Hanna Instruments HI 903 - Manual (Page 138)

9-10
METHODS OPTIMIZATION
The ideal sample size can be estimated using the table by drawing a line from the expected
water content to the amount of water in the sample corresponding to the desired titration
reproducibility (relative standard deviation). The ideal sample size is indicated by where the
drawn line intersects the ‘size of sample’ scale.
Consider the line on the table as an example. The line was drawn for a user with a sample
having approximately 1% water who required the best possible reproducibility. The intersection
of the red line with the size of sample column indicates that in order to introduce the optimal
10 mg of water into the titration cell the user must add 1g of sample.
The amount of sample required to introduce 10 mg of water into the titration cell can also be
calculated directly using the equation below.
9.2.3 Solid Samples
Sample water must be available to react with the titrant. This typically means that the sample
must be adequately dissolved in the solvent. This is achieved by choosing an appropriate
solvent system, proper preparation of the sample and optimization of the reaction conditions.
After ensuring that the sample is soluble in the choice of solvent or solvent mixture, dissolution
of a solid sample can be aided by grinding the sample into a fine powder, increasing the
pre-analysis stir time or heating the solvent during a titration with an optional jacketed
titration cell and water circulator.
Solid samples are added to the titration cell by removing the sample plug. The quantity of
solid sample added can be entered into the HI 903 as a mass or by number of pieces if, for
example, pills are to be analyzed.
The most accurate way to determine the mass of the sample added to the cell can be
achieved by an analytical technique called back-weighing. Back-weighing consists of the
following steps:
1. Measure the mass of a sample in a weigh boat.
2. Initiate the titration sequence on the HI 903 using the ‘start analysis’ option key
from standby mode. This will bring up the ‘add sample’ screen.
3. Slide the sample plug up out of the vessel top to open the sample port.
4. Rapidly add the sample through the sample port ensuring that ALL of the sample is
transferred to the solvent. Avoid any contact between the sample and the cell walls
or top.
5. Replace the sample plug into the vessel top.
6. Determining the mass of the ‘empty’ weight boat.
7. Calculate the mass of the sample added (subtract the mass of the emptied weigh
boat from the mass of the full weigh boat).
8. Enter the calculated mass of the sample into the HI 903.
9. Start titration using the option key ‘start analysis’ from the add sample screen.