Determine what diffusion coefficient tells. An informal definition is that diffusion coefficient corresponds to the rate "things mix." Diffusion coefficients have units of area/time (such as square centimeters per second). The coefficient depends on viscosity of each substance. Therefore, diffusion rate varies with temperature and pressure. The goal here is to ensure that the information given by diffusion coefficient is not confused with a "similar" term such as viscosity or absorptivity coefficient.
Calculate diffusion coefficient of two liquids. Diffusion in liquid has notation "D(AB)".Term A refers to the solute, and B refers to solvent. A very low diffusion coefficient means that the two liquids do not mix. The recent oil spill in the Gulf of Mexico illustrates this point---the oil "solute" does not diffuse into the surrounding water. Instead, the oil keeps to itself. CalculatorEdge and ChemEngineering Calculator sites both show that liquid-liquid diffusion coefficient depends on several variables. Molecular weight, solvent viscosity, and temperature constitute liquid-liquid diffusion calculation.
Make the relevant experimental measurements. Values may have to be determined experimentally. Note the accuracy and error magnitude of every instrument used in determining viscosity, molecular weight, temperature, and so on.
Follow a liquid-liquid diffusion calculation example. Remember, solute is substance "A", solvent is substance "B." The relevant variables are as follows:
Solvent molecular weight (W): 25 g/mol
Solvent association factor (F): 2.6
Solvent viscosity (V): 0.7 kg/second * meter
Solute molal volume (M): 0.34 cm3/grams * mol
System temperature (T): 300 Kelvin (26.85 degrees Celcius)
The equation is: D(A,B) = [7.4 * T * (F*W)^0.5] / [V * (M^0.6) * (10^8)]
Substitution gives: D(A,B) = [7.4 * 300 * (2.6 * 25)^0.5] / [0.7 * (0.34^0.6) * (10^8)]
D(A,B) = 17898.21 / 36642524.78 = 0.0004885 (4.885e-4).
Incorporate significant digits and error. Experimental variables will have measurement error. This is a factor to keep in mind when dealing with exponents. Exponents amplify error quicker than other mathematical operations. Significant digits were not taken into account in the example calculation so that clear steps and precise mathematical values were evident.