Volumetric KF titration
The sample size depends on the water content of the sample. In principle, the sample size should be selected in such a way that the titrant consumption lies between 10% and 90% of the buret volume. This means that if work is being done with a 10 mL Exchange or Dosing Unit, the consumption level of the titrant should be between 1 and 9 mL.
In addition, you should take care to ensure that the sample weight selected is not too low. The weighing error will be too great on the one hand, while on the other the sample will no longer be representative. In cases of high water content, it may be better to use a Dosing or Exchange Unit with a greater buret volume in order to avoid filling the Exchange Unit during the titration.
If the sample does not dissolve completely in the working medium, then the sample size has perhaps been selected too high and exceeds the dissolving capacity of the working medium. This can be remedied through the use of a smaller Dosing/Exchange Unit or a titrant with a lower water equivalent, e.g., 2 mg/mL or 1 mg/mL. Despite the lower initial weight, the titrant consumption will then achieve 10 to 90% of the buret volume.
As a general rule: When the titrant consumption is low, you should work with a lower titer (e.g., Titrant 1, Titrant 2), and with a high titer when titrant consumption is a high (Titrant 5).
Table: Recommended sample size
Sample
|
Approximate sample size [g] for 5 mL buret
|
Expected water content
|
Titrant 1
|
Titrant 2
|
Titrant 5
|
0.5%
|
0.1–0.9
|
0.2–1.8
|
0.5–4.5
|
1.0%
|
0.05–0.45
|
0.1–0.9
|
0.25–2.25
|
5.0%
|
–
|
0.02–0.18
|
0.05–0.45
|
10.0%
|
–
|
–
|
0.025–0.225
|
25.0%
|
–
|
–
|
–
|
50.0%
|
–
|
–
|
–
|
Sample
|
Approximate sample size [g] for 10 mL buret
|
Expected water content
|
Titrant 1
|
Titrant 2
|
Titrant 5
|
0.5%
|
0.2–1.8
|
0.4–3.6
|
–
|
1.0%
|
0.1–0.9
|
0.2–1.8
|
0.5–4.5
|
5.0%
|
0.02–0.18
|
0.4–0.36
|
0.1–0.9
|
10.0%
|
–
|
0.02–0.18
|
0.05–0.45
|
25.0%
|
–
|
–
|
0.02–0.18
|
50.0%
|
–
|
–
|
0.02–0.09
|
Sample
|
Approximate sample size [g] for 20 mL buret
|
Expected water content
|
Titrant 1
|
Titrant 2
|
Titrant 5
|
0.5%
|
0.4–3.6
|
–
|
–
|
1.0%
|
0.2–1.8
|
0.4–3.6
|
–
|
5.0%
|
0.04–0.36
|
0.8–0.72
|
0.2–1.8
|
10.0%
|
0.02–0.18
|
0.04–0.36
|
0.1–0.9
|
25.0%
|
–
|
0.016–0.144
|
0.04–0.36
|
50.0%
|
–
|
–
|
0.02–0.18
|
Coulometric KF titration
The sample size depends on the water content and should be small in order to allow you to titrate as many samples as possible in the same reagent and to keep the determination time short. However, ensure that the sample contains at least 50 µg of H2O.
The following table provides an indication of the sample size.
Water content of the sample
|
Sample size
|
H2O, to be determined (water content)
|
10000 ppm = 1%
|
10 mg–100 mg
|
100 µg–1000 µg
|
1000 ppm = 0.1%
|
100 mg–1 g
|
100 µg–1000 µg
|
100 ppm = 0.01%
|
1 g
|
100 µg
|
10 ppm = 0.001%
|
5 g
|
50 µg
|