Monitoring chromium - Application

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	Monitoring in the Chromium Wastewater

Industry : Electrical and Electronics
Product : pH/ORP Meter

Introduction

Reduction Monitoring in the Chromium Wastewater Treatment Process

Wastewater from electroplating facilities and certain types of chemical plants contains toxic forms of hexavalent chromium such as chromate and dichromate. The hexavalent chromium in this wastewater must be reduced before the water can be discharged. As the oxidation-reduction potential (ORP) and the speed of the reduction reaction are closely tied to the pH value, ORP and pH meters are used to ensure proper control of the reduction process through such means as the injection of reducing agents.
Both 2-wire and 4-wire meters are suitable for this measurement system. The 2-wire PH202G transmitter can measure both ORP and pH. (Two transmitters are required if pH and OPR must be measured simultaneously.) A gold electrode should be used for ORP measurement of chromium wastewater.

Reducing trivalent chromium process

Expected Benefits

-	Measures pH/ORP of chromate wastewater continuously
-	Reduces operating costs

Process Overview

First, hexavalent chromium, either in the form of chromate or dichromate, is reduced to trivalent chromium. Then, it is converted to chromium hydroxide, which can be removed by precipitation. The reducing agent may be a substance such as ferrous sulfate, sodium metabisulfite, or sulfur dioxide.

Solution Details

Measurement system

2-wire/4-wire ORP measurement system
2-wire/4-wire pH measurement system

Sensor
	KCl refillable ORP sensor:	OR8EFG-AU-☐☐-TT1-☐*A
	KCl refillable ORP sensor:	PH8EFP-☐☐-TN-TT1-N-☐*A
	Submersion type holder:	PH8HS-PP-☐☐-T-NN-NN*A

Terminal box (when needed)
	WTB10-PH1	(for 2-wire system)
	PH8TBG*A	(for 4-wire pH measurement system)
	OR8TBG*A	(for 4-wire ORP measurement system)

Converter/transmitter:
	2-wire transmitter (pH/ORP):	PH202G-1-E
	4-wire pH converter:	PH400G-☐-EA*B
	4-wire ORP converter:	OR400G-☐-E-A
	Distributor (for 2-wire system):	PH201G- A☐*B

Measurement Conditions

1.	Check solutions for ORP meter
	Oxidation-reduction potential of check solutions
	(quinhydrone/iron solutions)
	A check solution is used to determine whether the ORP sensor is operating correctly. There are two types of check solutions: quinhydrone and iron.
		Quinhydrone solution:	approx. 200 to 300 mV
		Iron solution:	approx. 420 to 520 mV
	Unlike a pH standard solution, an ORP check solution does not always indicate a constant reference value. Whether the sensor is operating normally can be determined by seeing whether its readings are within the acceptable range.

	Preparation of a check solution (250 mL)
	Quinhydrone solution: Dissolve the following reagent in pure water and dilute to 250 mL.
		Quinhydrone salts:	P/N K9024EC
	Ferrous & ferric: dissolve the following reagent in a 2-mol/L sulfuric acid solution and dilute to 250 mL.
		Ferrous & ferric salts:	P/N K9024ED

2.	Use of a gold electrode for ORP measurement
	For ORP measurement of chromate wastewater, the use of a gold electrode is recommended.

Measurement conditions

Utilities

PH202G
	Power supply:	17 to 40 V DC (from distributor)
PH400G, OR400G
	Power supply:	88 to 132/176 to 264 V AC, 50/60 Hz
	Power consumption:	approx. 8.5 VA
PH201G distributor
	Power supply:	100 V: 20 to130 V DC/80 to 138 V AC, 47 to 63 Hz
	Power supply:	220 V: 120 to 340 V DC/138 to 264 V AC, 47 to 63 Hz
	Power consumption	24 V DC: approx. 200 mA
		100 V AC: approx. 7 VA
		220 V AC: approx. 11 VA