Why would I see a 100% Transmittance (T) using the Low Range COD reagents?

Document ID

Document ID TE235

Published Date

Published Date 27/05/2017
Question
Why would I see a 100% Transmittance (T) using the Low Range COD reagents?
Summary
100% transmittance when using COD reagents
Answer
Generally this happens when the customer has built their own curve and has set the blank on the LOW RANGE to read 100% T. The blank is supposed to be set on most spectrophotometers to read at 35% T. Keep in mind, if there is COD present, the reagent reacts with it and changes from yellow to green. The higher the COD, the lighter in yellow color the sample will be and the higher the %T.  Because the COD of the sample causes a loss of yellow color, the meter scale is reversed; low range reagents measure the amount of yellow color remaining rather than the amount of green color produced..