Topical tips 15: Using aquarist test kits for simple analysis of 'pond dipping' samples.
brief notes on electronic pH and conductivity meters.
by David Walker, UK
A wander around larger pet supply stores will usually reveal a good variety of test kits for the aquarist to check various properties of aquarium water. These kits sell for a few pounds and are also useful for the microscopy enthusiast to assess some water properties of freshwater habitats studied.
A basic appreciation of a water body's chemistry is one step to put freshwater studies on a firmer ecological footing. In the author's home area for example, stone water troughs are a favourite habitat to study but it isn't always obvious whether a trough is filled from rainwater, a spring, run-off from roads, livestock fields or how it varies during the year. A few simple analyses using these kits can offer some insight into these factors.
Shown below are a selection of the type of kits the author has tried and found useful. The actual makers may vary depending on where bought. On-line dealers also offer a wide range if no local large pet store. Only a brief note on the value of each measurement type is made; the many books on limnology and freshwater ecology can provide a good introduction. The author also offers a few thoughts on some cheap electronic pH and conductivity test methods.
pH: Many freshwater organisms have specific pH preferences and tolerances.
pH - Broad range kits: This type of kit is my favourite and the one that gets most use, more so than an electronic pH meter. Very often all I wish to know is a rough idea of the acidity or alkalinity and this sort of kit does it well with clear colours for each pH. It's compact and very quick to do at pond/stream side to assess for this kit pH 4-10 to nearest unit. Fill the water to the 10 ml level, add four drops of indicator and check colour against the bottom row on box. (The pots of chemicals shown aren't needed, these are buffers for aquarists to adjust aquarium pH.) Maker 'Waterlife'. Indicator sufficient for ca. 150 tests. The colour range for their narrow range kit pH 6.6-7.8 is along top of box.
pH - Narrow range kits: These can narrow down the pH but only perhaps to the nearest 0.4 - 0.6 rather than the optimistic 0.2 of the scales as the colour changes can be subtle. If accuracy better than ca. 0.5 is required this is where a pH meter would be better. This type of kit can only be used of course when it is known from a broader measurement that the pH lies in the kit's range. Maker 'Aquarium Pharmaceuticals'. pH 6.0 - 7.6. 250 tests. ca. £4.
In the author's opinion the above two aqueous kits give easier to interpret colours than the rolls of broad / narrow range pH paper familiar from school labs. The colours often seem to fade on these papers with storage and colours weak. Although there are modern small plasticised strips with bonded indicators for pH measurement and a wide range of other tests, but these aren't always available outside of the lab. suppliers and can be expensive.
Water hardness: Some organisms require calcium for their shells and exoskeletons.
The kit above is an example of the colorometric tests for either total water hardness (calcium and magnesium) or calcium. The number of drops of the final reagent required to change the colour of the sample from red to blue are counted. The number of drops x10 gives, in this case total hardness, in ppm. This kit and nitrate one below are more expensive than pH kits at ca. £7 but would probably be used less regularly for a given water body. Maker of above kit 'Aquarium Pharmaceuticals'.
Nitrate: Nitrate is the end product of the aerobic degradation of organic and inorganic nitrogen compounds present in water. Fertilizer run off from surrounding land can also influence the nitrate levels. The nitrate content can provide an indication of the level of pollution, which in turn will affect the occurrence of organisms sensitive to or tolerant of such pollution levels. In general, nitrate concentrations from 25 - 100 mg / l or more indicate increasing levels of pollution.
The nitrate test kit above is a typical maker's example with three reagents and pot of 'powder' with spatula. After completing the test sequence the colour after 10 minutes is compared with that on scale on the box for nitrate concentration. Maker 'TetraWerke' ca £9-50 for 50 tests.
Electronic methods for pH and conductivity
The cost of pocketable pH meters and conductivity meters has plummeted in recent years and are widely available. A multi-purpose meter may be worth considering, some now measure temp, pH and conductivity. It is vital that meters are also bought with the recommended calibration solutions and, for pH measurements especially to be worthwhile, the user must be prepared to adopt a more rigorous approach, with initial and regular calibration, correct storage, standardised sample measurement etc. Metrohm, one of the leading makers of such equipment, are admirably succint with their opinion of pH measurements without calibration (see first link in resources below). It's worth keeping a calibration record sheet for each meter.
Electronic pH meters: It's worth comparing specs of models before purchase; some of the cheapest may have no temperature compensation or may resolve to two decimal places when its accuracy is 0.2. The model shown does have an accuracy of 0.02 but on reflection was overkill for casual studies, a one decimal place model would have been sufficient. The cheaper kits don't always come supplied with two calibration solutions as here (although these small pots will require replacement). I haven't found this design as practical as the shorter more pocketable designs.
Errors are more likely to creep into the often more casual and occasional studies the enthusiast may make cf. a lab's studies where an electrode will be correctly stored and invariably the calibration checked before each study. The hobbyist's meters may be left for some time and less than ideally stored. To demonstrate the potential inaccuracies, I measured a known calibration solution after a few months storage without performing the maker's recommended 4h reactivation procedure. The stabilised reading was 0.4 too low.
Conductivity: The electrical conductivity of water provides a measure of the total concentration of dissolved ions present and hence provides a general indication of the inorganic nutrient concentration of a water body.
The meter right is a typical model and very portable. Such meters, as did the author's, do not always come with a calibration solution and may give the impression that they are accurate straight out of the box. Not necessarily so, if the author's example is typical as it was reading 20% low before calibration.
Unlike pH meters, however, which typically have thin glass membranes which can change property with age and storage, the conductivity meters are measuring conductivity between two metal probes so are a lot more robust and less likely to change. After the initial calibration of the author's example it has remained fairly accurate with only occasional need for adjustment over nine years of intermittent use and storage.
A conductivity measurement of an unfamiliar water body is useful before pH measurements as it can show if it's of low ionic strength and thus potentially 'weakly buffered'. For such samples the pH can shift just by the act of measuring the pH so for more detailed projects studying such water bodies needs particular care in choice of equipment and measurement (as do accurate pH measurements of rainwater).
A temperature compensated meter is worthwhile as conductivity is strongly temperature dependent. An auto shut off model is also useful. Waters with high salinity may need a meter with higher range. Make / model shown, Hanna DIST WP3, range 0-1990 µS/cm. Resolution 10µS, automatic temp. compensation. ca £53 nine years ago, cheaper nowadays.
Comments to the author David Walker are welcomed.
On-line resources - a selection:
Metrohm's downloadable 'The background of pH measurement and hints for your daily work' (12 pages). Note section 3.4.1 on the validity of pH measurement without calibration!
Radiometer Analytical's MeterLab Documentation web page has superb free downloadable pdf articles on 'pH Theory and practice guide' (34 pages) and 'Conductivity theory and practice guide' (54 pages).
'Water pH' an article on the US Fish and Wildlife Service website is a useful overview of theory and practice of the pros, cons of different water analysis methods.
Metrohm's guidelines and recommended meters for pH measurement of water samples with low conductivity (defined by them as <200 µS/cm) and high conductivity.
An interesting thread on ChemTrail Central reporting and discussing the relative pros and cons of techniques for measuring the pH of rain water.
Water analysis equipment is one of a number of items for a microscopist's laboratory. Richard Howey gives a valuable discussion in his four part series 'Equipping a laboratory'.
Suppliers: An 'aquarist "test kits"' search in Google shows many online dealers. Electronic meter makers include Hanna Instruments who have a very wide range. The 'Checker' model sells frequently on UK eBay for <£20, but don't forget the calibration solutions.
Published in the March 2005 edition of Micscape.
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