Be a physical chemist, an analytical chemist, an organic chemist, if you will; but above all, be a chemist - Ira Remsen
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| Photo 1: Yazor Brook, Hereford (polluted by soil run-off and shopping trolley) |
As citizen scientists monitoring the health of the River Wye, we perform both qualitative and quantitative measurements. Qualitative measurements might include how the water looks (clarity, colour) and whether there is evidence for pollution (algae, foam, soil run-off, shopping trolleys, etc). You might be surprised how many shopping trolleys we have to extricate from brooks and streams!
Quantitative measurements would include the temperature, conductivity and pH of the water as well as the concentration of nutrients such as orthophosphates and ammonia. There is a third category which is semi-quantitative; this would include nitrates and nitrites when measured using a simple dip test. Most citizen scientists that are part of the Wye Alliance measure turbidity using a Secchi Tube, a semi-quantitative method, although some do use the quantitative nephelometer.
For quantitative and semi-quantitative analyses, it may be necessary to prepare standard solutions of known concentrations in order to check the equipment is working correctly (i.e. within specification). Occasionally, water samples may need to be diluted prior to measurement when the the concentration of the analyte (e.g. orthophosphate, ammonia, turbidity) falls outside the upper range of the method and/or instrument.
Approximate dilutions can be achieved using basic equipment such as measuring cylinders and syringes (Photo 2). Prepare a 1:10 dilution by syringing 1 ml of sample (e.g. river water) into a clean dry container before adding 9 ml purified water (distilled or de-ionised); gently swirl to mix the contents. Alternatively, add sample to the 10 ml measuring cylinder up to the 1 ml graduation mark and then add purified water up to the 10 ml graduation mark.
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| Photo 2: 10 ml Measuring Cylinder and 1 ml/10 ml Syringes |
For more accurate dilutions you could calibrate your measuring cylinders and syringes. To do this you will need a cheap (£20)
microbalance (
Photo 3) capable of measuring to at least three decimal places (0.001 g). To calibrate the measuring cylinder, place the empty cylinder on the balance and
tare; add purified water to the 1 ml graduation mark and record the weight; add more water to the 2 ml graduation mark and record the weight. Continue up to the 10 ml graduation mark. Water has a density of 0.998 g/ml at 20 ℃ so you should divide the recorded weights by 0.998 to give the corrected volumes for the measuring cylinder. The density of water
changes with temperature so you may need to choose a different correction factor if the ambient temperature differs from 20 ℃.
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| Photo 3: Cheap Electronic Top Balance |
Use the same procedure to calibrate your syringes. Alternatively, you can prepare the dilution directly using just the microbalance. Place a clean vessel on the balance and tare. Add the water sample and record the weight (W₁). Add purified water diluent and record the total weight (W₂). The dilution factor is W₂/W₁. This method only works when diluting a water sample with water.
Traditionally, volumetric dilutions were performed using glass pipettes (
Photo 4) and volumetric flasks (
Photo 5). Glass pipettes come in two forms (
bulb and
graduated); the former delivers an accurate single shot volume whereas the latter is more flexible and can deliver a range of volumes. Volumetric flasks (
Photo 5) come in a wide range of sizes (e.g. 1 ml to 5000 ml). Class A (high accuracy) and Class B (lower but still good accuracy) pipettes and flasks are available.
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| Photo 4: Pipette Pump and Graduated 25 ml Pipette (Class A) |
To prepare an accurate dilution, a known volume (e.g. 25 ml) would be pipetted into a volumetric flask (e.g. 250 ml) and made up to the mark with diluent. You can find more information on the accuracy and tolerances of volumetric glassware
here.
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| Photo 5: 250 ml (Class A) and 1000 ml (Class B) Volumetric Flasks |
For rapid and repeat dilutions, most scientists now use an adjustable volume autopipette (
Photo 6). A good quality autopipette can be
bought for £30-40 although the disposable tips (ca 0.05p to 0.15p each) are consumable extras. The autopipette range covers volumes from as little as 0.1 μl to 10 ml; the two Onilab instruments I bought had volume ranges of 100 μl to 1 ml and 1 ml to 5 ml. Both autopipettes came with QC certificates.
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| Photo 6: 100-1000 μl and 1000-5000 μl Adjustable Volume Autopipettes |
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