Hanna Instruments HI 903, Быстрый старт

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9-17
METHODS OPTIMIZATION
Due to the very water-sensitive nature of a Karl Fischer titration, only a few milligrams of
water are required for a typical standardization or system verification. A great deal of skill is
therefore required in determining the mass of the water introduced into the titration vessel in
order to achieve highly accurate results.
9.3.4 Standardizing the Titrant
Standardizing the Titrant, or determining the titer, is a routine and necessary part of
accurate Karl Fischer analyses. The titrant should be standardized daily for greatest
accuracy. Standardization serves to standardize the combination of parameters selected
as part of a particular method and serve as a system check. It is recommended that the
titrant be re-standardized if the method to be used for an analysis is very different from
that which was used to standardize the titrant initially. The titrant can be standardized
using hydrated salt, liquid water standards or tiny amounts of pure water.
A general procedure for titrant standardization:
1. Setup titrator according to the instruction manual. Ensure the titrator is set up with
the same reagents, solvents, working conditions, temperature and titrator settings
to be used for subsequent sample analyses.
2. Select the appropriate standardization method included with the
HI 903 .
Using a Sodium Tartrate Dihydrate Standard:
3. Back-weigh between 30 and 200 mg of standard. Be sure that the salt is a high
quality standard, which has been stored properly and exists as a fine, free flowing
powder.
4. Repeat the standardization at least three times and update the titrant concentration
using the averaged result value via the statistics screen if the variability between
the standardizations is small.
Using a Prepared Liquid Water Standard (Ampule):
3. Break open an ampule of standard. Rinse a syringe with a small portion of standard.
4. Draw up the remainder of the standard into the syringe, weight and titrate about
one third of the standard in the syringe.
5. Conduct two more standardizations with the standard remaining in the syringe.
6. Review the set of results on the ‘average results’ statistics screen. The titrant
concentration should be updated with the averaged results as long as there is not
excessive variability between standardization results.
With pure water standards:
3. Draw approximately 10
µ
L of pure water into a glass micro-liter syringe.
4. Introduce the water standard by back-weighing using an analytical balance with
0.01 mg resolution. Because of the extremely small sample size, it is important to
strictly follow the procedure for the addition of liquid samples outlined in the section
‘addition of liquid samples’ above.