In an effort to do short and intersting little end-of-the-year projects, I sent my chemistry students home for the weekend with small bottles to collect water samples for some simple tests. Someone told me to have them refrigerate the samples after collection. What else do you think I should know for them to get some decent results?
(I might be unavailable this weekend so if I don’t reply, don’t think I don’t appreciate the suggestions!)
It depends on what you’re testing the water for. We used to treat the water with nitric acid prior to tests for heavy metal contamination - but I imagine you won’t need to do that.
Temperature is a good measurement to get right when you collect the water - not only will the temperature of the water change after it’s collected, but it will affect other variables such as dissolved Oxygen and Biological Oxygen Demand.
Turbidity is also a good datum to collect just as you get the water.
If you’re testing for nitrates and phosphates, you should be warned that most of the commercial education reagent tests for this only detect fairly high concentrations of nitrates and phosphates - both freshwater and salt water tests I’ve done with these sorts of kits tend to show a lot of negative results, and just aren’t sensetive enough at the sort of concentrations I find in bodies of water local to me. Also be warned that the foil packets in these kits contain small amounts of heavy metals.
pH results can be interesting and pose few methodological difficulties for high school or middle school students, and the same can be said for salinity measurements.
If they are testing for bacteria, the sample should be taken first thing in the morning and before any spigot has been used. The spigot should also be sterilized first.
OK, thanks. I was thinking definitely pH, and maybe for a couple of different metals. I only have six students, and I’m trying to just do some things that are less theoretical at this point and a little more real-world. I know we have some probes somewhere that I didn’t think about pulling out. I’m not sure what types though. I hadn’t thought about bacteria, but that would be good.
You could look at conductivity, and if you have access to a good balance and a oven to dry with, do a gravimetric total disssolved solids. Graph all the student’s results with TDS on on axis and conductivity on the other and you should get a pretty good correlation. It won’t be perfect because the ionic activity will be different, but that’s what happens in the real world.
For heavy metal and nitrate testing (the two test my lab does regularly on our Water for Injection for pharmaceutical production) we evaporate the sample down to about 10% of it’s volume before analysis. These are qualitative tests (although designed as limits… if the final colour is less intense than a standard solution, we know the result is less than X ppm of material, and therefore passes the criteria we use).
If you can get your hands onto a copy of the USP/NF, the methods are all laid out in the Purified Water or Water for Injection monographs. Then you could obtain some purified water, distilled water, and your student’s sample waters and compare them. You might not have access to all the reagents to make the TS solutions, but it’s a start. This is a test done very frequently in the “real” world.
The key here is what are you testing for, what tests are you doing for these things, and where are their samples coming from? pH is unlikely to change much so is a good test.
You don’t really intend to accurately test for things, you just want to show the scientific method and scientific reasoning.