Hach APA 6000 Operation Manual

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Hach APA 6000 Operation Manual Manual pdf 13 page image

Page 136200018theory.fm Theory of OperationSection 2 Theory of OperationThe APA 6000™ High Range Hardness Process Analyzer is designed toprovide reproducible results through consistent solution handling. Every stepin an analysis is performed the same way each time, ensuring accurateresults. Accuracy can be verified and established by measuring grab sampleswith 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, thenthe detector.5. The mixture flows through the detector and the color is measured.2.1 Method of Analysis The sample is treated with a Magnesium-CDTA masking agent at acidic pH,to eliminate both carbonate/bicarbonate and heavy metal interferences. Thetreated 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 anddelivered to the detector via the reaction coil. The remainder of the mixturethen a slug of EDTA titrant is aspirated and delivered to the detector via thereaction coil.As the mixture travels through the reaction coil, the titrant becomes dispersedthrough 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 offluid on both sides of the titrant peak and where the EDTA quantitatively bindsall hardness present, the indicator changes color.As the fluid flows through the detector, its absorbance is recorded, producinga characteristic titration peak. The peak width is determined from theinflection point on both sides of the peak. The instrument calibrates using twostandards to establish a multi-point calibration curve.In the analysis, segments of untreated sample are kept acidic to assure thatno 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 twosteps 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 filledin two strokes of the autoburette to assure only representative sampleis in the mixer.