Michael Ian Lapsley

Refractive Index

Refractive index is a bulk material property. This property has a real and imaginary part and is dependent on wavelength. The real part describes how light "slows down" or "scatters" in a material and the imaginary part indicates how the material adsorbs light.

Typically, a single real value of refractive index will be attributed to a "transparent" dielectric material. If a material is transparent to your eye then we can assume that the imaginary part of the refractive index is close to zero. Also, in many cases, one may not want to consider the absorption of a material, only its scattering (reflection, refraction, transmission, diffraction, etc). Whenever a single value of refractive index is given with no wavelength specification, a good assumption is that the value was taken at a wavelength of 589.29 nm. This is the sodium D transition, and is typically used as the "standard" for refractive index values.

Now, for CaCl2, the refractive index changes with molar concentration. This is true for most salt solutions. CaCl2 is used in may optofluidic applications for its large refractive index range, and its stability. CaCl2 does not evaporate easily (so no crusty solids under the caps of your containers). So samples will remain good for long periods of time. The refractive index of CaCl2 is available in the CRC handbook of Chemistry and physics. Below is a plot of this data with two fitting functions. Many studies use a linear fit because it is convenient for calibrating devices. However, a second order polynomial has almost a perfect fit. * Remember this data is the refractive index at 589.29 nm. *

Linear Fit:


Polynomial fit:




The molarity of CaCl2 needed to match the RI of PDMS is about 3.5M (n ~ 1.41)

Molar Concentration 

Mass Needed [g] = desired Molarity [mol/L] * Molecular Weight [g/mol] * desired volume [L]
Example:
To get 0.25M solution of CaCl2*2H2O (MW = 147.01) in 0.05L
m [g] = 0.25[mol/L] *147.01[g/mol] * 0.05[L] = 1.838 [g]

Dilution of a solution

M1V1=M2V2
Where M1 is the initial concentration to be diluted, V1 is the volume of solution one to add to the dilution, M2 is the desired concentration and V2 is the total final volume of the dilute solution (V1 of M1 plus V2-V1 of DI water).

Converting from %Weight to Molarity

Molarity = (%Weight)/100*(Density)/(MolarMass)
This does not seem to account for the the change in density due to addition of the solute if you use the density of water. I tried to use an average for the density of water and of hydrated CaCl2 but this seem to be even worse. Use just the density of water seems to match well with other references.