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Last Login: 6/16/2008 4:32:34 PM
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| | Our lab is developing a colorimetric procedure for detemination of chemical oxygen demand, after many years of titration. We are using Hach tubes containing digestion reagents, and a Hach DR 5000 spectrophotometer. My question regards the proper means of zeroing the instrument absorbance prior to calibration. The information from Hach suggests the instrument should be zeroed with a digested blank (which I would consider a method blank). Standard Methods 5220 D suggests zeroing with deionized water. For some other procedures (e.g., total phosphate) we have been zeroing with a mixture of digestion solution and deionized water (some would call it a reagent blank, although others hold that a reagent blank can contain no matrix - i.e., no DI water). Any feedback, opinions, or information regarding what your labs are doing would be helpful. Thanks. |
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Supreme Being
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Last Login: 7/1/2008 2:01:12 PM
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| | First off, you have to notice that SM gives you two methods for analyzing COD. The first and most common is what you are using now that goes from about 50-100 up to about 1000mg/L COD. Use an undigested blank as your zero initially. SM says that once you determine your MB to be consistently clean you can use a digested blank as your zero. The part you see that says to use reagent water as your blank is for the low level reagents. You measure this one at 420nm, as opposed to 600nm, and the curve is backwards from what you expect. The color will actually get lighter the higher the concentration. This one has a range of 10-150 mg/L of COD. For everything, except low level COD's, we zero with a true method blank carried through the entire analytical procedure.
David |
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Last Login: 6/12/2008 9:19:35 AM
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| I have always zeroed the spec with the digested blank, for both methods - low and high range. Never had a problem with the calibration curves doing it this way.
Brandi |
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