A calibration curve is a graph used in analytical chemistry to determine the concentration of an unknown solution. It is created by measuring the concentration and absorbance of several calibration standards and can be used to determine the concentration of an unknown solution by measuring its absorbance and placing it on the curve. Spectrophotometry is the process of measuring absorbance and can be used to draw a calibration curve. The curve can be used to determine the concentration of an unknown solution by drawing a horizontal line from its absorbance and finding where it intersects with the curve.
A calibration curve is a method used in analytical chemistry to determine the concentration of an unknown sample solution. It is an experimentally generated graph, with the concentration of the solution plotted on the x-axis and the observable variable, such as the absorbance of the solution, plotted on the y-axis. The curve is constructed by measuring the concentration and absorbance of several prepared solutions, called calibration standards. Once the curve has been plotted, the concentration of the unknown solution can be determined by placing it on the curve based on its absorbance or other observable variable.
Chemical solutions absorb different amounts of light based on their concentration. This fact is quantified in an equation known as Beer’s law, which shows a linear relationship between a solution’s light absorption and its concentration. Researchers can measure the absorbance of a solution using a laboratory instrument called a spectrophotometer. This process as a whole is called spectrophotometry.
Spectrophotometry can be useful in determining the concentration of an unknown solution. For example, if a researcher has a sample of river water and wants to know its lead content, he can determine it by using a spectrophotometer to draw a calibration curve. First, the researcher creates several standard lead solutions, ranging from less to more concentrated. These samples are fed into the spectrophotometer, which records a different absorbance for each.
The experimentally determined absorbance values are plotted against the known concentration of each calibration standard. A set of points is created, which in the case of absorbance should be approximately linear due to Beer’s law. A line is drawn to connect these data points, forming the calibration curve. In almost all cases, the data points won’t be mathematically exact, so the line should be drawn to intercept the maximum number of points—that’s a best-fitting line. While the relationship between absorbance and concentration is linear, this is not always true for other experimentally determined variables, and curves must occasionally be employed to describe the relationship.
Now you can analyze the unknown solution. The sample is placed in the spectrophotometer and its absorbance is measured. Since this sample is measured against several standards containing the same compound, its absorbance and concentration must fall somewhere along the calibration curve for that compound. This means that once the absorbance of the solution is known, its concentration can be deduced mathematically or graphically.
A horizontal line can be drawn from the y-value of the unknown solution, i.e. its absorbance, which has just been measured. The point where the line crosses the calibration curve will indicate the x-value – the concentration. A vertical line, drawn from this point downwards, indicates the concentration of the unknown solution. The equation for the calibration curve line can also be used to mathematically determine the concentration of the solution.
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