Chemistry lab work - week 4
Pipette Calibration
"Fast is fine, but accuracy is everything"
Wyatt Earp
During fall quarter you will make several determinations that require volume measurements using a 25.00 mL transfer pipette. Critical to these measurements are the precision and accuracy with which you pipette the samples. Find a 25-mL transfer pipette that you like, label it and keep it while you are in the program for your water analyses. Calibrate the volume the pipette delivers by measuring the mass of water the pipette delivers. You will need to correct the mass of water for buoyancy of the air (Harris p28) and then convert the mass of water into a volume using the density of water (Harris p37 Table 2-7 column 2). You will want to do these calculations in two steps so you can see the magnitude of buoyancy and temperature-dependent density corrections. You can check your results by using the temperature corrected volumes for 1 g of water in column 4 of Table 2-7.
1. Obtain a beaker of deionized water. Measure the temperature of the water.
2. Tare balance with an empty container. (Does it make a difference whether the container is open or if it has a lid?)
3. Pipette 25.00 mL water into the container and accurately reweigh.
4. Calculate buoyancy corrected mass.
5. Calculate volume of water using density (g/mL) at measured water temperature.
6. Repeat steps 2-5, 5 times.
7. Calculate mean volume, standard deviation, relative standard deviation or coefficient of variation, and 95% confidence limits.
8. Place results in a clear table.
9. Write a short discussion that tells a reader how accurately and precisely you can pipette. Use the values you calculated in your discussion to make the discussion quantitative.
IC Analysis of Lake Samples
During week 1 you made standards for the ion chromatograph. By now your laboratory technique has improved. You should discard your previous standards making more room in the refrigerator. Prepare new IC standards with the other members of your lake sampling team. Use these new standards to analyze your lake water samples. There should be 6 sets of standards, one for each lake team.
For your alkalinity titrations you had the equipment and reagents available to do multiple titrations of each sample to evaluate your precision. For ion chromatography, each sample takes 12 minutes to run. For each lake sampling consisting of duplicate samples from 3 depths, you need to run a blank, 3 standards, your 6 samples, a replicate standard, and a spiked sample. That makes 12 IC runs per lake. If you are well prepared and don’t waste time between sample injections, you can complete 1 lake sampling in 3 hours with time left over for one extra run. Replicate standards help to determine whether the instrument calibration has drifted during sample analysis. A spike helps to determine if a matrix effect is affecting the sample results. The spike is prepared by adding a known amount of a standard to a sample that has been run. Ideally, when the spiked sample is analyzed, the concentrations will equal the sum of the sample and standard concentrations.
The concentrations of your standards range 0.25 to 10 mg/L for Cl-, NO2-, and NO3- and 1 to 40 mg/L PO43- and SO42-. Should I just inject undiluted Lenore Lake water into the IC? Not likely. Table I in Eugster and Hardie’s paper lists Lenore Lake water as containing 1360 ppm Cl- and 2180 ppm SO42-. If this 1963 analysis is representative of today’s ion levels in the lake, a 1:1000 dilution would give 1.36 mg/L Cl- and 2.18 mg/L SO42-. These values lie within your standard curve limits. Let me emphasize – The 1:1000 dilution represents a good starting dilution for the Lenore Lake samples. Dilutions from other lakes will be different. You will have to use whatever information you have available to estimate appropriate dilutions for other lakes. Table 1 has ion concentrations for 3 of our lakes. You have conductivity measurements for at least one of these and for your other lakes. You should be able to estimate dilutions based on these data. As you run samples you should be able to observe whether the concentrations exceed those of the standard and you should be prepared with pipette, water, and volumetric flask to prepare a further 1:10 dilution without having to exceed your reserved instrument time.
For Lenore Lake we would have the following samples to run:
1. Deionized water
2. Low Standard
3. Medium Standard
4. High Standard
5. Sample 1 (1:1000 dilution)
6. Sample 2(1:1000 dilution)
7. Sample 3(1:1000 dilution)
8. Sample 4 (1:1000 dilution)
9. Sample 5(1:1000 dilution)
10. Sample 6(1:1000 dilution)
11. High standard diluted 1:10 with sample 3(1:1000 dilution) – Spike
12. Medium Standard - Duplicate
Atomic Absorption Spectroscopy Workshops
We will cover the theory behind atomic spectroscopies in detail winter quarter. You need to be able to operate the atomic absorption spectrophotometer fall quarter to analyze sodium and potassium concentrations in your lake water samples. To do this you need an introduction to the theory behind atomic absorption spectroscopy. Read the following sections in Harris before you attend the first workshop.
Chapter 21 p 495-498 top
p 502 bottom – 505 middle
p 509 - 510