You can find UV concentration, typically of a substance in a solution, using spectrophotometry and the Beer-Lambert Law. This method relies on measuring the absorbance of UV light by the substance.
Here's a breakdown of the process:
1. Spectrophotometry:
- A spectrophotometer shines a beam of UV light through the sample.
- It measures the amount of light that passes through (transmitted light, I) and compares it to the amount of light that originally shone on the sample (incident light, I₀).
2. Transmission and Absorbance:
- Transmission (T): The ratio of transmitted light to incident light (T = I / I₀).
- Absorbance (A): A measure of how much light the sample absorbed. It's related to transmission by the equation: A = -log₁₀(T) = log₁₀(I₀/ I). A higher absorbance means more UV light was absorbed.
3. Beer-Lambert Law:
-
This law relates absorbance to the concentration of the substance, the path length of the light beam through the sample, and the molar absorptivity of the substance. The equation is:
A = ε L C
Where:
- A is the absorbance (measured by the spectrophotometer).
- ε (epsilon) is the molar absorptivity (also known as the molar extinction coefficient). This is a constant specific to the substance at a particular wavelength and indicates how strongly the substance absorbs light at that wavelength. You'll need to know this value, often found in literature or provided by the chemical supplier.
- L is the path length (the distance the light travels through the sample). This is usually the width of the cuvette holding the sample, typically 1 cm.
- C is the concentration (what you're trying to find).
4. Calculating Concentration:
-
Rearrange the Beer-Lambert Law to solve for concentration (C):
*C = A / (ε L)**
-
Plug in the values for absorbance (A), molar absorptivity (ε), and path length (L) to calculate the concentration (C).
Example:
Let's say you measure the absorbance of a solution at a specific UV wavelength to be 0.5. The molar absorptivity of the substance at that wavelength is 1500 M⁻¹cm⁻¹, and you are using a cuvette with a path length of 1 cm.
C = 0.5 / (1500 M⁻¹cm⁻¹ * 1 cm) = 3.33 x 10⁻⁴ M
Therefore, the concentration of the substance in the solution is 3.33 x 10⁻⁴ M (molar).
Important Considerations:
- Wavelength: Choose a wavelength where the substance absorbs strongly but the solvent and other components of the solution do not.
- Linearity: The Beer-Lambert Law is only accurate within a certain concentration range. Ensure your absorbance readings fall within this linear range for your spectrophotometer and substance. If the absorbance is too high, dilute the sample.
- Blank: Always run a blank sample (containing everything in the solution except the substance you're measuring) to zero the spectrophotometer and account for any absorbance from the solvent or cuvette.
- Units: Pay careful attention to the units of each variable and make sure they are consistent. Molar absorptivity is usually given in M⁻¹cm⁻¹, so concentration will be in molar (M) if the path length is in cm.
