I have performed COD analysis on Hach apparatus DR 200 (digester) and check its concentration and absorbance in DR 3900 (spectrophotometer). Here are the results:
Standard : 300mg/L Concentration measured: 327 mg/L absorbance 0.145
Standard: 200 mg/L Concentration measured: 218 mg/L absorbance 0.097
Standard: 100 mg/L Concentration measured: 100 mg/L absorbance -.0297
Negative absorbances have meaning and should not be discarded. A negative absorbance means that the intensity of light passing through the sample is greater than the intensity of light passing through the reference. If the experiment is performed correctly, a negative absorbance may have an important significance.
https://www.researchgate.net/post/What_is_negative_absorbance_and_why_am_I_getting_it
https://www.researchgate.net/post/Why_does_the_spectrophotometer_gives_negative_absorbance_valuescheck this link.
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