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Measuring of Nitrate and Phosphate using a build it yourself
Colorimeter
Don't be off by what you see; anybody
can build this colorimeter, even those who have little or no knowledge of
electronics at all.
The idea to make a
colours-/colorimeter, came from new ideas/ understanding about how to realize
better plant growth and to try and get rid of algae by putting into practice
the Redfield Ratio. More about this theory and others, you can find on the
sites of Chucks
Planted Aquarium Pages , Charles Buddendorf and Adriaan Briene The starting point of the colorimeter,
was the plan of Toni's
Site.
Test kits for measuring Nitrate- and
Phosphate values, are available from most aquarium-shops. These tests show a
certain colour, depending of the quantity Nitrate or Phosphate in the test
sample (e.g. water). By comparing the colour of the test solution with the
colour chart from the test kit, you get a certain result: ….....mg/l.
But here comes the difficulty: The colour
that the test solution must be compared with the colour chart at bright light,
and always from the same light source. But, this can be a problem, for example
many people, including me, are partial colour blind, so it is rather difficult
to read the correct values.
When I let other people look at the colours
of the test solution, we got different results, and that is why the idea was
born to build a colours-/colorimeter myself.
The principle of a colorimeter really is
quite simple, see image. The beam of light from the lamp (LED = Light Emitting
Diode) shines through a cuvette with the test solution. Depending of the
intensity of the light going through the solution, the LDR (receiver = Light
Dependent Resistor) will give a proportional signal. For another
explanation to Basic Principles Spectrophotometry Concepts take and look HERE
The colour of the test solution determines
the colour of the LED that you use, in our case a Green LED will do well with
the Sera test kits, when you use a multiple colours-LED, the matching colour of
the solution must be chosen for the LED-colour
Note:
For example, if you have a blue colour test solution,
then the matching colour is orange/red, and the red LED will give the best
results. For a blue test solution use a red LED, for a red test solution use a
green LED and for a green test solution use a blue LED. A little simple and
amusing (in Dutch) explanation about this can be found here and certainly worth while reading.
The plans and
The lamp, in this case a LED, shines through
the cuvette that contains the test solution and is partially absorbed by the solution;
the remaining light is measured by the receiver (LDR).
Such a set up can be realized in different
ways, Examples , I have also tried
several possibilities and came up with a few models, that are shown further
(see page down). For realizing these measuring principles, I came across some
problems that I don't want to keep from you, amongst others:
First of all the cuvette/tube, I took glass
ones, but they where of such a poor quality that they were unusable; (what I
only found out while testing: the glass appeared not to have the same thickness
everywhere, what gave various results, depending of the position of the tube in
its holder).
That's why I switched to disposable cuvettes,
they were not very expensive (100 pieces for € 7, -) supplier but
I could only buy square ones, more difficult than round ones, because the
holder is harder to make. But you can't have everything!
Besides it’s important that the
tube/cuvette used for measuring is always in the same stable position between
the lamp/LED and receiver/LDR/photodiode. That is why I have chosen for a round hole in
which the cuvette is placed. The four corners of the cuvette touch the wall of
the hole, so that it is well locked up and stable in the holder, and the flat
sides of the cuvette can not become damaged or dirty.
Instead of a round one you can also use a
square block of hardwood, sizes about 40x40x40 mm, drill a hole of 17 mm
diameter, (but rather a few tenth of mm smaller ), then widen the hole so far
that the cuvette fits into the hole, (almost) without tolerance. Preferably,
don't drill through the wood completely, but stop a few mm before the end. If
you use a bench drill it is easy, stop when the point of the drill guide hits
the end. There will be a thin bottom in the block, what might be handy later
on.
There are several possibilities for building
a holder a.o as here is described. For this example a 2 colours LED is used
and a wooden block, made in 2 parts, which is a big advantage because it is
easier to make the hole a better fit for the cuvette.
The lamp, we have often spoken about, is in our case a LED, which can be
obtained in a large number of sorts and colours.
There are LED'S that produce multiple
colours, that can reproduce superior colour, this looks like a perfect option.
But, unfortunately there is an undesirable side, colour consist in principle
off three base colours: Red-Green-Blue. These are generated in the LED in
different places, but not in the centre of the LED. In my case (I don't know if
it was accidental) the following problem occurred (see the schematic). The colour
that does not come from the centre of the LED occurred can cause flooding (the colours
out of the LED are passing by the receiver/LDR/Photodiode).
That is why I chose for the single green- or the
2-coloured (green/red) LED in a diffuse type (not clear), because these don't
have that problem, or in any case to a much lesser extent. More importantly, a
diffuse LED even has a much greater spread of light. The green LED gives the
best results, when using the nitrate- and phosphate test kits of Sera. By using
for example a 2- colour (red/green) LED you can get the colour orange/yellow,
by connecting at the same time 2 voltages to the 3-pins LED (red and green on =
orange/yellow). You can even get 3 colours with one LED, what occasionally
might be handy if you use other test kits or -solutions.
Connecting a 2-colour
LED:
Connecting a 2-colour LED comes
later on, meanwhile here you can see that there is no need for much more
expense. It can even be done a bit more simply, because the middle pin of the
LED is a combined one, by removing the 2 resistances of 330 Ohm and connect 1 resistance
to the middle pin. Take for the resistance of 330 Ohm minimally a 1-Watt type.
The last presentation if you are
going to use a single colour LED.
The 8 Volt stabilized power source can also
be made without knowledge of electronics. The plug adaptor is for sale from any
electrical shop (it must anyway be a continuous tension- type). The + 8 Volt
and the 0 terminal can be connected to the connection you see above.
The LDR-part also consists only
of a few parts.
The universal meter that is connected to the meter exit is cheap and can be
obtained at different addresses bev the e.g.
the B.DT-830B , and is
regularly a(n) (special) offer in the well known building markets.
The complete schematic, arranged for 3
colours red-green-yellow by using the switch and push button (red and green on
= yellow)
Note: Print and take to electrical
supplier and he knows what parts knows you need. See pictures
I am now changing something concerning the
receiver Photodiode/LDR of 5 mm, (see first image) I have taken a bigger type.
Besides that I placed de the Photodiode/LDR further in the block and the LED
also.
Another adaptation: Fixing/sticking/screwing
a small thin plate of synthetic material on top of the wooden block (the ribbed
side), so that the cuvette always comes in the same position and to make the fit
as tight as possible. By doing that you don't have to make a little flat fixing
side in the lid, this way works better and easier. Also you don't damage or
scratch the see-through side of the cuvette when moving in or out. (See image)
The use of cuvettes
The use of cuvettes: I use disposable cuvettes
that have ribbed or dim sides. Hold the cuvette always at the dim- or ribbed
sides.
The cuvette has two clear sides and two
ribbed/dim sides. The clear sides are the optical surfaces and the ribbed/dim
sides are used for handling the cuvette. There can be a slight difference in
transmission if the cuvette is rotated by 180 degrees.
Note: If you use cuvettes without marking
points, place a small mark on one of the clear sides of the cuvette; it will
help when placing the cuvette into the Colorimeter in exactly the same way and
place. Small scratches on the surface of the cuvette can affect results. The
optical side of the cuvettes will become foggy in time which leads to a loss of
transmission. When a cuvette is the only one used in an experiment, this is not
a problem.
Fill the cuvette with the liquid you want to
measure, if possible always with the same quantity. If necessary, mark the
height on the dim/ribbed side, besides, that mark can also do as a placing
mark. (See above)
Cuvettes also can have small mutual differences, always try to use cuvettes
with the same deviation. You can find out, by placing empty cuvettes into the
holder, select the cuvettes with the same values and give them a mark or
number. Always clean and dry the cuvette (but beware of scratches).
The arrow on the cuvette (clear) can as also
be useful for height size of the test solution.
A lot off info can you find on Ask Owners HERE
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WARNING.
This last bit is only a draft and not confirmed
yet (and can have mistakes !!!!)
Making a standard
solution:
Making a standard solution for
calibrating the meter can be done in the following way:
1 milligram = 1/1000 (0.001)gram
A solution with 1000 mg/l is the same as 1 gram/litre, but more accurate.
Making a standard
solution for Nitrate:
1.635 Grams KNO3 (potassium
nitrate) dissolved in 1 litre Demiwater (demineralised or distilled water) gives
an standard solution of 1000 mg/liter Nitrate
Making a standard
solution for Phosphate
1.83 Grams K2HPO4 (mono potassium
phosphate) dissolved in 1 litre Demiwater gives a standard solution of 1000 mg/litres
Phosphate.
Why the odd numbers?
K2HPO4 and KNO3 both consist of
2 matters in a particular proportion:
for K2HPO4 is that proportion 174/95 = 1.83 gram
for KNO3 is that proportion 101/62 = 1.63 gram
Ask at the pharmacist or chemist
if they can please weigh one or more portions of 1.635 gram KNO3 and 1.83 grams
K2HPO4. And if you are there anyway, ask for the smallest injection syringe and
pipet, very handy and not expensive.
The diluting scheme beneath shows
how to make to make different standard solutions for the meter, with thanks to Charles
Buddendorf for this schematic.
If you notice something wrong,
not clear or incorrect, please let me know, so I can correct it. I am just a
hobbyist without real knowledge. my e-mail.
This page is not complete yet, but
only a test, the definitive text will follow (I hope ;-)).
* As last, my special thanks, to
Agnes Zaal and
Wim van Eyk
helping me with the translation.
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