beer's law scatter plot and trend line (linear...
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Beer's law scatter plot and trend line (linear regression)
Eng. Haneen Nabil AL-Sbaihi
Environmental Engineering Department
Islamic University of Gaza
In mathematical equations you will encounter in this course, there will be a dependent
variable and an independent variable. Identifying the dependent and independent
variables in a mathematical equation will help you identify what you are solving for in
the equation. The independent variable is a variable whose value determines the value of
the dependent variables. Independent variable is plotted on the Xaxis, and the dependent
variable is plotted on the Y-axis. Other variables may also be present in equations.
These may be constants or other variables. They may be given to you or you may be
required to obtain them by performing curve fitting. The example below illustrates this
point.
Equation 1:
y = mx + C
y = dependent variable
x = independent variable
m and C = constants
A calibration curve or standard curve is usually using series of standard solutions known
concentrations is prepared and measured using spectrophometer instrument to determine
the unknown concentration of sample. It is represent a graph where concentration is
plotted against absorbance then a straight line (Beer's Law) is fit to the data that you
obtained and the resulting equation is used to convert absorbance of the unknown sample
into concentration. To plot any data in excel, you should enter the data, create a scatter
plot with only Markers from data, and then add linear regression (trendline) to make
Excel the better line fit to the data and can be used as formula (y=mx+c)to calculate the
concentration of unknown compound.
The linear equation on chart represents the relationship between Concentration (x) and
Absorbance (y) for the compound.
1. Open Excel program window
2.chane the direction of page by choosing “ left to right” option
3. Add Data Set in Excel
4. To graph the data set click on insert and Plot the graph as XY (Scatter) with data
points only.
5. format the axis’s by right click on the x- axis and choosing axis format option
6. remove the tick that on values in opposite Order option
7. To add a linear regression line on your scatter plot, right click on the data points, and
then click add "trend line". Choose "linear" as the type of Trend/Regression. Select check
boxes of "Display Equation on chart" and "Display R-squared value on chart", and then
click "OK". The R-squared value gives the square of the Pearson correlation coefficient of the linear regression. The closer value of
R to 1 means relationship between x and y is very exactly and line fit to data.
The equation for the function will be displayed on the chart as shown below.
9. Then format the graph