pH MEASUREMENT &ROSEMOUNT 5081-P-HTROSEMOUNT 5081-P-HT

Prasoon Prabhakar

HISTORY OF pH MEASUREMENT Its all began with food Tasting is the only way to know the food or drinks Potentiometry : Is to measure (meter) the voltage (potential)

caused by the Hydronium ion: H3O+ Potentiometry gave accuracy, reliability and faster results Potentiometry gave accuracy, reliability and faster results

than the taste of any human being p Scale : p stands for ‘POTENZ=Power’. pH,pNa,pOH,pCl

Scales Soren Peder Lauritz Sorenson, a Danish scientist introduced

pH term and pH scale in 1909

HISTORY OF pH MEASUREMENT The term pH simply stands for the negative logarithm of the

hydronium ion concentration pH is concerned with the acidity or alkalinity of a solution An acid dissociates into a positive hydrogen ion (H+) and a

negative anion. An alkali dissociates into a positive cation andnegative anion. An alkali dissociates into a positive cation anda negative hydroxyl ion (OH–). Pure water is unique in that itcontains both hydrogen (H+) and hydroxyl (OH–) ions inequal proportions

pH Scale: 0 to 14 with the acid region ranging from 0 to 7 pH,the alkali region from 7 to 14 pH, and neutrality at 7 pH

pH= -log10 [H3O+]HISTORY OF pH MEASUREMENT

HISTORY OF pH MEASUREMENT

BASIC pH MEASURING SYSTEM A pH meter (to accurately measure and transform the voltagecaused by hydronium ion into a pH value) A pH electrode (to sense all the hydronium ions and to producea potential) A reference electrode (to give

a constant potential no matterwhat the concentration of ourhydronium ion is).

MODERN pH SENSORS Modern sensors includes a

measurement and a reference sensor in a single probe

NERNST EQUATION• Derived by Walther Nernst, 1889, related electrical voltage to ion

concentration

U = Potential measured between indicator and reference electrode U0 = Standard potential of the electrode assembly, depends on its construction

(well made electrodes give 0 mV with pH 7 – neutral solution) R = gas constant (8.31441 J K-1 mol-1) T = absolute temperature in K (273.15 + t in °C) zi = charge of the hydronium ion (1+) (each ion carries a single positive charge

with it) F = 9.648 533 99×104 C mol−1 , Faradae’s Constant

mV = 0.1985 x (°C + 273) x pH

ROSEMOUNT 5081-P-HTAmbient Temperature: -4 to 149°F (-20 to 65°C)Analog Output: Two-wire, 4-20 mA output Superimposed with HART Signal pH Range: 0 to 14 pH Range: 0 to 14Automatic Temperature Compensation: External 3 or

4 wire Pt 100 RTd or Pt 1000 RTd located in thesensor, compensates the pH reading for temperaturefluctuations.

Automatically Slope Adjustment

ROSEMOUNT 5081-P-HT

ROSEMOUNT 5081-P-HT

ROSEMOUNT 5081-P-HT• How to check the transmitter functionality?

ROSEMOUNT 5081-P-HT• How to check the sensor functionality?

396P SERIES Used in 75-AT-6570C Available in Standard Hemi and Flat, We are using flat sensor. Specifically designed for improved life in harsh, dirty, and abrasive

applications such as lime slurry, waste treatment, paper machine headbox,and pigment/dye applications, where large quantities of suspended solidsare present.

Normal Cases RTD sensor is Pt100 Normal Cases RTD sensor is Pt100

3300,3500 SERIES We are using 3300 series in 75-AT-6038, 73-AT-5401, 45-AT-3726 We are using 3500 series in 73-AT-4039, 75-AT-7174_B1 Both 3300 and 3500 are using in high temperature applications 3300 can be used 50C-850C and 3500 can be used 50C-1200C

399 SERIES We are using 399 series in 59-AT-3726, 78-AT-4757A, 75-AT-3226, 75-AT-

4070, 75-AT-4270, 75-AT-4870 399 Series normally use to measure pH in pipelines, open tanks, or ponds.

3900 SERIES3900 SERIES Most used sensor in our plant General purpose pH sensor It responds to changes in pH at a minimum conductivity of 0.1mS/cm in

deionised water Sample flow rate must be controlled to 7.6L/hr

WIRING DETAILS TO 5081

CALIBRATION OF pH SENSOR The first buffer should be pH 7.00 (zero point adjustment) and the second

buffer should be near the expected sample pH (pH 4 or pH 9.21). If you willmeasure both acidic and basic samples, and only perform one pH calibration,you should use a multi-point calibration

Before starting calibration, be sure the sensor and the buffer solution are at thesame temperature. If not, allow time for temperature equilibration

Pour the necessary amount of buffer solutions into individual glass beakers.Buffer solutions will remain stable in a glass beaker for a maximum of 2 hours.Close the buffer containers promptly to avoid carbon dioxide absorption. Donot pour used buffer back into the storage container. Discard it.

CALIBRATION OF pH SENSOR Place the electrode into the first buffer. When the reading is stable, set the

pH meter to the pH value of the first buffer at the measured temperature Between buffers, rinse the electrode with distilled water and then with the

next buffer. If you, for some reason, not want to rinse the pH electrode withnext buffer. If you, for some reason, not want to rinse the pH electrode withthe next buffer; Rinse the electrode with distilled water and gently blot itdry with a lint-free tissue. Avoid rubbing or wiping the electrode bulb

Repeat calibration with next buffer When the pH meter calibration is done, rinse the electrode and place into

the sample and make the online pH measurement

CONCEPT OF SLOPE• Slope is related to a mathematical definition of the slope of the line.

Slope = ΔY/ΔX.• It is actually the change of gain• In case of an instrument slope can be define as ΔmV/ ΔRange(URV-LRV)• For example, suppose an instrument with a range of 0-100°C monitor with

a 4-20ma current loop. The 4ma corresponds to 0°C, and the 20macorresponds to 100°C. The slope is (20-4)/(100-0) = 0.16corresponds to 100°C. The slope is (20-4)/(100-0) = 0.16

• For a pH meter slope is theoretically 59.157 mV/decade change ofHydrogen ion activity, which came from Nernst Equation

CONCEPT OF SLOPE• Nernst equation can be rewritten as

• A new electrode should have a slope between 95-102% of theoretical

• As the pH membrane ages, the slope decreases. This affects

mV = 0.1985 x (°C + 273) x pH

• As the pH membrane ages, the slope decreases. This affects the accuracy

CONCEPT OF SLOPE• Modern transmitters calculate the slope using the

equation, which is stored in their microprocessor• E = A - B (t + 273.15) (pH - 7)

HOW A MODERN pH SENSOR WORKS?

https://www.youtube.com/watch?v=P1wRXTl2I

Courtesy : Endress+Hauser Youtube Page

REFERENCE• Emerson- Models 3300, 3400 and 3500, Instruction Sheet, PN 51A-PERpH-

X/revG, January 2011• Emerson- PN 44-6033/rev. D, December 2010, Theory and Practice of pH

Measurement• 396P + 396PVP Sensors Instruction Manual, LIQ_MAN_396P_396PVP, December

2014• Instruction Sheet General Purpose pH/ORP Sensor Sensors ,

LIQ_MAN_ABR_3900, October 2014• Temperature – Another Wild Card in pH Control, Technical Paper T-P95-T1,

Don Spriggs, Marketing Manager, ABB Automation• The background of pH measurement and hints for your daily work, Copyright

1999 Metrohm Ltd, Author: il@metrohm.ch• Nernst equation, From Wikipedia, the free encyclopedia• http://www.ionode.com/theory/phtheory• http://www.allaboutph.com/ph4.html