Why do the Test 'N Tube™ Methods for Total Organic Carbon (TOC) use two wavelengths?

Document ID

Document ID TE11175

Published Date

Published Date 05/21/2019
Why do the Test 'N Tube™ Methods for Total Organic Carbon (TOC) use two wavelengths?
Explanation as to why the TNT methods for TOC use two different wavelengths.
Hach offers several methods for Total Organic Carbon (TOC). Included in this list are these three methods which are all Test 'N Tube™ methods which only differ in range and procedure:
10129 LR, Test 'N Tube Direct
10173 MR, Test 'N Tube Direct
10128 HR, Test 'N Tube Direct

The summary of method for these methods are all the same and are explained in detail in this document: Total Organic Carbon for Water and Wastewater

In the chemistry of these methods, an indicator changes from "Color A" to "Color B". One wavelength measures the decrease of Color A, the other measures the increase of Color B. In spectrophotometers that can run mulit-wavelength such as the DR3900, they measure both and the relative absorbance change from the blank for both colors are added together and used to calculate the result. This is done because it increases the sensitivity of the test compared to using just one wavelength.

This is not true for the TNT+ methods for Total Organic Carbon (Product # TNT810 and TNT811). These methods are only measured at one wavelength which is the wavelength that measures the increase of Color B. The smaller diameter tubes and other benefits that come with using TNT+ chemistries compared to non-TNT+ chemistries make the increase sensitivity of using two wavelengths nominal. (See also: What are the benefits of using a TNT+ method over the non-TNT+ method of the same parameter?)

On Colorimeters such as the DR900, only one wavelength is used, and it is a wavelength that measures the amount of Color A remaining.

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