In various fields such as water quality monitoring, drinking water safety, and marine research, total dissolved solids (TDS), electrical conductivity (EC), and salinity are among the most commonly used water quality parameters. Although all three reflect the concentration of dissolved ions in water, they differ in meaning, calculation methods, and application scenarios. Understanding the relationship among them helps in selecting the right water quality sensors and scientifically analyzing water conditions.
This article provides a systematic overview of the definitions, measurement methods, conversion formulas, influencing factors, and characteristics of salinity, electrical conductivity, and TDS in different types of water. It also combines real-world examples and international standards to help readers fully grasp the relationship among these three key indicators.
What is salinity?
Salinity refers to the total amount of dissolved salts present in water. It is typically expressed in parts per thousand (ppt or ‰) or in a dimensionless unit known as the Practical Salinity Unit (PSU).
In the early days, salinity was determined based on chlorinity (Cl) – the concentration of chloride ions in seawater. The classic formula provided by the International Council for the Exploration of the Sea (ICES) is:
S (‰) = 1.80655 × Cl (‰)
Here, Cl represents the chlorinity, indicating the concentration of chloride ions, while the coefficient 1.80655 reflects the proportional relationship between chloride ions and other major ions in typical seawater, such as Na⁺, Mg²⁺, SO₄²⁻, Ca²⁺, and K⁺.
In modern oceanography, the Practical Salinity Scale (PSS-78) is more widely used. Instead of directly measuring chlorinity, it defines salinity based on the ratio of the electrical conductivity of seawater to that of a standard KCl (potassium chloride) solution. Through polynomial fitting, the salinity value can be accurately calculated from this conductivity ratio, providing a standardized and reliable measure of salinity.
What is electrical conductivity (EC)?
Electrical conductivity (EC) is a measure of a solution’s ability to conduct electric current. It is expressed in siemens per meter (S/m), though more commonly in microsiemens per centimeter (µS/cm) or millisiemens per centimeter (mS/cm).
A higher electrical conductivity indicates a greater concentration of charged ions in the water. EC is influenced by several factors, including ion concentration, ion valence, temperature, and solution viscosity.
To enable comparison between samples measured under different conditions, electrical conductivity is usually standardized to a reference temperature of 25°C, referred to as EC25.
What is total dissolved solids (TDS)?
Total Dissolved Solids (TDS) represent the total concentration of all dissolved inorganic salts and a small amount of organic matter in water. It excludes suspended particles and dissolved gases. The unit of measurement is typically milligrams per liter (mg/L) or parts per million (ppm). Common components of TDS include:
- Cations: calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺)
- Anions: carbonate (CO₃²⁻), sulfate (SO₄²⁻), chloride (Cl⁻), nitrate (NO₃⁻)
TDS can be measured directly using the gravimetric method (evaporating and weighing the residue). However, in routine water monitoring, it is usually estimated indirectly from electrical conductivity (EC) through conversion formulas.
The relationship between TDS, electrical conductivity, and salinity
Relationship between electrical conductivity and TDS
In most cases, there is a linear relationship between TDS and electrical conductivity. At 25°C, the conversion formula is:
TDS (mg/L) = K × EC₍₂₅₎ (µS/cm)
where EC₍₂₅₎ is the conductivity value standardized to 25°C, and K is the conversion factor, typically ranging from 0.5 to 0.9, with 0.65-0.7 being the most commonly used range. The exact value of K depends on the ionic composition of the water:
- For water dominated by sodium chloride (NaCl), K ≈ 0.50-0.55
- For natural freshwater, K ≈ 0.65-0.75
- For water rich in divalent ions such as Ca²⁺ and Mg²⁺, the coefficient tends to be higher
Some studies have proposed more detailed empirical relationships:
- When EC < 5 dS/m (i.e., < 5000 µS/cm): TDS (mg/L) ≈ 640 × EC (dS/m)
- When EC > 5 dS/m: TDS (mg/L) ≈ 800 × EC (dS/m)
For example: If the measured electrical conductivity is 1000 µS/cm, the corresponding TDS value can be estimated as 710 ppm, using a standard conversion factor of 0.71.
How to adjust the EC-to-TDS conversion factor based on different ion types
To adjust the conversion factor between electrical conductivity (EC) and total dissolved solids (TDS) according to the types of ions present in water, the following points should be considered:
- Different ion species contribute differently to conductivity. For example, divalent ions (such as Ca²⁺, Mg²⁺, and SO⁺²⁻) are more likely to form pairs than monovalent ions (such as Na²⁺, K²⁺, and Cl²⁻). These pairs may have lower or neutral charges, thus reducing the solution’s electrical conductivity (EC). Therefore, the relationship between EC and TDS varies in solutions with high concentrations of divalent ions.
- Temperature changes affect the dissociation pattern of ions, thereby affecting conductivity. EC is typically measured at 25°C to standardize for temperature effects, but temperature changes can cause conductivity changes, necessitating adjustments to the temperature compensation factor.
- Experimental data indicate that the relationship between TDS and EC can be described using polynomial and exponential fitting. The results of the polynomial fitting can be expressed as a general equation, where TDS is milligrams of total dissolved solids per liter, k is the conductivity in microsiemens per centimeter, and a, b, c, d, and e are coefficients. This approach allows for the conversion factors to be adjusted based on the specific ion species and concentration.
- A commonly used empirical conversion factor is 0.67, which best approximates the relationship between conductivity and total ion count in natural water. However, for certain water, such as highly concentrated sodium chloride or potassium chloride solutions, the conversion factor may need to be lowered to 0.55. Alternatively, a more appropriate conversion factor may be used depending on the sample’s specific conditions.
- The relationship between the two can be established by measuring the conductivity of the sample and using gravimetric methods, resulting in an accurate coefficient. Alternatively, chemical-thermodynamic computer codes (such as PHRREQC) can be used to calculate the conductive contribution of ionic species in water samples based on Kohlrausch’s law and the Debye-Hückel equation.
- In practical applications, the TDS value can be obtained by obtaining the water’s conductivity on-site, filtering, measuring, and evaporating the water, weighing the remaining solids, and dividing the weight by the amount of water. The TDS value is then divided by the water’s conductivity to obtain the conversion factor.
- Adjusting the conversion factor between conductivity and TDS requires comprehensive consideration of ion type, concentration, temperature, as well as specific experimental data and calculation methods.
Relationship between salinity and electrical conductivity
The relationship between salinity and electrical conductivity (EC) is complex, as different salts contribute differently to conductivity. In ocean monitoring, the Practical Salinity Scale (PSS-78) is generally used, which is defined by the formula:
S = f(R, T, P)
where:
R is the ratio of the seawater conductivity to that of a standard KCl solution,
T is the temperature (°C),
P is the pressure (dbar).
This formula is calculated through a polynomial expansion, allowing accurate determination of salinity across the global oceans under varying temperature and pressure conditions.
Relationship between TDS and salinity
TDS and salinity are not strictly equivalent:
- TDS (mg/L): Represents the mass concentration of all dissolved substances in water.
- Salinity (‰ or PSU): Emphasizes ionic strength and chemical composition rather than total mass.
In typical seawater, an approximate correspondence exists:
TDS (mg/L) ≈ 1000 × S (‰)
For example, seawater with a salinity of 35‰ corresponds to a TDS of about 35,000 mg/L. However, in non-marine water bodies, this relationship may not hold, as the ionic composition and proportion of dissolved substances differ significantly.
Effect of temperature on electrical conductivity (EC) and TDS measurements
1. Increase in electrical conductivity
As temperature rises, the viscosity of the solution decreases, enhancing ion mobility and thus increasing electrical conductivity (EC). For example, empirical data indicate that for most solutions, EC increases by approximately 2-3% per 1°C rise. In specific solutions like ultra-pure water, a 0.1°C temperature change can cause an EC variation of up to 0.55%.
2. Importance of temperature compensation
Due to the significant influence of temperature on EC, temperature compensation is necessary to ensure that measurements under different conditions are comparable. Measurements are typically referenced to 25°C, with compensation algorithms used to convert observed EC values to this standard temperature.
3. Increase in TDS
TDS (Total Dissolved Solids) is positively correlated with EC. As temperature increases, TDS also increases because higher temperatures enhance the solubility of salts and minerals. Studies have shown that a 1°C increase can significantly raise TDS, reflecting a higher concentration of dissolved solids.
4. Variation across materials and conditions
The effect of temperature may vary depending on the material type and experimental conditions. In some cases, increasing temperature may even lead to a decrease in EC, which can be attributed to the specific properties of certain substances or measurement conditions.
Salinity, electrical conductivity (EC), and TDS in different types of water
In different types of water, the relationship between salinity, electrical conductivity (EC), and total dissolved solids (TDS) varies significantly. These relationships depend not only on the type of water(such as freshwater, seawater, or groundwater), but also on seasonal changes, geographic location, and geological conditions.
1. Freshwater
- Salinity: Freshwater generally has very low salinity, typically below 0.5 ppt.
- Electrical Conductivity (EC): EC is relatively low, usually less than 1000 µS/cm.
- TDS: TDS values are also low, typically below 1000 mg/L.
2. Seawater
- Salinity: Seawater has high salinity, typically around 35 ppt.
- Electrical Conductivity (EC): Due to the high salt content, EC is significantly higher than in freshwater, usually 50-60 mS/cm.
- TDS: TDS in seawater is very high, generally above 35,000 mg/L.
3. Groundwater
- Salinity: Groundwater salinity varies widely depending on its recharge source and geological conditions. Coastal groundwater may have higher salinity due to seawater intrusion, while inland groundwater tends to have lower salinity.
- Electrical Conductivity (EC): EC also varies with salinity. Low-salinity groundwater exhibits lower EC, whereas high-salinity groundwater shows higher EC. EC can be used as an indirect indicator of salinity by measuring ion concentrations.
- TDS: TDS in groundwater is influenced by multiple factors. Low-salinity groundwater has low TDS, while high-salinity groundwater has higher TDS. For example, in the Niger Delta region, groundwater TDS ranges from 43.51-51.73 mg/L.
4. Surface water
- Salinity: Surface water salinity is strongly affected by seasonal variations and runoff. During the rainy season, large inflows of freshwater lower salinity, whereas in the dry season, seawater intrusion can increase salinity.
- Electrical Conductivity (EC): EC follows similar seasonal patterns, lower in the rainy season and higher in the dry season.
- TDS: TDS values are lower in the rainy season and higher in the dry season. For example, in coastal regions, surface water TDS ranges from 1,000-3,000 mg/L.
How to measure salinity, electrical conductivity (EC), and TDS
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