EUTECH INSTRUMENTS CYBERCOMM PRO FOR CYBERSCAN PH 2500 Dokumentacja Strona 2
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pH/ORP/Ion
About pH/ORP
Measurement
The movement of ions into the hydrated membrane changes the
electrochemical effect inside the glass which is measured in mV and
then converted via the pH meter to be reflected as a pH value.
Therefore depending on the concentration of ions in the solution, the
mV and hence pH varies.
The performance of an electrode is dependent on two parameters –
Offset and Slope.
Oset in pH Electrode
Theoretically, when placed in pH 7.00 buffer at 25 °C, a pH electrode
produces 0 mV which the pH meter reads as 7.00 pH. The difference
between 0 mV and the electrode’s actual reading is called the offset
error which can be as high as ±25 mV.
In other words, when the electrode is not in measurement or in pH 7
buffer solution, the output (or reading) will be known as the offset.
While in theory, the mV value should be zero, however in practice this
is rarely the case because of the following reasons:
• Liquiddierence
• Bulbcomposition
• Wiregeometrydierenceandotherfactors
In practice, it is unrealistic to achieve zero oset in electrodes.
The Eutech range of advanced micro-processor based
meters provides oset calibration abilities for consistent and
reliable measurements.
Slope in pH Electrode
A pH electrode produces different mV in different solutions. Therefore,
the slope of the electrode can be defined as
Slope = mV/pH unit
A perfect pH electrode, at 25 °C, produces a slope of 59.16 mV per pH
unit. For example, an electrode with 0 mV offset should read mV value
of 177.48 mV when placed in a pH 4.01 solution. The slope is hence
calculated as (177.48 mV - 0 mV) / 3 pH = 59.16 mV/pH. The difference
between this perfect slope reading and the electrode’s actual reading
is called the slope error.
These theoretical values are not always achieved, even with brand new
electrodes. The slope of a new pH electrode should fall between 92 %
and 102 % of 59.16 mV. If the slope falls below 92 %, cleaning of the
electrode may be needed.
The Ideal Electrode
An ideal electrode in theory has
the following properties:
Oset = 0 mV
Slope = 59.16 mV/pH @ 25 °C
▼
▼
mV
pH
0
7
ISO Potential Point
About pH Measurement
Why is pH Important?
pH is one of the most common parameters measured in a wide variety
of industries such as water and wastewater treatment, agriculture
research and production, environmental monitoring, chemical
and life sciences research, electronics production as well as other
industrial applications.
Here are examples of pH in a few common industrial and household
products:
pH Measurement
pH is always measured across a medium.
Although the litmus paper is one of the most common methods of pH
measurement, it can only provide a rough indication which might be
insufficient in most applications.
The more accurate method involves the use of a measurement system
that consists of a pH meter and a pH electrode that has a hydrogen ion
sensitive glass bulb. The most common sensing element used in the
electrode is the glass membrane as it is selective for H
+
ions i.e. H
+
ions
can permeate through the hydrated layer of glass membrane. However
the electrode body may not necessarily be glass.
Battery acid [0.3]
Vinegar [2.9]
Pickle processing [3.5]
Boric acid [5.0]
Corn [6.2]
Milk [6.7]
Fresh water aquarium [7.1]
Household dye [13.6]
[13.1] Bottle washing
Copper plating [12.8]
[12.6] Bleach
Ammonia [11.4]
[11.3] Brass plating
Milk of magnesia [10.3]
[9.4] Lime-soda softening
Borax [9.3]
[8.4] Baking soda
Saltwater aquarium [8.0]
[8.0] Seawater
Blood [7.5]
[7.4] Swimming pool water
[7.0] Distilled water
[6.3] Brewing process
[5.8] Nickel plating
[4.3] Orange juice
[3.2] Photo engraving
[2.3] Lemon juice
[5.0] Food processing
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About pH/ORP Measurement
page | 14
pH / ORP / Ion
- EUTECH 1
- INSTRUMENTS 1
- Why is pH Important? 2
- About ORP Measurement 3
- EcoTestr pH 2 5
- (Refer to page 106 for our 7
- ORP electrode selection) 7
- Pocket Testers 8
- page 9
- * CyberScan pH 110 only 12
- Ion 6+ ; pH 6+ ; pH 5+ 13
- Handheld Meters 14
- Specications 14
- Ordering Information 15
- page 15
- (software sold separately) 16
- Ion 2700 17
- Bench Meters 21
- page 22
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