What i want to do?

The aim of this project is to develop a simple but accurate gadget for the determination of a sample´s concentration. This instrument uses the optical interaction of the molecules with a source of light for determine the concentration of a sample. Design and how does it work? So , the construction and design is very simple, as I show in the following picture:

It consists in the following parts: Firstly, a light source , which consists in 3 primary LED diode (green, yellow and blue)and an additional one which is a multirange LED, which means that generates all the colors in a sequence , starting by red and finishing by blue , so this is use for scanning the full visible spectra. The leds are connected to a 3 v battery, this is no shown in the picture. Then , there is a small hole which is used as a support for holding the test tube that contents our sample. And finally immediately next to the test tube there is a photoelectric cell which turns light in to electric current , this cell is also connected to a small voltmeter which allows us to measure the voltage generated by the cell as a consequence of the non absorbed light.

So,how does it work?

Firstly the leds generate a continuous source of light of a determine wavelength range ,this light travels trough the chamber and gets the test tube, there the electromagnetic radiation interacts with the molecules of our sample , after that the molecules have absorbed part of the light emitted by the LED, the rest of the light leaves the test tube and impacts against the photoelectric cell, when that photons impact against the surface of the cell some electrons are released from the surface generating an electric current , which travels by the wires and finally, they get the multimeter(voltmeter), this apparatus has a small cooper coil designed for electric measurements called(galvanometer cell) when the electrons flow pass trough this small piece an electromagnetic force is generated, this is shown in the screen of the voltmeter as a measurement in volts. Is very important to choose the right light source color , which has to be the opposite of the sample´s color , for example if we want to determine the concentration of a cooper sulfate hydrate solution we have to use a red light source ,as the solution has an intense blue color, this means that the sample stuff absorb most of the visible light except the blue one. The right light color for a sample is shown in the next picture:

So , the idea is that the voltage generated by the cell is related with the amount of light non absorbed by the solution (trasmitance) so, as we increase the concentration of our sample , less voltage should be generated. This idea is very simple and it is actually one of the principal techniques used for determine the concentration of a sample, the modern instruments that are used for this propose are called colorimeters and are similar to the one I have designed but instead of a photoelectric cell they used a photoresistence and a diffraction grating as the way to choose their wavelength. The physical fundaments of this apparatus are two: The Einstein Photoelectric effect and The Beer-Lambert ´s law, I am not going to talk about them more because they are related with quantum physics and ,for me, it is quite difficult to explain them in a foreign language.

Calibration , sample preparation and analytical treatment

This is probably the most important part of the project Calibration: is important to know that each compound has an specific calibration, so for example if we want to determine the concentration of a solution of a compound A we have to calibrate the apparatus for this compound , if the sample is of a compound B we have to calibrate again the apparatus even both solution has the same color, as different compounds behave different with the light even we could think that they are quite similar. The calibration is very simple ,firstly you select the “color” you are going to use, for example the blue light, then you fill the test tube with your solvent(I use water) and you measure the voltage of the solvent (this is called a blank) , after that ,you prepare several solutions with different concentration of the compound ( I recommend to do around 20 different solutions) and finally you measure the values for each solution. Once you have done this , the next step is to do an analytical treatment of the data, you can use a software like excel for data treatment, you only have to find a function(mathematical formula) which establishs a relation between the volts and the concentration(I found that cubic functions works perfectly) once you have done this you have successfully calibrated the apparatus .And now you can determine with a lot of accuracy the concentration of a sample.

My attempt and results

Results I have test the apparatus with cooper sulfate pentahydrate ,which as you can see in the following picture ,absorbs significant amounts of red light, so I used as a light source the red light (which emits light of around 590-650 nm)

For the calibration I prepared around 20 solutions (5 ml each) which allows me to calibrate the apparatus with a lot of accuracy Here I show you the data I obtained after the calibration:

The results, were treated with Excel software , after the treatment I obtained the following relation between concentration and voltage:

I found that cubic function is just perfect for my apparatus (this depends on the photoelectric cell and its efficiency),as you can see the relation is very good and the error made by the apparatus is very small, which means it is very accurate.

Why I'm interested

Well , firstly I have to said that I decided to work in the developing of this apparatus because is more accurate and sensitive than the optic based one (using a spectrometer).And it has several uses, for example it can determine the concentration of proteins in a solution (using the biuret reaction) , we will also be able to determine the amount of free chlorine (a toxic chemical) on water by adding to the samples a reagent called Orthotolidine , or even determine the quantity of metals in solutions or contaminants by developing color-based detection techniques, so this could help us in the detection of some oil contamination or toxic chemicals in water or ground.

Interesting aplications

As i have said before , color-based detecting method is very important, so i have decided to show you some examples of detection techniques which allows us to determine its concentration:

Test for Chlorine: Free chlorine is a quite toxic compound which is present in low level in tap water and in swimming pool water.Chlorine gas has a greenish colour , but when is it solved in water , the colour almost disappear, so we can´t detect it , but if we add a reagent called Orthotolidine it turns orange.

As you can see in the picture, the first test tube is colorless even it contents high levels of free chlorine, the right one has the same amount of chlorine , but we add a drop of the reagent, so with this procedure we are able to measure chlorine level in water or some others.

Test for iron ions(Fe 2+):

iron (2+) solutions commonly have a light yellow-greenish color, but when is in very low concentrations it is almost transparent , however if we add a drop of a reagent called Potassium ferrocyanide a chemical reaction takes place , giving a complex called prussian blue, which has an intense blue color.

Test for Proteins:

A lot of proteins are colorless , but there is a reaction called "the biuret reaction" that allows us to detect the presence of proteins, firstly we add to the sample several drops of sodium hydroxide , after that we add a drop of cooper sulfate solution, then if there are proteins in the sample a violet color appears in the test tube

Test for Starch(Almidon): Starch is a white powder , almost insoluble in cold water , but quite soluble in boiling water, Starch solutions have a cloudy-transparent color , but it is not enough for the colorimeter , however if we add a reagent called "lugol", starch and iodine form a complex which has an intense blue color as shown in the image

Miguel Gallegos