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Environmental Chemistry Lab-Winter Quarter 2005
Week 6: Analysis of Ca, Mg and Cu by Flame Atomic Absorption Spectroscopy
Read in your text: Pg 101 + all of experiment 10
Metals play an important role in water chemistry and water quality. While some metals are toxic at high
concentrations, metals also play an essential role in natural stream biogeochemistry, and help buffer stream
waters. In this lab we will use flame atomic absorption (FAA) spectroscopy to quantify the concentration of
3 important metals, Ca, Mg, and Cu in North Creek water, an unknown and 2 tap water samples of your
choice. FAA is one of the most commonly used methods to quantify metals at moderately low (ppm)
concentrations.
Ca and Mg ions in stream waters are usually associated with CO32- ions. This means that Ca and Mg are the
primary buffers available in most natural waters. Higher concentrations of Ca and Mg are generally
associated with higher buffering capacities, which is important in resisting damage from, for example, acid
rain. Elevated levels of Ca and Mg also help attenuate the toxicity of other constituents, such as ammonia or
heavy metals to aquatic organisms. Generally waters in regions with Granitic soils, such as western
Washington or New England, are low in these metals and are referred to as “soft”. Natural waters in the
Midwest typically have high concentrations of Ca and Mg and are referred to as “hard”. Hard water also
interferes with the chemical action of soap. Hardness including the contribution from Ca and Mg ions, is
expressed as the equivalent CaCO3 concentration, expressed in mg/liter.
There are several common methods to determine hardness in surface waters. This includes EDTA titration
and portable water test kits (see experiment 8 in your text). However neither of these methods is very
sensitive and we have found that most waters in western Washington are too “soft”. Instead, we will use the
much more sensitive of flame AA to measure Ca, Mg and Cu.
Specific objectives for this lab are:
1) Learn to measure metals in water by flame AA;
2) Quantify the Ca, Mg and Cu concentrations in your samples and calculate the hardness;
3) Interpret your results with respect to the type of water sample;
4) Conduct a statistical analysis to determine the detection limit and the 95% confidence interval on the
unknown concentration.
Solutions and supplies provided:
Mixed stock standard containing 100 ppm of both Ca and Mg and 10 ppm of Cu.
DDW
200 ml volumetric flasks
North Creek water (acidified to 0.4%)
Two other water samples (This can be lab tap or your home tap water. Collect into a clean polyethylene
container).
1:1 concentrated Nitric acid
Pre-lab preparations:
Prior to coming to lab, you must complete the following steps:
1) Read through Experiment 10 and this entire lab handout. Note that the information in the text is for
background. We will not be following any of the detailed procedures given in the text.
2) Start on a new page in your labbook and record the usual information (title, goals, general approach);
3) You will prepare one set of working standards containing Ca, Mg (0-20 ppm) and Cu (0-2 ppm).
The set of standards should contain a blank, plus 4 concentrations spaced evenly through the
working range. Calculate the volume of stock standard needed to generate your working standards
(in 200 ml volumetric flasks). Put this information neatly into a table.
4) You will be provided with North Creek water, you should collect one additional sample for analysis
and document in your lab book the source, time and date of collection and any other pertinent
information.
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Specific Directions:
1) Goggles must be worn for the duration of this lab experiment;
2) For this lab, you will work in pairs.
3) Identify all unknown samples to be measured. This should include the north creek sample, the QC
unknown and your 2 unknown samples (one from each person). Using the 1:1 concentrated nitric
acid provided in the fume hood, acidify your 2 unknown samples to 0.4% (0.4 ml per 100 ml of
sample). Use caution in doing this step. Mix the samples gently and set aside.
4) You should now make the Ca, Mg and Cu working standards in 200 ml volumetric flasks.
5) Put your samples in safe, transportable containers in a random order. Record the order in your lab
book and proceed to the AA.
6) At the AA, you will get directions on its operation by the instructor or lab technician. You will need
to take some time to enter your sample information into the computer attached to the AA.
7) For each measurement, make 3 replicates and use the mean value in all calculations. While the AA
computer will do most calculations for you, it is essential that you get the absorbance data recorded
in your lab book. You can print out the absorbance data and tape this into your lab book and/or get a
floppy disk of the data.
8) As soon as you make your first measurements, check the response of the AA to ensure it is working
properly and your calibration standards are given a reasonable response. The absorbance of Ca and
Mg in your 10 ppm standard should be around 0.2 and 1.3, respectively.
9) Prior to cleaning up, make a quick plot of your calibration data to be sure they are reasonable. If any
calibration standard looks off, it would be a good idea to remake that solution.
Calculations:
1) Generate calibration curves for Ca, Mg, and Cu. Print out a plot of the calibration curve for each
element. Watch out for non-linearity. For each calibration curve, plot out the residuals to check that
the linear regression is a good fit to the data points and there is no evidence of non-linearity. If the
calibration data shows evidence of non-linearity at the higher concentrations, you could: (a) leave out
any higher concentration points and compute linear regression lines (but only if your unknowns are
within the linear region); or (b) calculate your regressions using a quadratic function. In either case,
your unknown samples must be bracketed by your concentration standards and the linear or
quadratic fit must be a good fit to your calibration data points.
2) Calculate the Ca, Mg and Cu concentrations of all samples. Report this information in a table. For
each element, state clearly how you did the calculation and give the equation for the calibration
function that you used.
3) Calculate the “method detection limit” for all elements. This is found by calculating 3 times the SD
of your lowest level standards. This gives the lower detectable absorbance value, which is converted
into a concentration using your calibration relationship. Report the detection limit for each element;
4) For Ca, calculate the 95% confidence interval for all unknowns using the information in Dr. Jaffe’s
“Survival statistics” handout and the spreadsheet that has been provided for this calculation. (you can
get a copy of the spreadsheet from the class website.
Report
Hand-in to Dr. Jaffe the following items by the deadline given on the syllabus:
1) A two page summary of your work;
2) Plots of the calibration curve for Ca, Mg and Cu and the residuals for each. (6 plots in total).
3) A table (table 1) showing the equation you used to calculate the concentration for each element
(linear or quadratic);
4) A table (table 2) showing the Ca, Mg, Cu concentration for each unknown, the detection limit for
each element in each unknown and include the 95% CI on all Ca results.
5) The hardness in ppm. You can use the following equation:
Hardness = Ca (mg/l) * 2.50 + Mg (mg/l) * 4.12
6) Comment on the hardness and Cu values of the 5 samples. Which ones have the greatest hardness,
why do you think? Are there any environmental implications of the Cu concentrations you
measured? What level of Cu is considered safe for drinking water?