20.0 ml
5
* 2.5 ml
22.5 ml
10
* 1.0 ml
24.0 ml
25
* 0.250 ml
24.75 ml
100
*Note: For sample sizes of 10 ml or less, a pipet should be used to measure the sample into the graduated
cylinder or volumetric flask.
b.
Titrimetric Test.
In conducting some tests the color change
takes place after using only a few drops of titrant indicating a
smaller amount of the substance being tested for.
It is then desir-
able touse a larger sample and additional titrant to obtain more
accurate results.
The number of indicator powder pillows (and amount
of liquid buffer,
if any) must be increased by the same factor as the
sample size.
The result, also, must be divided by this factor.
For
example, if it is desired to titrate with 4 measuring tubes in the
determination of total hardness, four indicator powder pillows and a 4
ml of buffer solution, are required and the result is divided by four
(table 2-2).
Dilution is recommended when a sample contains unusually
large quantities of the substance to be determined and, consequently,
requires many burettes full of titrant.
In this case, a procedure
similar to the one described previously (para 2-10) is recommended.
If the sample size for titration is less than 25 ml, use only that
portion of the dilution necessary to run the test.
The multiplication
factor will remain the same.
For example,
if it is desired to dilute
to 1/5 of the original sample concentration for the determination of
hardness, 5 ml of sample would be added to a graduated cylinder then
add demineralized water to fill to 25 ml.
The titration would then be
performed with one measuring tube full of this dilution.
The results
would then be multiplied by 5.
c.
Standard Additions is a widely accepted technique whereby an
analyst can check the validity of his test results.
Also known as
“Spiking” and “Known Additions” the technique can also be used to
check the performance of the reagents, the instruments and apparatus,
and the procedure itself.
2-8
