Two physical quantities, transmittance (T) and absorbance (A), are used to determine the ability of a solution to absorb light. Transmittance is defined as T= I/Io. I represents the number of transmitted photons (the intensity) per unit of time with the absorbing sepcies present. Io represents the number of transmitted photons per unit time with the absorbing species absent. If multiplied by 100, transmitance yields the percentage of photons that can pass through a solution. Since it is rather difficult to work with the number of photons, absorbance, defined as A= -logT= -log I/Io, is often used. In such an equation, absorbance uses the same definitions of I and Io as transmittance. Overall, absorption depends on wavelength, and is thus color specific. Therefor, the ability of a substance to absorb light is different for photons with different wavelengths. In order to determine such an ability in this lab, the absorbance of five solution, ranging in concentration, will be calculated using a calibration curve. The concentration of an unknown solution will also be determined, by finding thee slope of the calibration curve from the five known solutions. A colorimeter will also be used to measure the amount of light absorbed (the voltage is converted to an absorbance level in optical density units).
To determine the concentration of a colorful solution.