[kuberr]

Hello,
I am trying to learn some chemistry by myself and I had the question of what would we do if someone hands you data in absorbance or transmittance from IR over a period of time.  If the concentration isn't given and we are just going off of the absorbance change over time, could we get the rate of the reaction and the rate constant?  And what would we do if it doesn't follow any of the orders?

Selflearning

[Corribus]

Under conditions in which Beer's Law holds, the Absorbance value is proportional to the amount of substance that is there.  So, if the Absorbance doubles in some amount of time, then you know that the amount of the absorbing substance has doubled during that time. So, you can acquire some information about rate from just Absorbance measurements alone (for instance, half life) just by observing how the Absorbance value changes as a function of time.

However, without the constant of proportionality (extinction coefficient) or the starting amount of substance as a reference point, it is not possible to put any numerical value to the rate (in terms of concentration or mass of substance per time).

Edit: Changed Absorption to Absorbance throughout.

[marquis]

Corribus is correct.  However, IR frequently isn't done under conditions where beers law holds true. Its not the same as a UV/VIS instrument.

[marquis]

In addition, there are a number of unique prep methods.  For example, with polymers, it could  be hot press films or solvent cast films.  This is in addition to nojol blends (nujol is something you hear in IR and not much of anywhere else), KBr pellets, and straight liquid films.  The liquid films are usually put between two NaCl disk or two KBr disks and run.  If you don't control the thickness with a spacer or some other method, well, the absorb bands will be way off.  For this reason, often, peaks are referenced in IR work.  Not  straight absorb ance calculation, but the calculation is done relative to a standard peak in the spsctrum. A start for you to look up.  Good luck!

[Corribus]

Corribus is correct.  However, IR frequently isn't done under conditions where beers law holds true. Its not the same as a UV/VIS instrument.

The Beer-Lambert law is not something unique to visible and UV light.  Nonlinearity is certainly more common in FTIR experiments for a number of different reasons that lead to violation of the idealized assumptions of the law, but there are plenty of examples of linear Beer-Lambert curves in the infrared and near-infrared spectral region in the literature. Just as there are plenty of cases of nonlinear curves in the UV and visible region. The trick is always knowing what the assumptions of the Beer-Lambert law are and establishing that the system you are studying does not violate those assumptions, and then taking experimental steps to show that your curve is linear before doing any kind of analysis. And even if the curve is not linear, there are ways to modify the law to account for the causes of the nonlinearity if you know what they are likely to be.

[Babcock_Hall]

https://www.sciencedirect.com/science/article/abs/pii/S0015736888728273
For instance, Intoxilyzers use a set of infrared signals from ethanol to estimate breath (and therefore blood) alcohol levels.  From what I can gather, they operate in the region of concentration where the Beer-Lambert law holds.