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DPD No 4

DPD No 4 Tablets:

For those keeping fish and looking for DPD No 4 tablets to test for the presence of Chlorine or chloramine levels then the following information may assist. 

In the UK domestic water supplies are normally in the region of 0.5 mg/L but can be raised to about 4 mg/L. 

The maximum tolerable level for chlorine or chloramine for Koi is 0.02 mg/L. This is one tenth of the value at which rapid deterioration of the respiratory system begins so, even at normal tap water chlorine values of 0.5 mg/L, exposure to undechlorinated water can often lead to fatalities.

Proprietary dechlorinators have recommended dose rates that will typically fully dechlorinate water with chlorine/chloramine levels up to 2.0 mg/L and so will provide full dechlorination at normal supply values.  However, this dose rate can only provide partial dechlorination if there is an increased level due to the higher values sometimes added by water supply authorities.

Since the maximum value of chlorine that Koi may be exposed to without harm is 0.02 mg/L measuring such low values is difficult. 

Properly dechlorinated water will contain no chlorine; incompletely dechlorinated water (obviously) will contain some chlorine.  Therefore, in order to ensure water has been fully dechlorinated it isn’t important to know the exact value of any remaining chlorine in the water, all that is necessary is to know that there is either “no chlorine” or “some chlorine”.  The DPD 4 chlorine test is suitable to use as confirmation that water has been fully dechlorinated.

DPD 4 tablets are fast and simple.  All it takes to do the test is some DPD 4 test tablets, a sample tube and a piece of white paper.   The method is described below. 

How to Test

Clear results confirm that the water is totally free of any form of chlorine; any trace of pink indicates that the water hasn’t been fully dechlorinated and is therefore not safe to use until additional dechlorinator has been added and a second test indicates “clear”

Fill a standard 10 ml sample tube with the water to be tested and drop in a DPD 4 tablet. Don't look through the side in the usual fashion, place it on a white surface and look down through it. This will make it easier to see a faint colour change. If there is no chlorine or chloramine in the sample it will stay clear. The tablets begin to show a noticeable pink colour change at around 0.01 mg/L so, although this is too indistinct to be able to say what the actual level is, it is valid to say "no colour means no chlorine". If you can see the faintest pink colour in the sample, it means there is a trace of chlorine in it and there shouldn't be.

Strictly, a DPD 4 tablet doesn’t just test for chlorine, it is also looking for any other oxidising reaction, so, although it provides an instant colour change when any form of chlorine is present, it will then very slowly produce a pink colour change due to dissolved oxygen.  This is especially noticeable if the sample is left exposed to the air so that more and more oxygen can enter the solution. Look for the colour change immediately because even if there is no immediate pink colour, showing that there is no chlorine in the sample, after several minutes dissolved oxygen will begin to turn it pink anyway.

Types of DPD 4 tablets

There are two types of DPD 4 tablets. The "Rapid Dissolving" tablets do exactly that but the ingredient in the tablets that causes them to rapidly disintegrate and dissolve results in them tinting the water white. They still turn the sample anywhere from pale pink if there is a trace of chlorine or chloramine present to deep pink if there is a lot of it present.

There are also "Photometer Grade" DPD 4 tablets which are a little more expensive. These are difficult to dissolve but they leave the sample totally clear unless there is any form of chlorine present. Don't shake the sample tube vigorously in an attempt to dissolve these types of tablets; that will dissolve oxygen into the sample and may cause a false positive reading. The tablets should be crushed in the bottom of the sample tube with a specially made tablet crusher or with something that resembles the blunt end of a plastic knitting needle. After that, a couple of inversions of the tube will be all that is necessary to ensure good mixing.

Both tablets react identically to chlorine/chloramine the only difference is that the rapid dissolving types add the white tint in addition to any pink colour change due to chlorine, whereas the photometer grade tablets leave the sample clear except for any pink colour due to the presence of chlorine.

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