# How to measure chlorine content using ORP sensor?

In my project I need to measure pH, turbidity, and chlorine level of treated drinking water. I found sensors for measuring pH, turbidity, and also chlorine.

The actual problem begins is that the cost of chlorine sensor is too high, so I googled for an alternative solution, which led me to use an oxidation-reduction potential (ORP) sensor.

Can anyone tell me about the relationship between pH, chlorine, and ORP value in millivolts and how to measure chlorine content in treated water using ORP sensor?

• Homework. You need to do the literature study for your project.
– Karl
Dec 28 '20 at 14:45
• Can anyone tell me about the relationship between pH, chlorine, and ORP value in millivolts?, There is no relation because the ORP does not know what species it is sensing. Dec 28 '20 at 16:56

For a school project like yours, you can use iodometric titration. Iodometric titration is one of standard methods of water examination. The amount of free chlorine in tap water is really small on the order of < 1 parts per million therefore you need a lot of sample. One would typically take 500 mL of tap water (or more), add potassium iodide, and titrate with 0.01 M sodium thiosulfate. You can use an ORP and plot a potentiometric titration curve.

Consult your teacher for this option. He/she should explain to you the details.

There are spectrophotometric methods but the reagents may not be available in your school.

Based on the comment by M. Farooq with respect to the oxidation-reduction potential (ORP) sensor, I would suggest a pure chemical test.

In an inverted container containing the subject treated water, to test for dissolved chlorine and hypochlorous acid, simply inject a volume of stabilized hydrogen peroxide.

The following reactions should govern the system:

$$\ce{ Cl2 (d) + H2O (l) <=> HCl (aq) + HOCl (aq)}$$

$$\ce{ HOCl (aq) + H2O2 (aq) <=> HCl (aq) + H2O (l) + O2 (g)}$$

The last is a fast reaction and I would rapidly note (by marking) the volume of oxygen rapidly released. Subsequent elevation in released oxygen could be produced by other catalytic agents acting on the H2O2 itself.

I would recommend performing a trial run on distilled water to which is added a known amount of Sodium hypochlorite. Repeat with boiled tap water plus NaOCl and observe any growth in oxygen release with time.

• The amount of free chlorine in tap water < 1 ppm. This test will not work at very low concentrations. Dec 28 '20 at 23:05
• Per this table at google.com/… , assume since my tests would convert gaseous, dissolved, and chemically converted chlorine (as HOCl), say 3.5 ppm or about 1/1000 of a mole . As 1 mole equates to over 22,000 ml of O2, this here equates to over 20 ml of O2 gas. So employ an inverted container whose shape would facilely allow such a measurement. Agree, however, special equipment required. Hence, my testing recommendation. Dec 29 '20 at 3:54
• Thank you for sharing your knowledge. This is not a school project, its an industrial project, I need to measure ph, turbidity, and chlorine content and display the values using LCD in the wastewater treatment plant. I found ph and turbidity sensors and i wrote code for it, but now am struct on chlorine measurement, there is a sensor for measuring chlorine but its cost too high, ORP sensor measure the chlorine content but I don't know the relationship between chlorine ph and ORP sensor. I got one from google.(shorturl.at/lwQZ3)].can anyone explain the graph Dec 30 '20 at 5:49
• Again prepare test samples (using the existing water of interest) to which is added further known amounts of added say Ca(OCl)2 in acidified (with CO2 or very dilute mineral acid) solution. See how accurate your electronic sensor is and by repeating with varying amounts, proceed to construct your own graph. Some graphical fitting/extrapolation likely needed. For comparative purposes to assess accuracy, you can test alternate procedures as well (my answer is one such selection). Dec 30 '20 at 14:39
• Be mindful that Cl2, HOCl and even added H2O2 are impacted, in the water of interest, by the presence of organic matter and transition metals. In fact, HOCl is around 1,000 times more chemical reactive in this regard than H2O2 (so, your existing water ferrous content, and its exposure to say iron or copper pipes, organics,..., is very important). Dec 30 '20 at 14:51