Total Dissolved Solids (TDS) represents the combined concentration of all inorganic and organic substances that are dissolved in water. These substances include minerals, salts, metals, and certain organic matter. The measurement of TDS is a primary indicator of general water quality because the dissolved content affects aesthetic properties like taste and color. While TDS itself is not typically a direct health hazard, a high reading can signal the presence of concerning components like harmful metals or an excess of benign minerals.
Measuring TDS Using Conductivity Meters
The most common and accessible method for determining the total dissolved solids level involves using a handheld TDS meter. This device does not directly measure the mass of the dissolved solids but instead measures the water’s Electrical Conductivity (EC). Dissolved inorganic salts and ions allow water to conduct an electric current; therefore, a higher concentration of these solids leads to higher conductivity. The meter then uses this conductivity reading to estimate the TDS concentration.
To perform this estimation, the meter applies a conversion factor, often referred to as the \(k_e\) factor, to the EC measurement. This factor typically ranges between 0.55 and 0.8, with many meters pre-set to a common value like 0.65 or 0.7. The resulting value is expressed in parts per million (ppm) or milligrams per liter (mg/L), which are functionally equivalent in water testing. Submerging the probe into a well-mixed water sample and allowing the reading to stabilize provides a quick, useful estimate of the TDS level.
The TDS reading from a conductivity meter is only an approximation. The true conversion factor depends on the specific chemical composition of the water, as different ions conduct electricity with varying efficiencies. For instance, water containing mostly sodium chloride may have a different optimal conversion factor than water high in calcium bicarbonate. Consequently, handheld meters are best used for monitoring relative changes in water quality rather than for obtaining a precise, regulatory-grade measurement.
Calculating TDS Through Gravimetric Analysis
For the most accurate and scientifically definitive calculation of total dissolved solids, regulatory and research laboratories rely on the gravimetric analysis method. This process directly measures the mass of the dissolved solids by evaporating the water sample. The first step involves filtering a known volume of the water sample, typically through a glass-fiber filter with a pore size of two micrometers or less, to remove all suspended solids.
The filtered liquid, called the filtrate, is then transferred to a clean, pre-weighed evaporating dish. The dish and its contents are placed into a convection oven, where the water is evaporated to dryness. The residue is typically dried at a temperature of 180°C to ensure all water is removed, and the dish is repeatedly weighed until a constant weight is achieved.
Once the constant weight of the dish and residue is determined, the mass of the dissolved solids is found by subtracting the original mass of the empty dish. The final calculation converts this mass into a concentration using the following formula: TDS (mg/L) = [(A – B) x 1,000] / Sample Volume (L). In this formula, A is the final weight of the dish and dried residue (in milligrams), and B is the initial weight of the empty dish (in milligrams).
Interpreting Total Dissolved Solids Results
The final TDS value provides context for the water’s overall quality. Generally, water with a TDS level below 300 mg/L is considered excellent, while levels between 300 and 600 mg/L are considered good or acceptable for drinking. The United States Environmental Protection Agency (EPA) established a secondary aesthetic standard of 500 mg/L for drinking water, primarily due to concerns about taste and potential scale buildup.
As the TDS level rises above 500 mg/L, the water is more likely to exhibit an unpleasant taste, often described as metallic or salty. High TDS levels, particularly those exceeding 1,000 mg/L, are associated with significant aesthetic issues and can lead to increased scaling and mineral deposits. These deposits occur on household fixtures, pipes, and water-using appliances, which can reduce their efficiency and lifespan.
TDS is a quantitative measure and does not identify the specific substances present. A high TDS reading indicates that further, more specific testing may be necessary to identify if the dissolved solids include harmful contaminants like heavy metals or nitrates. Conversely, very low TDS water, such as distilled water, can taste flat and may lack beneficial minerals like calcium and magnesium.