Adding a new test to the suite

Posted by Samuel Rajkumar on 2018-09-04 00:00:00 +0000

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Bubble Nut Wash is a social enterprise based out of Bangalore. They make environment-friendly detergents and home-care cleaning products from soapnuts. The enterprise intends to use water test kits developed by ffem to compare parameters of after-wash waste water using synthetic detergents vis-á-vis soapnut-based detergents. When Manas, the cofounder, asked if our kit could test for Hydrogen Peroxide, I was tempted to say no. I am glad I did not, because what followed turned out to be a textbook example of how to add a new parameter to the ffem water test kit.

Bubble Nut Wash

Perhaps I am still stuck in the past where we used to think of every new parameter as a new feature, while what we have now is a system where adding a new parameter should be analogous to writing a blog post on, say, WordPress. Quite distinct from adding a feature to WordPress. Of course, the first question to ask before one writes a blog post is if the content lends itself to being addressed in such a format. In our case, we needed to find if a suitable colorimetric test exists for Hydrogen Peroxide, one that would lend itself to be used by our method.

Enter A colorimetric method for rapid and selective quantification of mixtures comprising peroxodisulfate, peroxomonosulfate and hydrogen peroxide by Benjamin. J. Deadman, Klaus Hellgardt and King Kuok (Mimi) Hiia. This paper lists a number of methods to measure Hydrogen Peroxide. Of interest to us is “TiOSO4 Redox Colorimetry Test for Hydrogen Peroxide Concentration” which is a colorimetric way of measuring Hydrogen Peroxide, and looks to be well suited for our method, as this image from the paper shows. The vials show reagent mixed with standard solutions of H2O2, starting with 0 on the left and 1M on the extreme right, with intermediate concentrations in between.

h202 test colours

Preparing the reagent is well documented in the paper, and so it was a matter of a few hours before we got a good answer if the test is good enough with the ffem method of smartphone based colorimetry. Below is a calibration screen for the Hydrogen Peroxide test.

h2o2 calibration colours

The calibration screen above shows progressive colour values from 0 M to 1 M. With ffem tests, we treat RGB colours as points in 3D space, and the difference between 2 calibration points is the Euclidean distance. The value that is of most value to us is ‘d’, representing the Euclidean distance between two succeeding calibration points. The larger the ‘d’, the better the test. Typically, ‘d’ values of 20 represent a good enough test. The ‘d’ values here of 0, 134, 30, 121, and 25 signify that this is indeed a good test.

Tests with lab mixed samples give excellent results.

H2O2 Standards (M) ffem results (M)
0.0 0.0
0.05 0.06
0.10 0.13
0.5 0.5
1.0 1.0

Having delivered this test to Bubble Nut Wash, we hope this will come in handy saving our city from its menace of frothing and burning lakes.

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