In physical and analytical chemistry, colorimetry or colourimetry is a technique “used to determine the concentration of colored compounds in solution.”We know when any intensity (say Io) of light fall on the surface of a substance, three processes occur : Absorption, Reflection, Transmission. So
These are the absorbed, reflected, transmitted intensity whose sum is incident intensity simple statement of Black body radiation.
In most of cases Iq<<Ir+It so we have reduced to Io=Ir+It, below is sample structure of a device we use to measure intensity of light.
Now, the amount of light absorbed is dependent on the following things,
- Path length
- Concentration of solution
- Wavelength of light
Light is nothing but a stream of photons. So, when a number of photons flow through a solution of molecules then,
- It depends on how long it had to travel in that region that means longer path more probability to get absorbed.
- Concentration is just a count of number of particles in given volume, so more is the concentration, higher is the chance of absorption.
- Now relation of λ is not like increase with increase, or decrease with increase, since we know every atom can take only a few λ of absorption spectrum so intensity of absorption is extremely high with λ corresponding to one of those gaps while is extremely poor if it is not.
Now from these explanations we have the relation that on increasing what kind of action will occur and why but for the mathematical formula we do series of experiment (we don’t have to) and get a relation of these factors called as
IT=(Ioe)e-Ecd simplified as ,
Ecd = A = log(Io/IA) a is absorbance.
E–> Molar extinction coefficient (Probability of availability of molecule to absorb the particular λ (chocolate or book) of monochromatic light)
c–> Concentration of sample
A = log(Io/Ia) = absorbance
T = It/Io = Intensity of transmitted light (I) (Transmittance)
Intensity of monochromatic light (Io)
So, in matter to get some unknown concentration we apply the simple principle, more concentration more absorbance A α C
So,Take a solution with amount of salt and everything else is same amount.(why? Because we are having same constituent in unknown solution so only one thing should be changed so that we can calculate affect only because of that, i.e. if I change amount of what concentration change if I don’t add ammonia the light absorbed because of will not be absorbed in our unknown so wrong reading)
Take same reading with some known conc. C1, C2 , C3 , C4 etc. and draw the calibration curve, (Why? As it is simply gathering the ratio of constant things like incident intensity, path length, wavelength. So when we change the concentration we get corresponding reading of A, so we can get concentration from graph.)Now put the unknown solution and get A on graph and get C(unknown).
Pointes to remember
- In many cases the absorbance of chemicals are very low in visible region, majority in transparent liquid that make it difficult to get appropriate λ easily.
i.e. yellow compound appears yellow because it absorbs blue light and therefore it must be estimate the λ in blue region for max A. So, when we have a transparent liquid we don’t know its taste that what company chocolate does it like so that problem of no reading can come. But for transparent solution we have a trick:
Whatever we added in whatever amount must be taken for all known reading also. (This explains why in estimation of Cu we add 5ml of NH3)
Eg. Cu2++ 4NH3 –>[Cu (NH3)4]2+
Which is we are converting the colorless solution into a known so that we can get the appropriate chocolate for it.
- Reference curette (that is blank) must contain everything except Cu I.e. all reagent in
Same amount as in c1, c2, c3 and c (unknown).
For safety take point, don’t extrapolate line in graph if concentration or absorbance is very high and make different batch of graph for different reagent. The reading with change in any of reagent used.
Abhishek Kumar Jha(Chemistry At UTKARSHINI) downlordable link –https://docs.google.com/document/d/1xGnyfCRzCXfUbpmQ4s2yZQn6FyB0OXNRvbbRWmA69W0/edit?usp=sharing