Laboratory Practices
Laboratory Practices
Page 24
5940022LabPrac.fm
2.10.3 Cleaning of Sample Cells
Most laboratory detergents can be used at recommended concentrations.
Neutral detergents, such as Liquinox, are safer when regular cleaning is
required. When using a detergent, you can clean faster by increasing the
temperature, or by using an ultrasonic bath. Finish by rinsing a few times with
deionized water and allow to air dry.
Cells may also be cleaned with acid, followed by rinsing thoroughly with
deionized water.
Individual procedures may require special cleaning methods.
2.10.4 Matching of Sample Cells
Although sample cells shipped with the instrument are distortion-free, nicks and
scratches from handling may cause an optical mismatch between two sample
cells and introduce error into the test results. This type of error may be avoided
by optically matching the sample cells as follows:
1. Turn the instrument on. In Instrument Setup, turn Display Lock
Off.
2. Return to the Main Menu and select
Single Wavelength.
3. Touch the λ key and select a wavelength of
510 nm, or the wavelength to be
used for the test.
4. Pour at least 25 mL (10 mL for 10-mL cells) of deionized water into each of
two sample cells.
5. Place one sample cell into the cell holder. Orient the fill mark toward the front.
6. Touch
Zero. The display will show: 0.000 Abs
7. Place the other sample cell into the cell holder.
8. Wait about three seconds for the value to stabilize and read the absorbance.
Record the resulting absorbance.
9. Turn the cell 180° and repeat step 8. Try to achieve an absorbance value within
±0.002 Abs of the first cell. Note the orientation of the cell.
If the sample cells cannot be matched within ±0.002 Abs, they may still be used
by compensating for the difference. For example, if the second cell reads
0.003 absorbance units higher than the first cell, correct future readings
(when using these two cells) by subtracting 0.003 absorbance units
(or the equivalent concentration) from the reading. Likewise, if the second cell
reads -0.003 absorbance units, add 0.003 absorbance units to the reading.
2.11 Other Apparatus
2.11.1 Boiling Aids
Boiling is required in some procedures. Under some conditions bumping may
occur causing sample loss or injury. Bumping is caused by the sudden, almost
explosive, conversion of water to steam as it is heated. Use of a boiling aid, such
as Boiling Chips (Cat. No. 14835-31), reduces bumping.
Make sure the boiling aids will not contaminate the sample. Do not use boiling
aids (except glass beads, Cat. No. 2596-00) more than once. Loosely covering the
sample during boiling will prevent splashing, reduce the chances of
contamination, and minimize sample loss.
Individual procedures will recommend the specific boiling aid to use.
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