Conductivity Meter Probe

Conductivity meters

For chemists, conductivity represents a fundamental parameter in general chemistry and the study of aqueous solutions in particular. In essence, a conductimetric probe, when immersed in a solution and connected to a "conductivity meter", provides the conductance [G]. The probe comprises two conductive plates and is typically manufactured from a non-corrodible conductive material, such as platinum.

A conductivity meter is, therefore, a device that is designed to measure resistance. This is achieved by measuring the intensity of the current passing through the solution when the electrodes are subjected to a voltage, and by applying Ohm's law (U = R.I). Alternatively, a conductivity meter may employ a different method that allows the measurement of a voltage proportional to the resistance of the solution.

The device, called a conductometer, is the electronic circuit but also the probe that will be used to measure the conductivity of the electrolyte solution.

Figure 1: Schematic of a conductometer

The conductance G depends on the surface area [S] of the electrodes, the distance [L] between them, and the chemistry of the surface. Each probe has a different coefficient [a] because the surfaces can change. It also depends on the conductivity of the solution [σ] :

The quantity G is expressed in Siemens (S), which is the inverse of the quantity Ohm. The expression is simplified by showing that G is proportional to the conductivity of the electrolytic solution [σ] according to a factor [k], which depends only on the probe. The quantity [σ] is expressed in Siemens per meters (S.m), and the expression for G is a function of the geometric characteristics of the probe and also the physical state and surface chemistry of the two electrodes. The constant [k] is of the order of magnitude from 10 to 1 cm.

Nevertheless, it is not possible to reduce a conductivity meter to a conventional multimeter with an Ohmmeter function. Indeed, an Ohmmeter applies a constant voltage between the electrodes, with the polarity always the same. The continuous voltage would cause the cations to migrate towards the negative plate and the anions towards the positive plate. This would result in a growing electric field that is antagonistic to the imposed field, leading to an increasing resistance that tends towards infinity. It is thus imperative to impose an alternating voltage of a rigorously zero-average value between the plates, with a specific frequency [F].

It is imperative that the frequency (F) is sufficiently high to prevent the phenomenon of ion migration from occurring during a half-period where the voltage is in the same direction. Furthermore, the frequency must not be excessively high to avoid disturbance by the parasitic capacitances of poorly conducting solutions, which will behave as a capacitor. Additionally, it is crucial to ensure that the electronic elements utilised in the conductivity meter are not adversely affected.

The most commonly accepted value falls within the range of 100 Hz to 1000 Hz.

Conductometry chemistry

The conductivity [σ] of an electrolytic solution is influenced by the chemical nature of the cations and anions in the solution, as well as their concentration [C]. The conductivity of an electrolytic solution is independent of the geometry or state of the electrodes of the conductimetry probe. This is because, as previously discussed, these parameters are incorporated into the probe constant [k], which must be determined through experimental means. Given that each cation and anion present in the aqueous solution contributes to the solution's conductivity [σ], we can consequently express this as follows:

This linear expression is only valid for low concentrations, that is to say, for anion and cation concentration values that are typically lower than 0.01 mol/l. The proportionality coefficient [λi] is the ionic molar conductivity, which is specific to the anions and cations under consideration. It should be noted that this ionic molar conductivity is also temperature dependent.

The [λi] are expressed in milliSiemens per square meter and per mole (mS m² mol⁻¹). The most well-known ions can be ranked with the values at 25°C according to the following table [[i]]

Figure 2: Molar conductivity of some ions

In the field of chemistry, two distinct measurement protocols are employed: relative measurement and absolute measurement. The relative measurement is by far the most prevalent. The method involves measuring the relative change in solution conductivity upon the introduction of a reagent, with the quantity of reagent added serving as the variable. Such assays include acid-base, complexation, precipitation, and flocculation assays for colloids, among others.

To illustrate, when a solution of hydrochloric acid is combined with sodium hydroxide, the following chemical reaction occurs:

H₃0⁺ + Cl^- + OH^- + Na⁺ → Cl^- + Na⁺ + 2 H₂O

The evolution of the conductivity of the solution during the introduction of the sodium hydroxide is illustrated in the following diagram:

Figure 3 : Relative evolution of conductivity during the induction of sodium hydroxide solution in a hydrochloric acid solution

The production of water and the disappearance of the H₃O⁺ cation and the OH^- anion decrease the conductivity of a solution because the production of water is no longer contributes to the overall conductivity of the solution. Therefore :

- Conductivity decreases as H₃O⁺ cations disappear before the equivalence point [E];

- At the equilibrium point [E], conductivity is minimal since there is no excess cation or anion;

- Beyond the equilibrium point [E], conductivity increases as we add excess OH^- anions.

The popularity of this protocol can be attributed to the fact that it is not concerned with the precise value being sought, but rather with the underlying trend. This may include identifying a minimum, discerning a change in slope, or other similar processes. In such cases, the calibration of a conductivity meter to a high degree of precision may not be a prerequisite.

In the case of absolute measurement, the objective is to achieve precise conductivity measurement. As has been demonstrated, this is a distinctive attribute inherent to the solution under examination. Consequently, this type of measurement necessitates the utilisation of previously calibrated conductivity meters and probes, for which the cell constant must be accurately defined.

Despite the high precision of the conductivity meter under construction, it is recommended that it be used exclusively for relative measurements, specifically during dosage. In this instance, the precision is not a result of the equipment itself, but rather the expertise of the chemist.

[i] Petr Vanýse, David R. Lide, CRC Handbook of Chemistry and Physics, 87th Edition, CRC Press, p. 5-78

In the manufacture of a two-electrode probe for a conductivity meter, the following materials are required:

Components for two-electrode probe production

1. The solvent glue or contact glue which exhibits a rapid setting time. The substance facilitates the adhesion of components while simultaneously ensuring the watertightness of the seal.
2. A DIN 72581 fuse. It is advised to employ a utilised fuse to refurbished it.
3. A previously used and now unusable pen or felt-tip pen may be employed. The tube will be retained to construct the body of the probe.
4. The probe head will be 3D-printed according to the dimensions of the blades of the fuse and the diameter of the felt-tip pen or pen.
5. Additionally, copper wires are required. The aforementioned cables are derived from a LAN cat5E cable, which is no longer in used due to its incompatibility with the current specifications.
6. The requisite materials include tin, a soldering iron, and the necessary connectors.

First, we need to choose a material for our electrode that is:

1. easy to get hold of;
2. made from a recycle item;
3. easy to shape or has standard shapes and sizes in order to prevent resizing;
4. has a good redox potential because it shouldn’t dissolve easily due to the voltage.

This final point is of great consequence. It is imperative to prevent the dissolution of the electrodes in aqueous solutions during the measurement process. In fact, platinum is employed for this purpose. The Pt²⁺/Pt(s) couple exhibits a standard potential of E° = + 1.2 V [[i]].

Zinc is an alternative metal for this purpose, given its favourable kinetic characteristics. The Zn²⁺/Zn(s) couple exhibits a standard potential of E° = −0.76 V [2], which is considerably lower than that of the H⁺/H₂(g) couple with a standard potential of E° = 0 V.

Figure 8 : Electrochemistry of Zinc, platinum and water. From Léo Fayard Thesis [[ii]]

Consequently, the theoretical reaction to be observed should be:

Zn(s) + 2 H₃O⁺ → Zn⁺² + H₂(g) + 2 + H₂0

Nevertheless, this reaction is not observed due to the presence of a considerable overvoltage (ηc = −0.80 V) during the reduction of H+ when electron transfer occurs on zinc. This overvoltage is indicative of significantly slow kinetic rates, which ultimately accounts for the absence of the chemical reaction.

In a practical sense, the decision was taken to use blade fuses, which are commonly employed in the automotive industry and which meet exacting standards (DIN 72581) in terms of both their dimensions and their surface chemistry, namely silver/zinc or tin/zinc alloy. Furthermore, the utilisation of refusbish fuse is recommended, as it is a valuable asset to this project [[iii]].

Figure 9 : DIN 72581 fuse

The surface chemistry precludes the possibility of tin soldering. However, the apertures on each of the blades will permit the attachment of a connection wire. The remaining requisite materials will be 3D printed in PLA, as the dimensions of the electrodes for the fabrication of the probe head and the body of the probe are already known. Alternatively, recycled materials may be employed for the latter component.

Figure 10 : The conductivity meter probe head is 3D printable using PLA. (a) Electrode housing. The distance between the two electrodes is 1 cm. The cable passage body housing is shown in (b), while the solute flow port is shown in (c).

[i] Libre Text Chemistry - Standard Reduction Potentials by Element

[ii] Léo Fayard ; « Étude d’un système d’électrolyse fractionnée basé sur l’électrochimie du zinc pour la production d’hydrogène: caractérisation et modélisation électrochimique: étude Technique, Economique et Environnementale », Thèse - 12 Jun 2024 – Web Hals

[iii]ATOF Series Blade Fuses specification. ATOF Web Site

The first step is to take the blades out of the fuse and connect them to the wires thank to the blade holes as below :

Figure 12 : DIN 72581 fuse connection

Prior to inserting the electrodes into the housings provided for this purpose in the probe's head, the connections are bonded and made watertight with a quick-setting solvent glue in order to prevent contact with the solution. An excess of glue is added in the cable passage in order to create a seal. The tube of the felt-tip pen or pen is then bonded into the housing in order to form the probe's body.

Figure 13:Housing of the blade and final conductometry probe

The assembly is completed with the appropriate connectors for your conductivity meter.

In application, the "probe constant" of the measuring chain can only be ascertained through calibration by submerging the probe in a standard solution of known conductivity at a specified temperature (for instance, an aqueous solution of sodium chloride, otherwise known as table salt). The actual cell constant of the probe is then determined by calculating the ratio of the displayed conductance value.

The second approach is to measure the conductivity of a solution of known nature and concentration and to compare this value to the theoretical value obtained by applying Kohlrausch's law. To provide an illustration, in the case of a saline solution, the conductivity of a sodium chloride solution with a concentration of c, where [Cl⁻] = [Na⁺] = [C], is equal to:

For a concentration of 8 10^-3mole/L of sodium chloride, the conductivity should be 1.01 mS/cm. The molar mass of sodium chloride (NaCl) is 58.44 g/mol. In order to achieve a solution with a concentration of 8 10^-3mole/L, it is necessary to dissolve 0.467 g of salt in one litre of water, or to make the requisite dilutions. In this instance, a solution of 23 grams of sodium chloride (table salt from the brand "La Baleine" with 97.7% sodium chloride) was prepared in 275 grams of water. A solution comprising 44 grams of the substance in question was diluted in 242 grams of water. Subsequently, 76 grams of the aforementioned solution were diluted in 224 grams of water.

Subsequently, known volumes of the solution were introduced to 141 grams of water, and the conductivity was measured using a conductivity meter constructed for this purpose (see step 4). The following values were obtained from the conductivity meter:

Figure 16 : Probe constant measurement

Despite the rudimentary nature of the electrode that has just been manufactured, a linearity that is consistent with Kohlrausch's law is observed, although it does not pass through the origin.

The rationale behind this shift can be attributed to the lower resistance exhibited by the electrode in comparison to that of water. The polymer employed (PLA; polylactic acid) is notably hydrophilic, which accounts for this low resistance. One potential avenue for further improvement would be to utilise a polymer with reduced hydrophilicity, such as ABS.

A comparison between the theoretical and actual values allows us to ascertain the electrode constant. The resulting value is 37 10^-3, with a shift (or onset) of 4.8 mS for 1 cm, which is linked to the conductivity of the electrode.

The electrode constant is then equal to :

G = 37 10^-3 s _theorical = k _electrodes _theorical

k _electrode = 37 cm

This value is somewhat higher than that of commercial electrodes, which typically range from 1 to 10 cm. Nevertheless, it is a valuable electrode for the measurement that will be carried out.

The resistance of the electrode when submerged in water is then:

R _electrode = 1/G_(C=0) = 1/4.8 10^-3 S

R _electrode = 1/G_(C=0) = 210 Ohm

This book is the first in a series of publications on the application of electronics and robotics to the manufacture of equipment for chemistry. It logically follows the previous 5 books on "Electronics for Chemists", which aim to introduce chemists and physicists to the design of laboratory equipment to create an autonomous and low-cost "ChemLab".

For chemists, conductivity represents a fundamental parameter in general chemistry and the study of aqueous solutions in particular. For electricians, a conductivity meter is a device that is designed to measure resistance. This is achieved by measuring the intensity of the current passing through the solution when the electrodes are subjected to a voltage, and by applying Ohm's law (U = R.I). This book therefore provides knowledge of the chemistry of conductimetry, explanations of the associated electronics, the components necessary to build a conductimeter, the circuits and programming codes, and also the 3D-printed accessories (probe) to have a complete and autonomous conductimeter.

This book then looks at measuring resistance with an alternating current and the effect of the frequency of the signal on the measurement. It also covers the use of astables, oscillator circuits and rectifiers to measure resistance in a complex system. The final measurement can be analogue, using a voltmeter, or digital, using a DAQ board. This book therefore deals with 4 different analogue voltmeters, depending on the oscillators and rectifiers used, and 2 different digital voltmeters with an Arduino card. From a pedagogical point of view, the book covers the basics of conductivity in chemistry, the effect of signal frequency on conductivity measurement, the robe design, the basics of astables, rectifiers, amplifying alternative signals, managing the clock and C++ language of an Arduino card.

Let's not forget that we cannot expect a chemist to become an expert in electronics and data processing ... However, we can educate him or her in the simple and basic principles. We invite chemists to participate in the open source movements to enrich himself and the community of "hackers" and "doers". In this sense, this book has been written in the hope that readers will fill in the missing pages!

The book : Gérard Bacquet- "Electronic for Chemists - The Conductivity Meter - SciencExpert Ed 2025