Which indicator is used to manually check the chlorine content in raw water before RO plant operation?

there is many indicators to check the chlorine content in water such as silver nitrate method , mercuric nitrate method , potentio metric method and ion chromatography .

N,N-diethyl-p-phenylenediamine(DPD).

Potential redox; addition to that and before feeding the RO train you have to netralise chlorine and making a PH adjustement

The indicator of choice is (DPD) N,N-Diethyl-p-phenylenediamine  Indicator Solution

the use of DPD 1 tablet and comparing with a control via color meter

The easiest and fastest way is to use the DPD indicator. If chlorine is present you will observe a pink coloration.

For instant checking of chlorine in field chlototex reagent is the best.

For Laboratory accurate checking DPD method with the hep of photometer is best.

And apart from above method one can always check the chlorine level by comparig it with ORP reading.

DPD = N,N-diethyl-p-phenylenediamine

Using ORP device Or DPD method 

using Oxidation-reduction potential online device ORP

DPD INDICATOR SOLUTION (N,N-Diethyl-p-phenylenediamine)