INDUSTRIAL ANALYTICAL INSTRUMENTS

BY: Al MarasiganBSEE-5A

IntroductionThe maintenance and servicing of

analytical instruments presents one of the major problems in industries. pH and conductivity measurement and control problems are very common in steam boiler plants, wastewater treatment facilities, and others. Frequently, pH control in sugar mills and refineries are perennial problems.

IntroductionIndustrial control problems of pH can

be minimized if not entirely eliminated by taking three important steps:

1. applying a basic knowledge of pH to the process.

2. proper design of the loop and selection of the right type and model of instruments for the particular application.

3. calibration and servicing at the proper intervals and maintaining them in tip-topcondition.

The Concept of pHTo a layman, pH is the measurement of a

liquid to find out whether it is acidic, neutral or alkaline. But technically speaking, it is the measure of the amount of dissociation(separation of chemicals into their associated ions) of ions in a liquid.

Mathematically,

Range of pH MeasurementThe pH scale is 0 to 14. Midway is 7 w/c

is neutrality, meaning, it is neither acidic nor basic(alkaline). From 7 down towards zero, represents increasing acidity, and from 7 to 14 represents increasing alkalinity. See table pH scale.

How pH Motor WorksThe pH of an aqueous is measured by

means of two pH electrodes: the glass or measuring electrode, and the reference w/c develops a reference voltage that is constant regardless of any changes in the solution characteristics. The electrodes generates a millivolt potential, which is proportional to the degree of ionization of the hydrogen ion in the solution being measured.

(Cont..)In most cases, a third electrode called the

temperature compensating electrode or thermo-c0mpensator is added. It is used to automatically compensate for errors due to temperature fluctuations.

Remember that with the same pH value, a variation of temperature produces a different millivolt value. This effect is separate from the fact that temperature hasa significant effect on the degree of ionization of a solution due to change in temperaaturre.

CalibrationIt is the major maintenance job for the technician.

Initial calibration of a pH meter should be done using a pH simulator or a millivolt source.

To calibrate the pH meter w/ its sensor, we use standard soluti0ons called buffers of known pH value. It is advisable to calibrate at two points. For ex., if the operating value where it is used acidic, then use a 6.86 pH NBS traceable buffer for the first point and a 4.0 pH for the second point. If the typical process is basic, then use 6.86 pH for the first point and a 9.18 pH for the second point.

The pH Scale

Practical Techniques1. Always use 2 sets of standards or buffer solutions

when calibrating your pH meter. The values should be one that is slightly lower than your normal operating value and the other slightly above.

2. If you use 1 standard solution., then you have not checked your span.

3. If you use 3 diff. solutions, you may not get a linear value and it will confuse you.

4. If you make any adjustment in your calibrate pot, then do it only in one value of the standard, either the low or the high.

Practical Techniques5. If unable to calibrate properly because of the

first problems, then check the electronics portion of your pH meter. Either the electrodes are weak or the electronic circuits need re-calibration.

pH SimulatorA high voltage source specifically

incremented to follow the logarithmic pH scale.

Electrolyte ConductivityMeasure of the ability of a solution to carry

an electrical current. This ability to conduct electricity is the reciprocal of the resistance to the flow of electricity and is called conductance. The unit in which it is measured is the reciprocal ohm, (1/ohm) commonly called mho; the symbol is inverted ohm

Conductivity of Solutions1. Ultra Pure Water-------- 0.05 µmho/cm or

µSiemens2. Distilled Water---------- 1 µmho/cm or

µSiemens3. Cooling Water----------- 2 µmho/cm or

µSiemens4. Rain water--------------- 50 µmho/cm or

µSiemens5. 0.05% Salt Solution---- 1000 µmho/cm or

µSiemens6. Sea Water--------------- 20000 µmho/cm or

µSiemens7. 30% Sulfuric Acid------ 1 mho or 1

Siemens

Practical Applications/Uses1. The concentration of sodium chloride, or

sodium hydroxide, or sulfuric acid in simple water solutions are determined by conductivity measurements.