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 R
^{2} 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″.