Section 2 Theory Of Operation; Method Of Analysis - Hach APA 6000 Operation Manual

Section 2

2.1 Method of Analysis

6200018theory.fm
Theory of Operation
The APA 6000™ High Range Hardness Process Analyzer is designed to
provide reproducible results through consistent solution handling. Every step
in an analysis is performed the same way each time, ensuring accurate
results. Accuracy can be verified and established by measuring grab samples
with a known concentration of analyte.
These steps describe how the analyzer works:
1. The instrument aspirates sample into the holding coil.
2. Appropriate reagents are aspirated.
3. Sample and reagents are premixed in a mixing chamber.
4. The sample-reagent mixture is dispensed into the reaction coil, then
the detector.
5. The mixture flows through the detector and the color is measured.
The sample is treated with a Magnesium-CDTA masking agent at acidic pH,
to eliminate both carbonate/bicarbonate and heavy metal interferences. The
treated sample is then mixed with an indicator/buffer solution at a higher pH.
Some of the sample/buffer/indicator mixture is aspirated from the mixer and
delivered to the detector via the reaction coil. The remainder of the mixture
then a slug of EDTA titrant is aspirated and delivered to the detector via the
reaction coil.
As the mixture travels through the reaction coil, the titrant becomes dispersed
through the sample/buffer/indicator mixture on both sides of the titrant slug,
and forms a gradient of EDTA concentration. There will be a small volume of
fluid on both sides of the titrant peak and where the EDTA quantitatively binds
all hardness present, the indicator changes color.
As the fluid flows through the detector, its absorbance is recorded, producing
a characteristic titration peak. The peak width is determined from the
inflection point on both sides of the peak. The instrument calibrates using two
standards to establish a multi-point calibration curve.
In the analysis, segments of untreated sample are kept acidic to assure that
no precipitation occurs in the system.
The measurement cycle follows these basic steps:
1. The analyzer rinses out the holding coil, mixer, and reaction coil in two
steps with acidified sample.
2. Sample is pulled into the holding coil, followed by masking reagent.
3. This mixture is dispensed into the mixer and stirred. The mixer is filled
in two strokes of the autoburette to assure only representative sample
is in the mixer.
Page 13
Theory of Operation