Hanna Instruments HI 84431, Инструкция по эксплуатации, Страница: 8 / 44

8
PRINCIPLE OF OPERATION
Water alkalinity determination is performed by titration with acid reagent to pH end point.
Alkalinity is expressed as:
• Strong alkalinity (phenolphtalein alkalinity) determination by titration at 8.3 pH (in this
reaction hydroxide ions are converted to water and carbonate ions to bicarbonate ions):
2HO
¯
+ H
2
SO
4
→ 2H
2
O + SO
4
2
¯
2CO
3
2
¯
+ H
2
SO
4
→ 2HCO
3
+ SO
4
2
¯
Since bicarbonate ions can be converted to carbonic acid with additional sulfuric acid, the
phenolphtalein alkalinity measure total hydroxide ions and one half of the bicarbonate
ions contribution.
• Total alkalinity determination by titration at 4.5 pH (completly convert of the bicarbonate ions):
2HCO
3
¯
+ H
2
SO
4
→
2H
2
CO
3
+ SO
4
2
¯
It is important that the samples are protected against atmospheric factors (store them in sealed
bottles that are completely filled) and avoid vigorous shaking, mixing and long exposure to air.
The
HI 84431 minititrator is designed to determine the alkalinity of waters by potentiometric
titration in a low range (10-500mg/mL as CaCO
3
) and in a high range (400-4000mg/mL as CaCO
3
).
Selection of the alkalinity range is limited by the pH sample: if the measured pH is greater than 8.3
both total and strong alkalinity can be determined. If the sample pH is lower than 8.3 only total
alkalinity can be determined.
The
HI 84331 utilizes two different titration solutions to cover the entire measurement range. If the
range is changed, the titrant must be changed and the peristaltic pump must be primed and
recalibrated.
HI 84431-55 is used for pump calibration; the volume used will vary for the selected range.
Titratable Alkalinity in waters, as performed on the HI 84431 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 calibration
procedure is required. The 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.
REQUIRED REAGENTS
Code
Description Quantity/Test
HI 84431 - 50 Titrant (Low Range) 1 mL
HI 84431 - 51 Titrant (High Range) 1 mL
HI 84431 - 55 Pump Calibration Solution 2 mL for low range
20 mL for high range