EUTECH INSTRUMENTS CYBERCOMM PRO FOR CYBERSCAN PH 2500 Specifikace stáhnout pdf (Strana 3)

pH/ORP/Ion

About pH/ORP

Measurement

Temperature Compensation

In a perfect pH electrode – one that measures zero at exactly pH 7 – there

is no temperature eﬀect on the electrode sensitivity at pH 7 regardless

of temperature change. Most pH electrodes are not perfect, but the

errors from changes in temperature are still very minute when near pH

7, plus or minus one-tenths of a pH, and can be disregarded. However,

the further from pH 7 the solution is and the greater the temperature

changes, the greater the expected measurement error due to

changes in the electrode’s sensitivity. For most electrodes, the error

is approximately 0.003 pH/°C/pH away from pH 7.

For example, if a pH meter is calibrated at room temperature (25 °C)

and is measuring a sample around pH 4 at around 5 °C,

Temperature diﬀerence: 25 °C - 5 °C = 20 °C

pH away from neutral: 7 pH - 4 pH = 3 pH

Total error: 0.003 x 20 x 3 = 0.18 pH

To overcome this error, pH meters require some form of temperature

compensation to ensure standardized pH values. Meters and controllers

with Automatic Temperature Compensation (ATC) receive a continuous

signal from a temperature sensing element and automatically correct

the pH value based on the temperature of the solution. Manual

Temperature Compensation requires the user to enter the temperature

of the solution in order to correct pH readings for temperature. ATC is

considered to be more practical for most pH applications.

Most Eutech meters oer ATC capabilities. Models with this feature

include the pHTestr® 10, 20, 30 and all the handheld and bench

pH meters.

Single and Double Junction Electrodes

For many applications, a single junction reference electrode is

satisfactory. However, if samples contain proteins, sulﬁdes, heavy metals

or any other material which interacts with silver ions, unwanted side

reactions may occur. These reactions can lead to erroneous reference

signals or to precipitation at the reference junction leading to a short

service life.

A double junction reference design aﬀords a barrier of protection to

combat the above interactions. When in doubt about using single

or double junction designs, the safest approach is to use the double

junction as they can be used anywhere a single junction design can

be used. Conversely, single junction designs should not be used where

double junction designs are needed. In most process applications, it is

recommended to use double junction electrodes.

Eutech’s new range of large screen pocket testers pHTestr® series

feature double junction electrodes that extend useful life and

provide long term cost savings for users.

Normal Aging

As electrodes are used or stored for long periods they will experience

some deterioration in performance. Oﬀsets will change and slope

errors will increase. By using the calibration controls these errors can

be corrected. If an electrode is able to be calibrated and is stable and

responsive, it is still a functional electrode and may be used in service

even though it no longer meets “new” electrode speciﬁcations.

About ORP Measurement

ORP – Oxidation Reduction Potential

Oxidation-Reduction Potential (ORP) or Redox Potential measurements

are used to monitor chemical reactions, to quantify ion activity, or to

determine the oxidizing or reducing properties of a solution. ORP is a

measurement of the electrical potential of a redox reaction and serves

as a yardstick to judge how much oxidation or reduction takes place

under existing conditions.

ORP electrodes measure the voltage across a circuit formed by the

measuring metal half cell and the reference half cell. When the ORP

electrode is placed in the presence of oxidizing or reducing agents,

electrons are constantly transferred back and forth on its measuring

surface, generating a tiny voltage. The ORP measurement can be made

using the millivolt mode of a pH meter.

Major areas of usage include the treatment of industrial wastes, study of

biological systems, oxidation of cyanide, bleaching of pulp, manufacture

of bleach and reduction of chromate wastes.

The measurement of ORP is also useful in pool water treatment as

an indication of sanitation in relation to free chlorine parameter. ORP

technology has gained recognition worldwide and is found to be a

reliable indicator of bacteriological water quality. The table below

illustrates the Kill Time of E.Coli bacteria as a function of ORP value.

With a value of 600 mV, the life of the bacteria is almost 2 minutes; at

650 mV it reduces to 30 seconds. Above 700 mV the bacteria is killed

within a few seconds. It is therefore necessary for the water to have an

ORP value of at least 700 mV to ensure good water quality.

ORP value also depends on the pH of pool water. Normal values lie

between 7.2 and 7.6 pH with a tendency to increase to around 8.0 to

9.0 pH depending on the level of contamination. The pH of the pool

water has to be maintained at the optimum level between 7.2 and 7.6

pH by dosing appropriate chemicals. If the pH of the swimming pool

water is acceptable and the ORP value is below 700 mV, hypochlorite

or other oxidising chemicals should be added.

Eutech oers a wide range of meters that measure both pH and

ORP values in various educational, laboratory and industrial

applications. These include ORPTestr® 10 pocket tester, handheld

meters CyberScan Series pH 300, pH 310, pH 11 and pH 110,

pH 6+ and the CyberScan bench meter series 510, 1100, 2100, 1500

and the colour touchscreen research-grade series 6000.

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pH / ORP / Ion

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