pH Meter Working

IC Controls Quality Water Solutions for pHwww.iccontrols.com

R1.0 © 2004 IC CONTROLS

pH / ORPConductivityDissolved OxygenChlorineStandards

An Overview of

pH and ORP

CONTROLS




H2O

Water

H+ + OH- Hydrogen Ion Hydroxyl Ion




1 Mole = 6 x 1023

Hydrogen Ions Hydrogen Ions




Dissociation Constant of Water = 6 x 1016

1 Mole = 6 x 1023





H2OWater

6 x 1023

H+ + OH- Hydrogen Ion Hydroxyl Ion 6 x 1016 6 x 1016

Dissociation Constant of Water = 6 x 1016

1 Mole = 6 x 1023





Concentration of Hydrogen Ions

There are 6 x 1023 molecules in a mole of Water

Water Dissociates to produce 6 x 1016 molecules of Hydrogen Ions

Therefore the concentration of Hydrogen Ions per Mole of Water is:

6 x 1016 = 10-7 6 x 1023




Mathematical Definition

pH means pondus Hydrogenii or Hydrogen Exponent

pH = log10 1 = -log10 H+

H+ Therefore:

pH = -log10 (10-7) = 7




Logarithmic Nature of pHBecause the pH scale is a logarithmic the change in concentration is also logarithmic. This makes controlling pH in a process more difficult because of the large concentration difference between pH units as seen in the chart on the next slide. The concentration difference in one pH unit is a factor of 10 and 2 pH units a factor of 100. Therefore, when controlling pH it is suggested that one pump or valve can only control accurately to 3 pH units. 3 pH unit is a factor of 1000 and most pumps or valves do not exceed an accuracy of a 1000 to 1 injection capability.

pH pH ChangeConcentration

Change

6

7 1 10

8 2 100




pH H+ OH- pOH

0 100 1 0.00000000000001 10-14 14

1 10-1 0.1 0.0000000000001 10-13 13

2 10-2 0.01 0.000000000001 10-12 12

3 10-3 0.001 0.00000000001 10-11 11

4 10-4 0.0001 0.0000000001 10-10 10

5 10-5 0.00001 0.000000001 10-9 9

6 10-6 0.000001 0.00000001 10-8 8

7 10-7 0.0000001 0.0000001 10-7 7

8 10-8 0.00000001 0.000001 10-6 6

9 10-9 0.000000001 0.00001 10-5 5

10 10-10 0.0000000001 0.0001 10-4 4

11 10-11 0.00000000001 0.001 10-3 3

12 1012 0.000000000001 0.01 10-2 2

13 10-13 0.0000000000001 0.1 10-1 1

14 10-14 0.00000000000001 1 100 0

Hydrogen Ion Concentration




How a pH ElectrodeWorks




Glass Stem

Reference Wire

Internal Buffer Solution

Glass Membrane

The Glass Electrode




How Glass Electrodes Work

H+H+ H+

H+

H+

H+ H+

H+

When the glass is put into solution it undergoes a chemical reaction which forms a leached layer on the inside and the outside of the glass. The Hydrogen Ions cause a charge to be produced on the inside and outside on the glass. As the pH changes so does the charge differential between the inside and outside of the glass. This charge differential is a POTENTIAL difference which is read as millivolts. Thus, the change in Hydrogen Ion concentration will be measured as a millivoltage. Since the internal fill solution is pH 7, when a process is pH 7 the output from the electrode is ZERO millivolts because the charge on the inside is the same as on the outside and there is no potential difference.




Styles of Glass Electrodes

The glass membrane itself is not limited to any configuration or shape; its only requirement is contact with solution. Therefore, different styles of glass electrodes have evolved to maximize pH sensing ability and extend longevity in some of the more difficult applications.




Temperature Effect on GlassConductivity of the internal layer of the glass is a key factor in the responsiveness of the electrode. The more conductive the glass is, the quicker the potential difference will be transferred across the membrane which result in quicker pH response. Thicker, ruggedized styles of glass are slower to respond in ambient conditions because it takes longer to transfer the charge and set up the difference between the Leached Layers.

The conductivity, which is the reciprocal of resistivity, is highly temperature dependant as seen in the graph on the following slide.

Because of the temperature effect on the conductivity of the glass, a thinner glass with a low impedance (resistivity) is used in ambient conditions, and a thicker more rugged glass with a higher initial impedance can be used for high temperature applications.

Therefore, when selecting a glass electrode for industrial pH applications, the normal operating temperature must be considered to maximize response and longevity of the electrode.




Temperature vs Resistance

1

10

100

1000

10000

0 20 40 60 80 100

Temperature (Celcius)

Re

sis

tan

ce

(m

eg

oh

ms

)

From the above graph it is evident that as the temperature increases the conductivity of the glass also increase, which is why thick glass are sluggish at ambient temperatures but very responsive when the temperature increases.




The Reference Electrode

A pH electrode works like a battery with the glass being one half and the reference being the other half of the battery. The pH is proportional to the potential difference between the two sides of the glass. The reference is a constant millivoltage thus, giving the glass a reference point to distinguish changes in Hydrogen Ion concentration seen a potential across the Leached Layers.




The Reference Electrode

Silver/Silver ChlorideWire

Porous Frit

Solid Spool Piece

Porous ReferenceJunction

Potassium Chloride (KCl)Internal Fill Solution




The Reference Junction

The Silver/Silver Chloride reference has a porous junction between the KCl reference solution and the process. This porous junction adds a barrier t keep the reference internals and the process from readily mixing and contaminating the the reference, which would render the reference useless.

As with a glass there are different materials that can be used for the porous junction. However, in considering which junction is used for an electrode the chemical compatibility of the junction with the process must be considered




The Reference JunctionCeramic

•High Accuracy - Low Junction Potential•Prone to Clogging•Brittle

Wood•Resists Clogging•High Surface Area•Effective liquid transport across wood•Higher Junction Potential•Lower Chemical and Temperature Resistance

Porous Plastic•Good for high Temperature and high pH•High Surface Area •Higher Junction Potential



pH / ORPConductivityDissolved OxygenChlorineStandards What is the output from a pH electrode based on the potential difference

discussed? The relationship between pH and potential is millivolts (mV) and is a linear relationship as seen below.

At 25ºC an electrode the is 100% efficient will output 59.16 mV per pH unit

pH Electrode Response

millivolts per pH

-500-400-300-200-100

0100200300400500

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

pH

mil

livo

lts

(mV

)




The pH ScaleWhen discussing pH it is important to understand the relationships the are present in the pH scale which can become confusing.

pH is the concentration of Hydrogen Ions expressed in Logarithmic terms by a linear millivolt scale. However, understanding this relationship will help control pH in the plant very effectively.

Hydrogen Ion Concentration pH millivolts (mV)

10-6 6 59.16

10-7 7 0

10-8 8 -59.16




Slope and OffsetThe slope indicates how efficiently the sensor is responding and the offset corresponds to how close to theoretical perfect the probe is responding.

In general slope is an indicator of glass performance and offset is an indicator of reference performance.

A perfect electrode has a slope of 100%. If the slope is decreasing the pH electrode is not responding as efficiently as it should. In this case the potential difference between the inside and outside leached layers is not being transferred efficiently which is an indicator of poor glass performance.

A perfect electrode has an offset of 0 millivolts. If the offset is increasing it indicates that the reference point is shifting or being contaminated. Thus the offset is an indication of the health of the reference electrode. As the offset moves farther from 0 millivolts the reference is becoming more contaminated and the design of the reference should be investigated to see if the are ways to improve reference life. (Please see PowerPoint Presentation on Reference Contamination).

I




The Basics of pH

pH is the -log of Hydrogen Ion Concentration

A pH electrode works like a battery

Glass measures potential difference

Reference is the other half of the battery

Different Styles of Glass Available

Different Junction Materials Available

Temperature will affect electrode response

Slope is an indication of glass performance

Offset is an indication of reference contamination

Proven TechnologySuperior DesignBetter Results

pH Meter Working

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