Metrohm 756 KF Coulometer - Manual (Page 17)

2.6 Tips for working with water standards
756/831 KF Coulometer, Instructions of Use 9
2.6 Tips for working with water standards
For validation of the instrument, as a fully integrated measuring sys-
tem, commercial, certified water standard solutions with water con-
tents of 1.00
± 0.003 mg/g and/or 0.10 ± 0.005 mg/g should be ap-
plied (The 1.0 mg/g Standard is easier to handle and therefore to
prefer).
Recommended initial weight range:
Liquid standard 1.0 mg/g 0.2-2.0 g
Liquid standard 0.1 mg/g 0.5-1.5 g
2.6.1 Recommendations for practice
For validation of the system very accurate handling is needed. To
minimise possible measuring inaccuracies the sample preparation
and handling should run accordingly to the following procedure:
1. Wear gloves (As always in KF Titration).
2. Take a fresh plastic syringe and open it.
3. Take a fresh ampoule of KF standard and shake it for 10
seconds.
4. Open the ampoule and suck 1 ml of the standard into the
syringe
5. Pull the piston of the syringe up to the end and shake the
syringe for a few seconds, so that the inner part of the sy-
ringe is rinsed with standard and gets rid of water
contamination.
6. Splash the used standard into a waste bottle.
7. Repeat the same procedure with another ml of the standard
solution.
8. Suck the whole rest of the standard into your syringe.
Thereafter, verify that there is no more solution in the needle
by sucking a small amount of air into the syringe.
9. Clean the needle by wiping it with a soft tissue. Close the
needle with the corresponding cap.
10. Place the syringe on the balance and press TARA.
11. As soon as the drift at your Coulometer is stable, you can
take the syringe, press <Start> at the Coulometer and in-
ject around 1 ml of the standard. This can be done in two
different ways:
a. The standard is injected without dipping the needle. If
a small drop keeps hanging at the needle, aspirate it
back into the needle, before pulling the needle out of
the septum.
b. The standard is injected directly under the surface of
the KF solution.
Furthermore, make sure that the standard doesn’t splash
on the wall of the vessel or on the electrode.