The key difference between calibration curve absorbance and concentration is that calibration curve is a graph of absorbance and concentration, absorbance is the amount of light absorbed by a sample whereas concentration is the amount of a substance distributed in a unit volume.
Spectroscopy is an analytical technique which is useful in determining the concentration of an unknown compound present in a given sample. Therefore, it is a quantitative analysis. In this technique, we can determine the concentration of the compound using a curve. We should draw this curve between absorbance and the concentration. And we can draw the graph for several absorbance values obtained for different known concentration values. Then, we can use the absorbance value for the unknown sample to determine the concentration of that sample using the graph.
CONTENTS
1. Overview and Key Difference
2. What is Calibration Curve
3. What is Absorbance
4. What is Concentration
5. Relationship Between Calibration Curve Absorbance and Concentration
6. Side by Side Comparison – Calibration Curve Absorbance vs Concentration in Tabular Form
7. Summary
What is Calibration Curve?
A calibration curve is a standard graph which shows the change in the response of an analytical instrument towards different concentrations of the analyte. The analyte is the substance in which we need to find the concentration. In order to draw the calibration curve, we should use the known concentrations of the substance present in our sample to get a set of responses or signals. For spectroscopic techniques, the responses or signals are the absorbance values. Then, we can draw a graph between absorbance and concentration.
Figure 01: The Structure of a Calibration Curve
We should draw the graph using the absorbance values obtained for each known concentration. We can use this graph to determine the unknown concentration of the same compound in a given sample by measuring the absorbance of that sample. The concertation value at the point of that absorbance value in the curve is the concentration of the compound in the sample.
What is Absorbance?
Absorbance is the response of a spectrophotometer towards the concentration of a sample. It measures the amount of light absorbs by the sample. This value depends on the amount and nature of the compound present in the sample. We can give this value using the following equation;
A = log(I/I0)
Where A is the absorbance, I is the intensity of the incident beam, and Io is the intensity of the transmitted beam through the sample. We can give this relationship in another way as follows:
A = –logT
Where T is the transmittance. Therefore, the absorbance is related to the transmittance. for the same substance, if the concertation is high, the absorbance is also high and vice versa. That is because, if the concentration is high, the sample has a high amount of the compound that absorbs the light from the light beam. Moreover, when measuring the absorbance from a spectrophotometer, we should not use very high or very low concentrations. This is because, if we use very high concentrations, the intensity of the incident light beam may not sufficient for the total amount of compound present in the sample to absorb. If we use a low concentration, the sensitivity of the instrument may not sufficient enough to detect the low amount of compound in the sample.
What is Concentration?
Concentration is the amount of a substance that distributes throughout a unit volume of a sample. We can measure this in the unit of mol/L in which we give the amount of substance as a mole value and volume of the sample in liters. We call this the molar concentration. It is the most common unit for the measurement of the concentration.
Apart from that, we can determine the concertation as mass concentration, number concentration or a volume concentration.
What is the Relationship Between Calibration Curve Absorbance and Concentration?
We draw a calibration curve from two data sets: absorbance values and concentrations. We should take the absorbance as the y-axis and concentration as the x-axis because we can change the value of the concentration. Hence, it is the independent variable. But the absorbance changes according to the value of the concentration. Therefore, it is the dependent variable. Then, if we measure the absorbance of a sample, we can obtain the concentration of that sample using this calibration curve.
What is the Difference Between Calibration Curve Absorbance and Concentration?
A calibration curve is a standard graph which shows the change in the response of an analytical instrument towards different concentrations of the analyte. It indicates absorbance in y-axis and concentration in x-axis. Absorbance is the response of a spectrophotometer towards the concentration of a sample. It has no units. Concentration is the amount of a substance which distributes throughout a unit volume of a sample. Its unit is mol/L.
Summary – Calibration Curve Absorbance vs Concentration
Calibration curves, absorbance and concentration, are widely used terms in spectroscopy. The difference between calibration curve absorbance and concentration is that the calibration curve is a graph of absorbance and concentration and absorbance is the amount of light absorbed by a sample whereas concentration is the amount of a substance distributed in a unit volume.
Reference:
1. “Calibration Curve.” Wikipedia, Wikimedia Foundation, 15 June 2018. Available here
2. “Absorbance.” Wikipedia, Wikimedia Foundation, 15 June 2018. Available here
Image Courtesy:
1.’Calibration curve’By Kkmurray – Own work, (CC BY-SA 3.0) via Commons Wikimedia