Hanna Instruments HI 903 - Manual (Page 139)

9-11
METHODS OPTIMIZATION
Care should be taken to add a solid sample as fast as possible in order to minimize the
amount of time that the sample port is open. It is also important to be sure that all of the
sample reaches the solvent and does not make contact with, or stick to, the inner sides of the
vessel cap. Losing even a small fraction of the sample mass will result in a high sample water
content.
In some cases solid samples may require one of the additional preparatory steps listed in the
sections that follow. Specific sample preparation instructions are included with each standard
method.
9.2.4 Liquid Samples
As with solids, the water contained in liquid samples must be available to react with the
titrant. It is important to select a solvent system or mixture with which the sample is miscible.
Liquids are typically added through the septum in the sample port via a syringe and needle
using the following steps:
1. Attach a long needle (approximately 6 cm long, 21-gauge) to a syringe large
enough to hold at least one complete sample volume.
2. Rinse the syringe and needle with sample several times by drawing in a small
portion of sample, fully extending the plunger, shaking to coat the syringe interior
and expelling the sample into a waste collection container.
3. Draw enough sample into the syringe for at least one titration.
4. Dry the outside of the needle with a lint free wipe or tissue.
5. Determine the mass of the syringe and sample.
6. Initiate a titration from standby mode by pressing the ‘start analysis’ option key.
7. Insert the needle through the septum in the sample port. Push the syringe through
the septum until the end of the needle is approximately 1 cm from the surface of the
solvent.
8. Steadily dispense the contents of the syringe ensuring that the sample is introduced
directly into the solvent and does not splash or spatter onto the wall of the titration
vessel electrode or dispensing tip.
9. Draw a small amount of air from inside the cell into the syringe to ensure that no
sample drops remain on the tip of the needle.
10. Remove the syringe and needle from the septum taking care to not touch the
needle to the solvent or other internal cell components.
11. Determine the mass of the syringe and needle.
12. Calculate the mass of the sample added to the titration cell (subtract the mass of
the syringe after the sample has been added from the mass of the syringe before
sample addition).
13. Enter the calculated mass of the sample into the HI 903.
14. Start titration using the option key ‘start analysis’ from the add sample screen.
As indicated above, when adding a liquid sample with a needle and syringe, it is important that
the sample is introduced directly into the solvent. Sample that is deposited on the sides of the