Transcript Document
pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
CONTROLS
An Overview of pH and ORP
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IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
H
2
O
Water
H +
Hydrogen Ion
+ OH -
Hydroxyl Ion www.iccontrols.com
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
1 Mole = 6 x 10
23
Hydrogen Ions Hydrogen Ions
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IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
1 Mole = 6 x 10
23
Hydrogen Ions Hydrogen Ions Dissociation Constant of Water = 6 x 10
16 www.iccontrols.com
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
1 Mole = 6 x 10
23
Hydrogen Ions Hydrogen Ions Dissociation Constant of Water = 6 x 10
16 H
2
O
Water 6 x 10
23
H +
Hydrogen Ion
6 x 10
16
+ OH -
Hydroxyl Ion
6 x 10
16 www.iccontrols.com
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
Concentration of Hydrogen Ions
There are 6 x 10 23 molecules in a mole of Water Water Dissociates to produce 6 x 10 16 molecules of Hydrogen Ions Therefore the concentration of Hydrogen Ions per Mole of Water is:
6 x 10
16 =
10
-7
6 x 10
23 www.iccontrols.com
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
Mathematical Definition pH = log 10 1 = -log 10 H +
Therefore:
H + pH = -log 10 (10 -7 ) = 7
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pH means pondus Hydrogenii or Hydrogen Exponent
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
Logarithmic Nature of pH
Because 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 Change Concentration Change pH 6 7 8 1 2 10 100
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IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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Hydrogen Ion Concentration
pH
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10 10 -11 10 12 10 -13 10 -14
H+
1 0.1
0.01
0.001
0.0001
0.00001
0.000001
0.0000001
0.00000001
0.000000001
0.0000000001
0.00000000001
0.000000000001
0.0000000000001
0.00000000000001
OH-
0.00000000000001
0.0000000000001
0.000000000001
0.00000000001
0.0000000001
0.000000001
0.00000001
0.0000001
0.000001
0.00001
0.0001
0.001
0.01
0.1
1 10 -14 10 -13 10 -12 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0
pOH
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
How a pH Electrode Works
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IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
The Glass Electrode
Glass Stem Reference Wire www.iccontrols.com
Internal Buffer Solution Glass Membrane
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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.
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IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
Styles of Glass Electrodes
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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.
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
Temperature Effect on Glass
Conductivity 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.
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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Temperature vs Resistance 10000 1000 100 10 1 0 20 40 60 Temperature (Celcius) 80 100
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.
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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.
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IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
The Reference Electrode
Silver/Silver Chloride Wire Potassium Chloride (KCl) Internal Fill Solution Porous Frit Solid Spool Piece Porous Reference Junction www.iccontrols.com
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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 www.iccontrols.com
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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The Reference Junction
Ceramic
• 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
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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pH Electrode Response
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 millivolts per pH
500 400 300 200 100 0 -100 -200 -300 -400 -500 0 1 2 3 4 5 6 7 pH 8 9 10 11 12 13 14 IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
The pH Scale
When 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 10 -6 10 -7 10 -8 pH 6 7 8 millivolts (mV) 59.16
0 -59.16
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IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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Slope and Offset
The 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.
I 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).
IC Controls Quality Water Solutions for pH
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pH / ORP Conductivity Dissolved Oxygen Chlorine Standards
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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
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IC Controls Quality Water Solutions for pH
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