The molar extinction coefficient (ε) is a measure of how strongly a molecule absorbs light at a specific wavelength. Molar extinction coefficient values are unique to each molecule and wavelength and have units M^-1 cm^-1.  As ε is a fixed constant at a specific wavelength, it can be used to determine the concentration of a solution from its absorbance values (using a fixed path length of 1cm ) using the Beer-Lambert equation:

Concentration (M) = Abs / ε

Molar extinction coefficients are reported at specific wavelengths (λ max) with a range of 2-3nm. For example, Erythrosin B (red food dye # 3) has a reported extinction coefficient of 82500 at 524-528 nm - i.e. with a 4 nm range. The LEDs in the colorimeter have a much broader bandwidth of 18, 33 and 25 nm for the red, green and blue LEDs respectively. Therefore it is necessary to determine the molar extinction coefficient with this instrument to get accurate concentrations. Below we desribe how we have determined the molar extinction coefficient of erythrosin B. Erythrosin B is a very useful tool in educational labs as it is completely safe to work with, can be used in a variety of labs and is inexpensive.

Experimental results

A dilute solution of Erythrosin B (Sigma, Cat # 198269) was prepared and absorbance values scanned using the Amersham Biosciences Ultrospec 2100 Pro. As shown in the image above, maximum absorbance (λ max) was at 526 nm.

A series of dilutions of erythrosin B was prepared from a stock solution to yield a final concentration of 0 - 7.4 µM. Absorbance was measured with both the spectrophotometer and colorimeter and a plot of concentration versus absorbance prepared (see below). The slope of absorbance/concentration is molar extinction coefficient (Beer-Lambert Law).

• ε for erythrosin B = 82,211 M^-1 cm^-1 at 526 nm with 3nm bandwidth (spectrophotometer)
• ε for erythrosin B = 55,929 M^-1 cm^-1 at 523 with 33nm bandwidth (colorimeter)

The spectrophotometer value was very close to the reported value as the wavelengths used were extremely close. The colorimeter green LED wavelength is much broader, so we would expect the molar extinction coefficient to be lower.

Redesigned the colorimeter LED and sensor PCBs to exchange the through hole terminal blocks for surface mount connectors. This will allow the boards to be mounted directly onto the enclosure walls.

Redesigned the Arduino shield board to a more simplified version. Shown on the left is an image of the new version 2.0 with an Arduino Uno.

Tartrazine (yellow food dye #5) has an absorbance in the 400-460 nm range with a peak absorbance of 425nm. A solution of tartrazine was diluted in water and absorbance values were measured using the colorimeter. An image of the result is shown on the far left. As expected, tartarazine absorbed light from the blue led (470 nm) but not red or green.

The same stock solution of tartrazine was diluted further to obtain a standard curve. Absorbance was measured at 470 nm using only the blue led. The linear relationship between absorbance and concentration is shown in the plot on the right.

The image on the left shows a plot of concentration versus absorbance measured with the colorimeter.

A series of dilutions of green dye was prepared from 0-100%  - the 10-90% dilutions are shown above. Absorbance was measured with the red led (625nm). The graph shows the linear relationship between absorbance and concentration. 

Below are screenshots of the colorimeter GUI which show the colorimeter readings with a blank/water cuvette (top left image) and the absorbance measurements for three food dyes (red, green and blue).

The first prototype of the Arduino Colorimeter shield is now in and we are currently testing the shield with the colorimeter. The shield provides a convenient way to connect the colorimeter to an Arduino Uno or Duemilanove. The colorimeter attaches to the shield via a ribbon cable between the 5x2 pin headers on the shield and the sensor board. In addition the shield provides connections for an LCD display, a potentiometer (display brightness) and a pushbutton for developing a standalone colorimeter.

The colorimeter sensor board was also modified to swap the 6x2 header for a 5x2 header. Design files for both the shield and the newer sensor version 1.1 can be found in the design files.

The original enclosure prototype was modified further to include a sliding cover and a cuvette holder. Below are some images of the new design.

I recently posted a new instructable for making the UV transilluminator. This instructable was featured by the editors and even made it onto the front page briefly and into the weekly newsletter, which generated quite alot of views over the weekend. Anyway, here is the link: http://www.instructables.com/id/UV-Transilluminator/

We have put together a 2-page quickstart assembly guide for the pendulum sensor classroom kits. Laminated copies will be provided with each of the kits for students to use as a reference when they are assemling the pendulum in the classroom. A copy of the guide can de downloaded below or under the Documentation tab on the pendulum website.

pendulum_quickstart.pdf

Some images of the first prototype of our programmable colorimeter. The enclosure parts were laser cut from 1/16" black acylic and assembled with stand-off and machine screws. The prototype was designed to fit standard 1.5 mL disposable semi-micro cuvettes (12.5mm x 12.5mm x 45mm). These cuvettes have a 10mm pathlength and 4.5 mm × 23 mm window.

The images below show the inside of the prototype with the RGB color sensor and LED PCBs mounted on opposite ends of the enclosure. Also visible is an inner wall with a slit the same dimensions as the cuvette window. The cuvette sits flush against the wall to ensure that only transmitted light reaches the RGB sensor. In the next round of enclosure designs we will include a light-tight hinged lid.

The prototpe enclosure has a 2.6" x 2.5" x 1.5" footprint.To design the enclosure for the colorimeter, we used Py2SCAD -  a python library that creates a file for viewing in OpenSCAD.