Test kits for water analysis
Test kits for water analysis

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Test kits for water analysis

6.3 Electrical conductivity measurements

The electrical conductivity of water arises from the movement of dissolved ions in solution.

In general, the greater the concentration of dissolved solids, the greater the conductivity of water.

For low concentrations of dissolved ions, the relationship between concentration of cations (expressed as millimoles of charge, mmolc, per litre) and conductivity is approximately linear (Figure 11).

Figure 11  Relationship between the electrical conductivity of dilute salt solutions and cation concentration, expressed as millimoles of cationic charge per litre. 'S' stands for siemens, the unit of electrical conductivity, where 1 siemens stands for 1/(1 ohm).

As a measure of dissolved ion concentration, we tend to use total dissolved solids (TDS), which is the amount of solids dissolved in a water sample. This can be determined by gravimetric analysis but is more usually determined by measurement of electrical conductivity, taking the raw conductivity value and multiplying by a conversion factor.

There are two types of conductivity sensor: contacting and electrodeless. The former have their electrodes in direct contact with the test solution. Electrodes were originally made from gold or platinum because these metals could tolerate solutions that were highly acidic or caustic. Today, however, graphite, titanium or stainless steel are used and have been shown to have good resistance to such harsh test solutions.

One of the problems with contacting sensors is that some solutions tend to damage or coat the electrodes. Electrodeless sensors, however, contain two coils within a waterproof housing surrounding a tube which is open to the test solution. They induce an alternating current in the solution and measure the magnitude of this current to determine the solution conductivity.

The conductivity of a solution increases with temperature as ionic mobility increases, and so test results are reported at a specified temperature, usually 25 °C. Instruments usually have built-in temperature compensation, either manual or automatic, to allow the readout to be corrected to 25 °C.

Activity 4  MCERTS

Timing: Time allowed: 1 hour

The UK Environment Agency established its monitoring certification scheme called MCERTS to deliver quality environmental measurements. MCERTS covers the monitoring of emissions to air, land and water.

Visit the website of the Environment Agency [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] and find out what the requirements are for portable test kits for water analysis.

Activity 5  Test kits in action: health and safety considerations

Timing: Time allowed: 2 hours

Watch the Videos 7-9 showing water quality test kits being used in three locations.

Download this video clip.Video player: Video 7 
Skip transcript: Video 7  Measuring the quality of leachate from a landfill site.

Transcript: Video 7  Measuring the quality of leachate from a landfill site.

Narrator
This is a landfill site at Brogborough in Bedfordshire, southern England. We are going to look at two sorts of onsite analytical test, used to monitor the composition of the leachate, formed by rain and surface water draining through the landfilled waste. Leachate is sampled at a series of boreholes. We'll first look at some electrochemical measurements being carried out in-situ.
What I'm doing now is taking the multi-parameter sond, hand-held meter. I'm going to put this down the borehole to take a reading. These sonds are specifically designed to fit down this borehole - nice snug fit. We need to make sure that the sond is constantly moved in the liquid in order to get again the flow for the pH and the D.O. measurement. We note that the temperature is 12.6. so it's somewhat cooler than some of the other samples we've taken. Dissolved oxygen measurement at the moment is hovering around about the 12 to 15% level, quite low. The conductivity round about 4500 to 5000 and the total dissolved solids again round about the 3000 to 3100 mark. The pH value here in this borehole is round about 6.6 - so it's very low level dissolved oxygen in this borehole sample, as perhaps one would expect if it's been under pressure and sealed off against the open atmosphere.
We're now going to measure the ammonia concentration of the leachate photometrically - for this we need to take a sample. The level of water is first measured using an electrode on the end of a tape measure - you can hear the beep when we reach the water. A sample collector is then lowered to the required depth.
Thank you very much. So what we do now, is we need a tube - we're doing an ammonia test here.
Take the tube;
Take the pipette and we need 100 microlitres of sample;
We carefully pipette that into the tube;
Then we add one dose of the reagent;
Make sure that reagent is fully dissolved;
Leave the tube for 15 minutes.
After 15 minutes the diphenol blue reaction occurs, and we get a nice turquoise blue colour which indicates the concentration of ammonia present in the sample. Insert the tube again into the photometer, press it down. We wait a couple of seconds and again we get a reading of round about 79 mg/l-1 of ammonia as nitrogen in that borehole sample. So it's very straightforward, very quick process.
End transcript: Video 7  Measuring the quality of leachate from a landfill site.
Video 7  Measuring the quality of leachate from a landfill site.
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Download this video clip.Video player: Video 8 
Skip transcript: Video 8  Water quality testing at Caldecotte Lake.

Transcript: Video 8  Water quality testing at Caldecotte Lake.

Narrator
Here we demonstrate how a commercially available test kit can be used to measure the water quality of a lake. In fact, we'll measure the concentration of a representative chemical species - sulfate ions (SO42-). You'll see how test strips are used not only to establish the presence of the sulfate ions, but also to give an approximate value of their concentration. We will follow this with more accurate analysis using a portable spectrophotometer.
The preliminary test strip measurement is particularly useful as it can tell us if the concentration of the analyte is outside the linear range of the spectrophotometer, and thus, if a dilution is required prior to analysis.
But before we get started, we look at aspects of good practice when obtaining a representative sample.
The best way of taking a sample is to make sure you have an appropriate sample point where you can get a representative sample. Perhaps try and do it against the current in order to make it easier and to stop too much organic matter from floating into the bottle. Make sure you rinse the bottle out a couple of times, in order to get out anything that might have contaminated the bottle prior to sampling. When filling, as well, try to ensure that you don't get too much agitation into the bottle and as much air out of the bottle as possible if you're sampling for dissolved oxygen or BOD. Once you have the sample you can then take it back to the analysis station, or if need be, the laboratory.
At the moment we are perhaps unsure of the levels of the particular parameters we're looking for in the sample, so the easiest and quickest way of doing that is to take a test strip - these are dip strips - to give us a broad idea of perhaps what levels of sulfate and ammonia that we have in the sample. Take the strip, dip it into the sample for a couple of seconds, and shake off the excess liquid. We'll leave that for a minute or so to develop. If we look at the sulfate - if we compare it - sulfate levels are very low. We're looking at levels with a very, very small colour change in the pad at the bottom of the test strip. So we're looking at a result possibly somewhere round about 200 mg/l-1 of sulfate.
We then take a sulfate tube here; take the reagent we need; take the top off the tube. What we then do is take 1 ml of our sample, pipette that into the tube. Then take a reagent; this is designed to give you a barium sulfate solution which is measured as a precipitate. Make sure that - it says add one micro-spoon - all the reagents you add slightly to excess in order to make sure that there's plenty in there; again, you don't have to be hugely precise. You then shake the tube; leave it for a couple of minutes for the precipitate to form.
So once we have the tube - we've left it the appropriate two minutes for the precipitate to form, to give us a reliable reading - you take the tube, insert it into the machine, again making sure that the little black line is lined up with the front of the machine so that the photometer reads the barcode correctly. Insert the tube. It takes a few seconds to measure - in this case 105 mg/l-1. So the lake sample is 105 mg/l-1 of sulfate.
End transcript: Video 8  Water quality testing at Caldecotte Lake.
Video 8  Water quality testing at Caldecotte Lake.
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Download this video clip.Video player: Video 9 
Skip transcript: Video 9  Water quality testing in the River Ouzel.

Transcript: Video 9  Water quality testing in the River Ouzel.

Narrator
This is a stretch of the River Ouzel - a small river near Milton Keynes, in southern England. Here we will use a commercially available test kit to measure the concentration of nitrate ions in the water. You'll see how test strips may be used not only to establish the presence of the NO3- ions, but also to give us an approximate value for their concentration, prior to a more accurate photometric analysis. The preliminary test strip measurement is particularly useful, as it can tell us if the concentration of the analyte is outside the linear range of the spectrophotometer, and thus, if a dilution is required prior to analysis. You'll also see how potential problems from interfering nitrite ions (NO2-) are avoided.
But before we get started, we look at aspects of good practice when obtaining a representative sample.
David Hall (VWR)
We need to take a sample from the river. At this point you need to find an appropriate, safe and easy sample point. Looking at the flow of the river, it's going from left to right here, so it's, if we try and take a sample that's with the flow. It's important that you rinse the bottle out a couple of times to get a representative sample. Once we have the sample we take it back to the work station. Make sure it's labelled correctly so you don't get it mixed up with other samples.
What we're going to do now is perform some very quick tests. So the first thing to do is take the sample, take the nitrate tube, take a strip out, leave it in the sample for a couple of seconds, shake off any excess liquid. You then need to leave that for a few minutes to develop.
Now the strips have had enough time to develop. If we look at the colour development here, the bottom strip is the nitrate result and the second pad - the one from the bottom - is the nitrite result. Nitrite is showing zero or as close to zero as it's possible to be, but we do have some nitrate in this particular sample, which would go alongside the fact that the river contains a lot more plant life, a lot more algae, giving a much higher level of nitrate. In this case we'd anticipate a level around about 25 - a little bit more perhaps - between 25 and 50 mg/l-1 of nitrate in the river sample.
Right, we're now going to do a nitrate test to get a slightly more accurate result. What we need is to take one of these empty tubes, with the reagent already in the tube. First thing we do, is we need to pipette 0.5 ml of the river sample, add it to the tube. We then need to take 1 ml of nitrate reagent, put the tube cap back on again, shake and then that needs to be left for approximately ten minutes for the colour to develop.
If we then take our nitrate sample, we then insert this into the machine. The machine is saying that in this particular sample we've got 2.6 mg/l-1 of nitrate.
Finally, we carefully record our results.
End transcript: Video 9  Water quality testing in the River Ouzel.
Video 9  Water quality testing in the River Ouzel.
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Comment on the health and safety considerations to be taken into account when sampling and carrying out measurements outside. Identify at least three hazards associated with this activity, along with the potential harm these may cause. You need to think not only about any substances or equipment being used, but also about the environment in which the activity takes place and the activity itself.

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