hi, i am posting this because of the need of knowing how to obtain a chlorine(Cl) sensor compatible with arduino.
If you know how to buy one to some company or something like that please answer this post
Also, I let open the posibility of making the sensor myself in my house( i dont mind if the elements to do it are expensive)
Thank you at least for reading this and also to people who answer.
Try looking at this -- then give a better definition.
Do you mean as a gas or in water? There is a difference in the technology.
http://www.sensorex.com/products/more/process_free_chlorine_sensors
Do a Google Search for Chlorine Sensor Electrode ... (DAGS)
There are a few out there...
To your question is chlorine disolve in water for a pool or a fishbowl.
James:
Now you need to do your part. I gave you some links.
Here is what you do. You look at the links. You see how the sensors work.
Next you do some research on the concentration of chlorine you have to measure. A swimming pool should have lots. A fish tank almost none.
Dig out the range of measurements you need to make. The make some judgments about the sensors available.
Then you look at whether you are able to interface the sensors electronically.
The you try to imagine what type of program you would write to read the sensors -- then to display the sensors.
The you think about alarms.
For a pool -- you need an alarm if the concentration is too low.
For a fish tank you need an alarm if the concentration is too high.
by "Alarm", I mean the obvious -- flashing lights, beeping speakers, a Robot Screaming "Danger Will Robinson!" etc
Is this clear now? 
James-
Measuring chlorine in water electrically is a bit complicated. It depends on things like the pH of the water. Why you want to quantify the chlorine matters. For example, if you are trying to kill wee beasties in your pool, you are interested in the HOCl (hypochlorous acid).
Anyway, if you measure pH and use an ORP (oxidation/reduction potential) electrode, you can get this kind of measurement.
These electrodes aren't cheap, they are kind of a pain to maintain, and you need to calibrate them.
However...
A pretty good way to measure chlorine is colorimetrically. You might rig some kind of system to suck up a bit of water, then add DPD (N, N-diethyl-p-phenylenediamine), available at pool supply places as solid or in solution. The degree of color change is a function of chlorine concentration.
Then, how to measure? Absorbance of 530nm light is how it is done in some commercial meters. This is a green, and there are LEDs at this wavelength. You could thus
a)get a sample with a little pump controlled by the arduino
b)measure the light that goes through with some sort of light sensor
c)have a device add a few drops of the DPD solution
d)measure again.
You would need to calibrate against either known samples or use a guide published by the maker of the test solution to quantitate.
This is a pretty sensitive test. How you go about getting the water, and adding the reagent, well, that's certainly going to be half the fun. At first blush, it might sound like an ORP electrode is easier, and it might be. Usually, these have both silver and platinum in them, they are often fragile and fussy, and they are expensive.
In freshwater pools, useable chlorine is either HOCL or OCL ions.
I'm no expert, but, that last one looks like it affects conductivity.
You might find that a simple conductance meter will work just fine? Calibrate it using a plain old test kit at the start, then look for changes and set an alarm when it's out-of-range ?
qwerty12345:
To your question is chlorine disolve in water for a pool or a fishbowl.
Erm, chlorine is poisonous to aquatic life, you can't mean a fishbowl!