Hanna Instruments HI 84430 - Manual (Page 5)

9 8
PRINCIPLE OF OPERATION
Water acidity determination is based on the neutralization of all acidic species contained in water
samples by titration with base solutions (the titrant alkaline reagent):
H + +OH - H
2
O
In an ideal solution, the end point of an acid titration corresponds stoichiometrically to the complete
neutralization of the acids present.
Water acidity results from strong and weak acids, and from the hydrolysis of salts. Due to these
more complex mixtures, end points for water acidity are arbitrarily fixed on practical considerations.
Unpolluted surface waters contain carbon dioxide as the main acidic component and titration of
these samples corresponds to stoichiometric neutralization of carbonic acid (titration to 8.3 pH at
25°C degrees). Strong acidity of the polluted waters is determined by titration to 3.7 pH. If the
samples are suspected to contain hydrolizable salts or polyvalent cations (such as chromium, iron,
manganese), it is necessary to pretreat the samples with a HI 84430-58 reagent (see Sample
preparation /Titration).
A potentiometric acid-base titration employs an indicator electrode (such as a pH sensor) to establish
the end point of the titration.
The HI 84430 minititrator is designed to determine the acidity of waters by potentiometric titration
in a low range (15-500mg/mL as CaCO
3
) and in a high range (400-4000mg/mL as CaCO
3
).
Both Strong and Total acidity can be determined by the HI 84430, however the actual sample pH
may limit your choice. If the measured pH is lower than 3.7 both total and strong acidity can be
determined. If the sample pH is greater than 3.7 only total acidity can be determined.
The HI 84430 utilizes two different titrant solutions to cover the entire measurement range. The
titrants used have different concentrations, appropriate for each work range. Low Range measurements
use HI 84430-50 titrant. High Range measurements use HI 84430-51 titrant. If the range is
changed, the titrant and the tube set must be changed and the peristaltic pump must be purged and
recalibrated. The standard solution (HI 84430-55M) is used for pump calibration for both working
ranges, but the pump calibration solution volume used is different.
Titratable Acidity in waters, as performed on the HI 84430 minititrator, utilizes a simple sample
preparation, a high quality peristaltic dosing pump for titrant, potentiometric endpoint detector and
instantaneaous computations. To maintain the high precision of the titrator, a simple pump calibra-
tion procedure is required. A range change requires different titrant and a pump recalibration. The
pump calibration involves the analysis of a known volume of a known solution (standard provided)
and compensates for changes in pump dosing that may occur due to many factors including tube
stretching or aging. This procedure should be performed regularly.
OVERHEAD VIEW OVERHEAD VIEW OVERHEAD VIEW OVERHEAD VIEW OVERHEAD VIEW
1. Titrant bottle
2. Graphic LCD
3. Keypad
4. Dedicated guide tube
for Temperature Probe
5. Electrode holder
6. Dedicated guide tube for titrant tip
7. Peristaltic pump tube
FRONT VIEW FRONT VIEW FRONT VIEW FRONT VIEW FRONT VIEW
REAR VIEW REAR VIEW REAR VIEW REAR VIEW REAR VIEW
8. Temperature probe
9. pH Electrode
10. Beaker
11. BNC electrode connector
12. Temperature probe socket
13. USB connector
14. Power switch
15. Power adapter connector
16. Peristaltic pump
FUNCTIONAL AND PHYSICAL DESCRIPTION