Hach AutoCAT 9000, Инструкция по эксплуатации

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AutoCAT 9000 Validation Study
50081_AutoCAT 9000 Validation Study.fm
Page 161
Appendix E AutoCAT 9000 Validation Study
An AutoCAT 9000 chlorine amperometric autotitrator was tested in a validation study to
see whether the instrument could in fact routinely measure chlorine concentrations in the
low ppb (µg/L) range, according to Standard Method 4500 Cl D.
The instrument was a production unit without modification. All glassware used was
pretreated for chlorine demand. Reagent grade, organic free, deionized water
(> 18 Mohm) was used in the preparation of standard solutions. Chlorine standards were
prepared from stock solutions that were kept under a high-purity nitrogen headspace
blanket, under positive pressure, and diluted with reagent grade water using Class A
volumetric pipettes for samples. The PAO titrant was standardized prior to the tests. In
these low µg/L tests, the smallest volume increment was selected, 0.3µL, for titrant
delivery.
Data from the instrument was output through an RS232 serial port, pairing dispensed
titrant volume with current output. Endpoint determination and results calculation were
performed on the instrument using the instrument's embedded endpoint determination
algorithm and embedded equations. Additionally, these data pairs were then imported
into a spreadsheet program for graphing and analysis, to validate the instrument
calculated results. A representative titration curve, from data imported into the
spreadsheet program and graphed, is shown in
Figure 1 .
Linear regression analysis was conducted on the data segments prior to, and after the
'dead-stop' endpoint, with the intersection of these line-fits used to calculate the endpoint,
in µL of titrant; the sample volume and titrant normality were then used with this
endpoint volume to calculate a chlorine concentration of 6.4 µg/L. The r
2
correlation
coefficient was found to be 0.973 prior to the endpoint, and 0.840 after the endpoint, with
a signal to noise ratio of approximately 25:1. The instrument itself showed a calculated
result of 6.3 µg/L, using the embedded endpoint determination and results calculation
routines.
Figure 1 Titration Curve—6.4 µg/L Chlorine