Conductivity, total dissolved solids (TDS) and salinity are commonly measured parameters in water and other liquid samples. Since all three are related to the presence of dissolved ions, they can sometimes seem interchangeable. However, each parameter provides a different perspective on the sample.

One important distinction is that conductivity is a directly measured parameter, whereas TDS and salinity are typically calculated or derived values based on conductivity measurements and conversion algorithms. Understanding these differences can help you determine which parameter is most relevant for your application and how to interpret the measurement correctly.

Conductivity measurement helps determine the concentration of dissolved ions in the sample.

What is conductivity?

Conductivity indicates how well a solution can conduct an electrical current. This ability is influenced by the concentration, charge and mobility, and temperature of the ions present in the sample.

Conductivity can also be described as the reciprocal of resistivity and is commonly expressed in µS/cm, mS/cm or S/cm. In laboratory applications, µS/cm and mS/cm are the most widely used units. Conductivity provides useful information about the overall ionic characteristics of a solution. Rather than identifying individual ions, it reflects their combined contribution to the electrical conductivity of the sample.

Conductivity can be particularly useful when:

• Monitoring water quality or purity, where changes in dissolved ionic impurities can result in measurable changes in conductivity. This makes conductivity a useful parameter for detecting variations in water and other low-conductivity samples.
• Checking consistency between samples or processes, as a change in conductivity may indicate a change in the ionic composition of the solution. This can support routine quality checks and process monitoring.
• Analyzing electrolyte solutions, where the presence and mobility of ions are important characteristics of the sample.

Temperature is an important consideration because conductivity changes with temperature. For this reason, conductivity measurements are often referenced to 25° C, and modern conductivity meters commonly use automatic temperature compensation (ATC) to improve comparability between measurements. Appropriate temperature measurement or compensation is therefore necessary when comparing conductivity results.

What is TDS?

While conductivity describes the electrical behavior of a solution, TDS represents an estimate of the amount of dissolved substances present in the sample. TDS is generally expressed in mg/L or ppm.

TDS can be derived from conductivity using a suitable conversion factor. Many dissolved substances form ions in solution, and these ions contribute to electrical conductivity. Conductivity can therefore provide a practical basis for estimating the overall concentration of dissolved solids.

However, it is important to remember that TDS derived from conductivity is an estimate rather than a direct measurement of every dissolved substance in the sample. Different ions have different charges and mobilities and therefore contribute differently to conductivity.

For this reason, two samples with similar conductivity values do not necessarily have exactly the same composition or concentration of dissolved substances.

TDS is particularly useful when the objective is to express the overall dissolved content of a sample in a familiar concentration format such as mg/L or ppm.

What is salinity?

Salinity is also closely related to conductivity, but it answers a more specific question: how much salt is present in the sample?

When salts dissolve in a solution, they dissociate into charged ions. These ions enable the solution to conduct electrical current, establishing the relationship between conductivity and salinity.

However, conductivity and salinity should not be interpreted as the same parameter. Conductivity indicates how effectively a sample conducts electrical current, whereas salinity uses conductivity measurements to estimate the salt content of a sample.

Like TDS, salinity is generally calculated from conductivity rather than measured directly. In many applications, salinity calculations are based on established conductivity-salinity relationships, such as those developed for seawater measurements.

Salinity is therefore particularly relevant when the salt concentration of the sample is the information of interest rather than its overall ionic behavior.

Which parameter should you choose?

Although conductivity, TDS and salinity are related, the appropriate parameter depends on the analytical question you want to answer.

• Choose conductivity when you want to understand the overall ionic behavior of the sample. It is suitable for applications such as monitoring water purity, detecting changes in ionic concentration, checking process consistency or characterizing electrolyte solutions.
• Choose TDS when you need an estimate of the overall dissolved-solids concentration. Expressing the result in mg/L or ppm can be useful when the amount of dissolved material is more relevant to the application than the electrical conductivity value itself.
• Choose salinity when the focus is specifically on salt content. It provides a result related to dissolved salts rather than simply reporting the electrical conductivity of the sample.

Selecting the parameter based on the information required from the sample helps ensure that the measurement result is relevant and meaningful.

Choosing the right sensor

Selecting the right parameter is only part of obtaining a reliable result. The conductivity sensor should also match the sample and its expected conductivity range.

Samples can range from low-conductivity pure water to highly conductive solutions and may differ considerably in chemical composition, sample volume and measurement conditions. Therefore, sensor selection should also account for factors such as sensor material, sensor geometry, cell constant, temperature measurement and chemical compatibility.

The conductivity cell constant (for example, K = 0.1, 1.0 or 10.0) is an important consideration because it influences measurement performance across different conductivity ranges.

For a deeper understanding of conductivity measurement, the Conductivity Measurement Theory Guide explains the underlying principles, key factors that influence measurement accuracy, and good practices for calibration and measurement, with step-by-step guidance to help achieve reliable results.

Download the Conductivity of Li-ion Battery Electrolyte application note to explore practical considerations for accurate and reliable conductivity measurement of li-ion battery electrolytes.