Many important relationships can be expressed by graphing. For the analysis of materials, a standard curve is used to determine the concentration of a substance (i.e. – the unknown sample).
The analyst first prepares samples of the substance in various known concentrations (i.e. – the standards). The method of preparation should be similar to the unknown that is being measured. The analyst subjects the standards, one at a time, to an analytical instrument and records the data pair (Sample Concentration, Instrument Response) for each standard.
The following is data collected for a set of standards. A sample of some unknown concentration gives an instrument response of 0.78 mV.
First, plot a standard curve using the following data. A graph of the instrument response is plotted on the Y-axis and the concentration on the X-axis. The known variable is always plotted on the X-axis and the measured variable is always plotted on the Y-axis. Use the standard curve to determine the unknown concentration of the sample.
| Sample Concentration (g/mL) | Instrument Response (mV) |
| 0.0 | 0.00 |
| 10.0 | 0.14 |
| 20.0 | 0.28 |
| 30.0 | 0.41 |
| 40.0 | 0.52 |
| 50.0 | 0.71 |
| 65.0 | 0.84 |
| 75.0 | 1.04 |
| 80.0 | 1.15 |
How to generate an acceptable graph to determine the concentration of the unknown sample.
- Use the whole sheet.
- Label the axes. (Note: the axes have units.)
- Write the title of the graph. (Note: “Y versus X”)
- Draw the best straight line through the points. Force the line through the origin, (0,0). (Note: Do not connect the dots.)
- Determine the equation of the line. (Note: For hand-drawn graphs, pick two points that the line goes through. These points may or may not be the data points. For computer-generated graphs, add the trendline and the R2 value.)
Write the equation of the line on the graph. - For computer-generated graphs, need to add minor tick or grid marks on both axes.
- Write the slope of the line on the graph. (Note: slope has units.)
- Use the equation of the best fit line to calculate the unknown concentration of the sample.
- Label the unknown sample point on the graph. Draw a line from this point to the y-axis. Draw a line from this point to the x-axis. These lines MUST be perpendicular to the axes.
- Write the concentration of the unknown sample on the graph.
Let your instructor sign your hand-drawn graph before you leave. Graph the same set of data using Excel. Print a copy of the graph and let a classmate do a written peer review on the graph. Discuss the peer review together. Fix your graph in Excel. Reprint the fixed graph and hand in BOTH Excel-generated graphs (classmate critiqued graph and the corrected graph) next week.
Next week, this is what you hand in:
- A cover page that clearly indicates your name and the peer reviewer(s)’ names.
- Print and label your original graph : GRAPH 1.
- Print GRAPH 1 for each peer reviewer to review your graph. Reviewer must use a different colour pen to write comments on the graph to make your comments stand out. Call this graph “Graph 2″.
- An Excel-generated graph with the corrections. Call this graph “Graph 3″.