MicroLAB™ and Calibration Curves Lab 4 Pre-lab Help MicroLAB™ MicroLAB™ is a computerized system that allows us to acquire real-time data. In addition MicroLAB™ has a spreadsheet program with data analysis tools. Your lab manual has the instructions for the completion of this exercise. Double check your work onscreen before printing out anything. Pitfalls Always type a “0” before a decimal point for numbers smaller than 1, e.g. 0.123. Label columns correctly the first time. If you don’t, you will have to delete and redo them, which will result in a loss of data. If you are told to label a column [Fe], M the “[Fe]” refers to the label and the “M” the unit. If there are no units present, then that variable does not have any units. More Pitfalls Do not give two columns the same label. Do not label a column for which you will need to create a formula. The digits of precision you set your column properties to should reflect the digits of precision of your tabulated data. Select “Accept Data” often! When asked to look at a graph to determine certain values, always reference the spreadsheet for the exact values instead. Label / Variable Graphing Graphing is an important tool used to represent relationships in experimental outcomes and to set up calibration curves. It is a modeling device. Don’t worry about the meaning of the labels / variables yet. You will get to know these as the semester progresses. Let’s look at the following data: Units [Ni2+], M Absorbance 0.200 0.041 0.300 0.063 0.400 0.085 0.500 0.101 Graphing Graph Title Absorbance vs. [Ni2+], M Absorbance Equation 0.110 0.100 y = 0.20 x + 0.002 0.090 0.080 0.070 0.060 Best-fit Line 0.050 0.040 0.200 0.250 0.300 0.350 0.400 0.450 0.500 Graph Axis Labels [Ni2+], M Graphing Always add a title to your graph. On the previous slide, the title was Absorbance vs. [Ni2+], M because absorbance was graphed on the y-axis and [Ni2+], M was graphed on the x-axis. (Always y vs. x!) If a graph title is Temp F, degrees F vs. Temp C, degrees C what should be graphed on the x-axis? Graphing Always label your axis appropriately. The label for the y-axis is Absorbance. Absorbance has no units, so none are listed. The label for the x-axis is [Ni2+] and the units M. What does “M” stand for? If an axis is labeled with Temp F, degrees F which one is the unit? Graphing If your data points look like they fall on a line, be sure to add a “linear” calibration curve to them. If they don’t appear linear, DO NOT add a linear line. When you add a regression line to a set of data points, an equation results. This equation describes the line and can help you solve unknown values. The equation on our graph was: y = 0.20 x + 0.002 Equation y = 0.20 x + 0.002 (y = mx + b) The number (and sign) before the “x” is called the slope of the line (m). The number (and sign) without the “x” is called the y-intercept of the line (b). If a value is substituted in for x, do you know how to solve for y? If a value is substituted in for y, do you know how to solve for x? Calibration Curves A calibration curve gives you a graphical representation of an instrument’s response to a particular analyte. If we were to declare your 1992 Ford Escort an “instrument” and the gas it uses an “analyte,” we could construct a calibration curve for: Distance Driven, miles vs. Gas Consumed, gallons Calibration Curves The measurements that are made should all made using the same parameters: with the same vehicle using the same set of tires driving under similar road and environmental conditions using the same type of gas Data Table of Standards Distance Driven, miles Gas Consumed, gallons 25.3 1.0 49.2 2.0 73.9 3.0 98.2 4.0 122.8 5.0 Calibration Curve Distance Driven, miles vs. Gas Consumed, gallons Distance Driven, miles 140.0 y = 24 x + 0.7 R2 = 1 120.0 100.0 80.0 60.0 40.0 20.0 0.0 0.0 1.0 2.0 3.0 4.0 5.0 Gas Consumed, gallons Notice that the average gas consumption of this vehicle is 24 mpg! Unknowns Unknown analyte refers to an unknown measurement, not an unknown identity. To analyze three unknowns related to our previous example…we know we will be analyzing gas, the unknown measurement refers to either the gallons of gas consumed, or the distance driven. No matter which unknown we are trying to determine, our analysis must be made under the same conditions as previously so that we don’t introduce new variables. Do you remember what they were? Data Table of Unknowns Distance Driven, miles Gas Consumed, gallons 41.5 ? 82.1 ? 103.6 ? Calibration Equation Use the previously determined calibration equation to determine how many gallons of gas it would take to drive the number of miles indicated on the previous slide. Since Distance Driven was plotted on the y-axis and Gas Consumed was plotted on the x-axis, the new equation becomes: Distance = 24 (Gas Consumed) + 0.7 Calibration Equation Distance = 24 (Gas Consumed) + 0.7 Let’s solve this equation for Gas Consumed: Gas Consumed = (Distance – 0.7) / 24 Solving Unknowns Gas Consumed = (Distance – 0.7) / 24 Substitute in the measured values: Gas Consumed = (41.5 – 0.7) / 24 = 1.7 gallons Gas Consumed = (82.1 – 0.7) / 24 = 3.4 gallons Gas Consumed = (103.6 – 0.7) / 24 = 4.3 gallons Hand-Entry Template After you access MicroLab™ (in one of the computer labs or on your computer) do the following: In the dialogue box that tells you the MicroLab™ hardware could not be found, click on “Hand Enter / Simulate” Click on the “Hand Enter” tab. Open the “MicroLab Experiment” file. Follow the instructions in your lab manual to complete your pre-lab questions.