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Guide to On-line Conductivity Measurement - 48 Pages

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Guide to On-line Conductivity Measurement

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Conductivity Guide School experiments METTLER TOLEDO Process Analytics On-line process and pure water systems A Guide to On-line Conductivity Measurement Natural science laws experience “live” – Theory learn easily and Pract

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2.7.4    Geometry Related Errors - Field Effects    23

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3.6.2    Power Industry Cycle Chemistry    35 3.6.3 Industrial Boiler Neutralized Conductivity Measurement    36

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1. Introduction Electrical conductivity has been measured for more than 100 years and it is still an important and widely used analytical parameter today. The high reliability, sensitivity, fast response, and relatively low cost of the equipment make conductivity a valuable, easy-to-use tool for quality control. In some applications, the same measurement is made as resistivity which is the inverse of conductivity and with reciprocal units. Electrical conductivity is a non-specific sum parameter, responding to all dissolved ionic materials (salts, acids, bases, and some organic substances)...

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2. Theory 2.1 Electrical Conductivity – Basic Information Electrical conductivity is the ability of a material to carry an electrical current. The term conductivity can also be used in other contexts (e.g., thermal conductivity). In this guide the term “conductivity” always refers to electrical conductivity. The transport of electricity through matter always requires the presence of charged particles. Conductors can be classified into two main groups based on the nature of the charged particle. Conductors in the first group (primarily metals) consist of a lattice of atoms with an outer...

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Formeln Conductivity Guide: To measure the resistance or the conductance of a sample, a measuring 1. V is required. The measuring cell consists of at least two electrically cell R  I conductive electrodes (sometimes called poles) with opposite charge. The electrodes together form a measuring cell. The cell and the support V 2. body together make up the sensor. R Formeln Conductivity Guide: The conductance measured depends also on the geometry of the mea1 suring cell, which is described by the cell constant (K). This is the ratio 3. V  R  I 1. G of theR distance between electrodes (l)...

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Theory 2.3.1 Dissolved Ions The dissolution of materials that form ions is called electrolytic dissociation. Some examples of electrolytic dissociation are as follows: NaCl ➝ Na+ + ClHCl ➝ H+ + ClFormeln Conductivity Guide: CH3COOH CH3COO- + H+ 1. V  R  I There is a distinction between strong and weak electrolytes. Strong electrolytes like sodium chloride and hydrochloric acid dissociate comV 2. pletely to form sodium and chloride ions or protons and chloride ions. R I In contrast, weak electrolytes like acetic acid dissociate only partially, symbolized by the two-way arrow above. This...

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Table 1: Equivalent conductance of different ion types The equivalent conductance generally increases with the charge number and decreases with the size. However, the predicted values can differ from the actual measured values listed in Table 1. For example, the small lithium ion is much less conductive than the larger ammonium ion. This is because the size of the solvated ion, and not the size of the ion itself, is the determining factor in mobility. A smaller, highly charged ion like lithium can attract more water molecules and build up a bigger solvation shell than a bigger and less...

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Temperature also influences the equivalent conductance. Higher temperature increases the motion of the particles and lowers the viscosity of the solvent. This leads to increased mobility of ions and to higher conductivity. Because viscosity is a property of the solvent, the mobility of all ions tends to have the same increase with temperature which simplifies temperature compensation for general purpose measurements. However, in the case of weak electrolytes, a higher temperature may also change the amount of dissociation and therefore influence the conductivity further. This complicates...

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A conductivity measuring cell consists of an electrode pair, to which a voltage is applied. The instrument measures the flowing current and calculates the conductivity (see section 2.2). However, this is an oversimplified explanation of the measuring principle. Figure 2: Schematic set-up of a conductivity measuring cell When a direct current (DC) is applied to the electrodes, the positively charged ions (cations) migrate toward the negatively charged electrode (cathode) and the negatively charged ions (anions) move toward the positively charged electrode (anode) (see Figure 2). This can...

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Even when using AC the accumulation of ions is not fully eliminated. All effects which occur at the boundary surface of the electrodes and the solution when a current is applied are summarized as polarization. The main effect is the formation of a double layer of ions that impedes the mobility of the ions near the surface. This layer has the same effect as contamination of the electrode surface and causes an additional resistance. Polarization negatively influences the measurement of samples with moderate to high electrolyte concentration and reduces the linearity at the upper end of the...

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rises correspondingly. Even more important in process measurements, a long cable connecting the sensor to the measuring instrument can add very significant capacitance. The influence of capacitances can be reduced or prevented by: • djusting the measuring frequency: The lower the measuring frequenA cy, the lower the impact of capacitance. • sing a conductivity cell with low capacitance: The smaller the elecU trodes’ area and the greater the distance (l) between the electrodes, the lower the capacitance. However, this is not a viable option because for measurements in a low conductivity...

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Theory 2.5 Conductivity Sensor Types Given the diversity of applications, it is not surprising that there is no single measurement technology which is ideal for every situation. The following three technologies are used for process conductivity measurement: • 2-electrode conductivity cell • 4-electrode conductivity cell • Inductive conductivity cell Classical 2-electrode conductivity cells consist of two parallel plates. The plates are supported by a structural insulator(s) which protects them from mechanical damage and reduces the errors caused by field effects. Early designs used platinum...

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