Portable Conductivity Meter ES

Technical note

■

Technical note

●

Conductivity measurement

Conductivity is an index that expresses the ease with which electric current flows through a material. Conductors are categorized either as electron conductors (such as metals and other substances which use free electrons to conduct electricity) or ion conductors

(such as electrolytic solution or fused salt, which use ions to conduct electricity). This section describes the kind of conductivity that pertains to ions, especially the conductivity of electrolytic solution that uses water as the solvent. As shown in figure, two pole plates with an area "a" are positioned parallel to each other, separated by distance "l". Then solution is poured into the cell until it is full and alternating current is run between the plates. Each positive and negative ion in the solution will migrate toward the oppositely charged pole. The result is that current flows through the solution by means of ion conductivity. When this occurs, resistance "R" is in inverse proportion to the area "a" of the pole plates, as is the case with metal and other conductors, and is proportional to the distance "l" between the two pole plates. These relationships are expressed by equation ①

, below.

............................................................. ①

R: Resistance (Ω), r: Resistivity (Ω·m), a: Pole plate area (m

2

), l: Distance between pole plates (m), J: Cell constant (m

-1

)

Resistivity is an index that indicates the difficulty with which current flows and is a constant determined according to the solution. The inverse of "r", which is "L" (and is equal to 1/r), is called the specific conductivity and is widely used as an index to express the ease with which current flows. Specific conductivity "L" is generally referred to as simply conductivity and is expressed in units of S/m.

Inserting conductivity "L" into equation ① results in equation ② , below.

............................................................................... ②

60 HORIBA

Technical note

As is clear from equation ② , when a conductivity cell having a cell constant "J" of 1 m

-1 is used (in other words, when a conductivity cell having two pole plates that each have an area "a" of 1 m

2

and are positioned parallel to each other such that the distance "l" between the two plates is 1 m is used) the inverse of the resistance "R" of the solution between both pole plates is the conductivity. Conductivity is defined in this way, but it changes according to the temperature of the solution. The conductivity of a solution is generally expressed as the value when the solution is 25°C.

Solution

Polar plates a m

2

x2

ES-71 61