Hach DR/850, Procedures Manual, Page: 25 / 460

25
CHEMICAL ANALYSIS INFORMATION
, continued
Testing for metals requires acid and heat to pretreat the sample. Since
these conditions destroy filter paper, vacuum filtration with glass fiber
filter discs is recommended. Also, glass filter discs, unlike paper, do not
retain colored species.
Temperature Considerations
For best results, perform most tests in this manual with sample
temperatures between 20 °C (68 °F) and 25 °C (77 °F). If a test requires
closer temperature control, notes in the procedure will indicate this.
Sample Dilution Techniques
Ten and 25 mL are the volumes used for most colorimetric tests.
However, in some tests, the color developed in the sample may be too
intense to be measured. Unexpected colors may develop in other tests. In
both cases, dilute the sample to determine if interfering substances are
present.
To dilute the sample easily, pipet the chosen sample portion into a clean
graduated cylinder (or volumetric flask for more accurate work). Fill the
cylinder (or flask) to the desired volume with deionized water. Mix well.
Use the diluted sample when running the test.
To help with dilutions, Table 5 shows the amount of sample used, the
amount of deionized water used to bring the volume up to 25 mL and the
multiplication factor.
The concentration of the sample is equal to the diluted sample reading
multiplied by the multiplication factor.
More accurate dilutions can be done with a pipet and a 100-mL
volumetric flask (see
Table 6 for more information). Pipet the sample and
dilute to volume with deionized water. Swirl to mix.
Table 5 Sample Dilution Volumes
Sample Volume
(mL)
mL Deionized Water Used
to Bring the Volume to 25 mL
Multiplication
Factor
25.0 0.0 1
12.5 12.5 2
10.0
1
15.0 2.5
5.0
1
20.0 5
2.5
1
22.5 10
1.0
1
24.0 25
0.250
1
24.75 100
1
For sample sizes of 10 mL or less, use a pipet to measure the sample into the graduated cylinder or
volumetric flask.