Conductivity, a physical concept, can also be called conductivity.This amount in the medium is equal toelectric field intensityThe product of E is equal to conductioncurrent densityJ。aboutIsotropic medium, conductivity is scalar;aboutAnisotropic mediumThe conductivity is a tensor.In ecology, conductivity is the ability of a solution to conduct electric current expressed in numbers.In Siemens per meter（S/m）Represents.[1]
Conductivity is used to describechargeParameter of flow difficulty.In the formula, the conductivity is written in Greek lettersσTo represent.conductivityσThe standard unit of isSiemens/m(abbreviated as S/m), is resistivityρThe reciprocal of, that isσ=1/ρ。
When the current of 1 ampere (1 A) passes through the cross section of the object and there is a voltage of 1 volt (1 V), the conductivity of the object is 1 S.Siemens is actually equivalent to 1 amp/volt.If G is conductance (in Siemens), I is current (in amperes) and U is voltage (in volts), then:
G = I/U
conductivityσThen it can be calculated by conductivity G, conductor cross-sectional area A, conductor length l:
σ = Gl/A[4]
Usually, when the voltage remains constantelectric currentIt is proportional to the conductivity.If the conductance is doubled, the current is also doubled;If the conductance decreases to 1/10 of its initial value, the current will also change to 1/10 of the original value.This rule also applies to many low frequency AC power systems, such as home circuits.In some AC circuits, especially inHigh frequency circuitThe situation becomes very complicated because the components in these systems store and release energy.
conductanceIt is also related to the resistance. If R is the resistance (in ohm Ω) of a component and equipment, and the conductance is G (in Siemens S), then: G=1/R[2]。
influence factor
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temperature
Conductivity is highly correlated with temperature.The conductivity of metal decreases with the increase of temperature.The conductivity of semiconductors increases with temperature.In a temperature range, the conductivity can be approximately proportional to the temperature.In order to compare the conductivity of substances at different temperatures, a common reference temperature must be set.The correlation between conductivity and temperature can often be expressed as the slope of conductivity to the upper temperature line graph.
Doping degree
The doping degree of solid semiconductor will cause great changes in conductivity.The conductivity will increase with increasing doping degree.The conductivity of the aqueous solution depends on the concentration of the solute salt contained in it, or it will be decomposed intoelectrolyteChemical impurities.The conductivity of water sample is an important indicator for measuring the salt content, ion content, impurity content, etc. of water.The purer the water, the lower the conductivity (the higher the resistivity).The conductivity of water is oftencoefficientTo record;The conductivity coefficient is the conductivity of water at 25 ° C.[3]
anisotropy
Some substances will haveanisotropyThe conductivity of (anisotropy) must be expressed by a 3 X 3 matrix (in mathematical terms, the second order tensor is usually symmetric).
measuring method
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The measurement of conductivity is usually the measurement of conductivity of solution.The resistivity of solid conductor can be determined byOhm's lawAnd resistance law measurement.electrolyte solutionThe conductivity is generally measured by the AC signal acting on the two electrode plates of the conductivity cellConductivity cell constantK and the conductivity G between the two electrode platesσ。
The AC bridge method was first used in conductivity measurement, which directly measured the conductivity value.The most commonly used instruments are constant regulator, temperature coefficient regulator and automatic temperature compensatorTemperature sensorComposition, can be measured directlyelectrolyte solutionConductivity.
measuring principle
The principle of conductivity measurement is to put two parallel plates (or cylindrical electrodes) with a fixed distance L into the solution to be measured, and add a certain amount ofpotential(In order to avoid solution electrolysis, it is usually a sine wave voltage with a frequency of 1~3 kHz).Then throughConductivity meterMeasure the conductivity between plates.
Conductivity measurement requires two aspects of information.One is the conductivity G of the solution, and the other is the conductivity cell constant Q.Conductivity can be obtained by measuring current and voltage.
The conductivity value can be obtained according to the relationship K=Q × G.This measuring principle is widely used in direct display measuring instruments.
And Q=L/A
A -- Effective plate area of measuring electrode
L - distance between two plates
This value is called the electrode constant.Between electrodesUniform electric fieldThe electrode constant can be calculated through the geometric dimension.When the two areas are 1 cmtwoThe constant Q of this electrode is 1 cm when the square electrode plate with an interval of 1 cm is used to form an electrode-1。If the conductivity value measured with this pair of electrodes is G=1000μS. Then the conductivity of the measured solution K=1000μS/ cm。
In general, the electrode often forms partial non-uniform electric field.At this time, the electrode constant must be determined with standard solution.The standard solution generally uses KCl solution because the conductivity of KCl is very stable and accurate under different temperatures and concentrations.The conductivity of 0.1 mol/L KCl solution at 25 ℃ is 12.88 mS/cm.
so-calledNon-uniform electric field(Also called stray field, leakage field) has no constant, but is related to the type and concentration of ions.Therefore, a pure stray field electrode is the most complex electrode, which can not meet the needs of a wide measurement range through a single calibration.
type
Conductivity electrodeIt is generally divided into two types: two electrode type and multi electrode type.
The two electrode conductivity electrode is the most widely used type of conductivity electrode in China. The structure of the experimental two electrode conductivity electrode is to sinter two platinum sheets on two parallel glass sheets or on the inner wall of a circular glass tube. By adjusting the area and distance of the platinum sheets, conductivity electrodes with different constant values can be made.Generally, there are K=1. K=5. K=10 and other types.andOnline conductivity meterThe two electrode conductivity electrode used on the is usually made of cylindrical symmetrical electrode.When K=1, graphite is usually used. When K=0.10.01, the material can be stainless steel or titanium alloy.
Multi electrode conductivity electrode, generally there are several annular electrodes on the support body, through the different combinations of the serial and parallel connection of the annular electrodes, the conductivity electrodes with different constants can be made.The annular electrode can be made of graphite, stainless steel, titanium alloy andPlatinum。
Conductivity electrode also has four electrode types andelectromagnetismType.The advantage of four electrode conductivity electrode is that it can avoid the measurement error caused by electrode polarization, and it is widely used in foreign experimental and online conductivity meters.Electromagnetic conductivity electrode is suitable for measuring solutions with high conductivity. It is generally used forIndustrial conductivity meterOr use its measuring principle to make a single component concentration meter, such as hydrochloric acid concentration meternitric acidConcentration meter, etc.
Conductivity electrode constant
According to the formula K=S/G, the electrode constant K can be obtained by measuring the conductivity G of the conductivity electrode in a certain concentration of KCl solution. At this time, the conductivity S of KCl solution is known.
Due to the different concentration and temperature of the measuring solution, as well as the different precision and frequency of the measuring instrument, the conductivity electrode constant K sometimes has a large error. After using for a period of time, the electrode constant may also change. Therefore, the electrode constant of the newly purchased conductivity electrode and the conductivity electrode after using for a period of time should be re measured and calibrated,Pay attention to the following points when measuring the conductivity electrode constant:
1. The matching conductivity meter shall be used for measurement, and other conductivity meters shall not be used.
2. Measure the temperature of KCL solution with electrode constant, which should be close to the temperature of the actual measured solution.
3. Measure the concentration of KCL solution with electrode constant, which should be close to the actual concentration of the measured solution.
temperature compensation
Conductivity measurements are temperature dependent.The influence of temperature on conductivity varies with solution, and can be calculated by the following formula:
Gt= Gtcal{1 + α(T-Tcal)}
Including:
Gt=Conductivity at a certain temperature (° C)
Gtcal=Conductivity at standard temperature (° C)
Tcal=Temperature correction value
α = Temperature coefficient of solution at standard temperature (° C).
The following table lists theαValue.To get other solutionsαValue, as long as the conductivity within a certain temperature range is measured and plotted with the temperature as the vertical axis
solution
（25°C）
concentration
Alpha
(α)
hydrochloric acid
10 wt%
one point five six
Potassium chloride solution
10 wt%
one point eight eight
sulphuric acid
50 wt%
one point nine three
Sodium chloride solution
10 wt%
two point one four
hydrofluoric acid
1.5 wt%
seven point two zero
nitric acid
31 wt%
thirty-one
Corresponding conductivity change curve, the curve point corresponding to the standard temperature isαValue.
All conductivity meters sold in the market can be adjusted or automatically compensated with reference to the standard temperature (usually 25 ° C).Most conductivity meters with fixed temperature compensationαAdjust it to 2%/° C (approximately 25 ° Cα）。The conductivity meter with adjustable temperature compensation canαAdjust it to be closer to the measured solutionα。
Conductivity reference
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The conductivity benchmark in China and many countries is a national sub benchmark, which is established by the relative measurement method.
Make a high purity of 99.99%potassium chlorideAs a conductivity reference material conforming to international recommendations, the reference solution prepared by it shall have the conductivity value recommended internationally.Take the conductivity of the solution at 25 ℃ as the starting point, measure each conductivity constant accordingly, and then calculate the conductivity constant K at other temperatures according to the following formula
K=Kzero(1-at)
Where, KzeroIs the conductivity cell constant at 0 ℃;A Glass used for making conductivity cellLinear expansion coefficient；T is the solution temperature, in ℃.
The above formula is an approximate derivation result, which can not exceed plus or minus 1xl0 at most when considering complex situations-5Differences.Then, the conductivity of each solution at different temperatures is calculated according to the measured resistance value of each solution on the corresponding conductivity cell at different temperatures.Because the temperature coefficient of relative change of conductivity cell constant is -8.49x10-6℃-1While the temperature coefficient of conductivity of KCl solution is about+2x10-2℃-one。Therefore, if the measured conductivity of 1D, 0.1D and 0.01D solutions at 18 ℃ and 20 ℃ is consistent with the internationally recommended values, it can be considered that this relative measurement method is reliable, which has been verified in the subsequent international sample comparison.The international recommended value of 20 ℃ is 1972 and 1976IUPACRecommended value.
