Determine the concentration of an unknown sample with your smartphone

One thing that can be frustrating in the classroom is access to high quality instrumentation - even if you are lucky enough to possess equipment, inevitably students are forced to either share in large groups or watch a teacher led demo. But with the increasing power of smart phones and some nifty software, you are able to democratize access to laboratory grade equipment.

Using the excellent Physics Toolbox Suite from (Android and iOS versions available) you are able to turn your smartphone into a sensor suite - one of the functions of which is to measure the light intensity falling on the camera's light sensor - in effect turning your phone into a portable colorimeter.

Determining the concentration of an unknown sample of "orange soda"
Imagine the scenario - you are working in a fast food restaurant and you have a machine that can make orange soda from concentrate. The manufacturer recommends a concentration of between 130 and 150 grams per litre for that "ideal taste". As a consumer, you visit the restaurant and that soda tastes watery to you. What do you do?

Well with the help of Physics Toolbox Suite, you can confidently complain and rest assured that crimes against soda are wiped out.

  1. Buy a jar of soda concentrate* (usually sold as syrup) and make up a range of concentrations (from 300g/l down to 50g/l)
  2. Fill a range of cuvettes with the solution
  3. Fill one cuvette with pure water (or plain soda water)
  4. Fill one cuvette with the sample that you wish to determine the concentration
  5. Place each cuvette in turn over the light sensor of your phone and using the light meter function of Physics Toolbox Suite measure the light transmitted to the sensor. (I used a standard desk lamp as a source of illumination in a dark room, so that the light source intensity was constant)
  6. Let the intensity measure for pure water be I0 and the intensity for each cuvette be I. Calculate the absorbance, Log10(I0/I)
  7. Plot concentration (x axis) against absorbance (y axis) - this should be a straight line (for reference you can look up the Beer-Lambert law)
  8. From the equation of the straight line and the absorbance of the unknown sample, determine it's concentration.

*Works just as well with soda from a bottle - your favorite soft drink for example, but the default concentration is 100% and proceed to dilute to make 90%, 80%, 70% solutions - you dont know the absolute concentration, but working in percentages will be fine.

(For the purposes of this exercise, the "unknown sample" given to students was 110g/l - visually almost identical to the 100g/l reference sample used.

In this picture - pure water is to the far left, the unknown sample to the far right. The first sample has less fluid in - when the experiment was undertaken, all cuvettes has the same fluid amount so that when placed horizontallyex over the phones sensor, the depth of liquid was the same (the full width of the cuvette)

The equation has an offset of 0.0127 - probably due to light leakage under the cuvette - my phone has a rubber case on it and the cuvette does not fit totally flush to the sensor.
For the unknown sample, Log10(I0/I) = 0.134799485 - a simple rearrangement of the best fit line gives: Concentration of unknown sample = (0.1348 - 0.0127) / 0.0011 = 110.99 g/l (compared to an actual concentration of 110g/l) << pretty nifty for a smart phone and piece of software.

If you are interested in giving this a go, or to turn your mobile phone into physics toolkit, grab Physics Toolbox Suite.

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