Colourimetry sac (TOMORROW)

[Bozo]

Ok so i'm doing a sac tomorrow on colourimetry analysing the mass of iron in a sample of fertilizer.

In doing this we add K3Fe(CN)6 with small volumes of water to make up our standards which gives a deep blue colour. This is also added to our sample.


I'm predicting one of the questions to be why doesn't K3Fe(CN)6 affect results.

Another concept i'm trying to understand is if the solution is blue, that means that a RED LED is used because that is absorbed the most. Lets say you used a BLUE LED with the blue solution, what exactly happens? Does it completely transmit?


Anyway thanks for your help guys!

[schnappy]

I don't think they use LED's, since you'd only get a small handful of discrete wavelengths to play with. Hence they use an incandescent light and a monochromator (variably positioned prism and a slit) to give you your light. (Unless you've rigged up your own kit in which case LED's would work in most cases, just means you only get one type of blue.)

If you use blue light with blue solution, all light incident on the sample will be reflected. The colour gizmo compares the light input to the output, so it knows how much goes in, and how much comes out. The difference is the amount your blue solution gobbled up, ie. the amount absorbed.

The K_3Fe(CN)_6 will certainly affect the reading of absorbance (I'm guessing atleast). However you find the absorbance of a set of known standards, then you prepare a calibration curve. From this you find the concentration of your sample by seeing what its absorbance will be.

So your sample's solution must be within  the range of your standards, if need be dilute your sample and then bump up your answer later (Similar to volumetric analysis techniques).

[Bozo]

So in using the RED wavelength which in this case is a RED LED, the blue absorbs the RED, and depending on the strength concentration some light may be transmitted if its low conc, but if its high conc a large number will be absorbed

[schnappy]

The more concentrated the solution is, the more light it will absorb.
The less concentrated, the less light is absorbed.