What are total dissolved solids and why should they be measured?

The term Total Dissolved Solids (or TDS) refers to the total amount of minerals, salts, and/or metals dissolved in water.

When drinking water has a high level of TDS, it will be unpleasant to drink; therefore, many countries have established a maximum recommended level for TDS in a drinking water. TDS is also used to monitor the quality of watershed source waters, such as rivers, lakes, and ponds. High TDS can indicate hard water, brackish or saline water, and/or nutrient loading of water. Hard water may be unsuitable for industrial, aquarium, spa, swimming pool, and reverse osmosis water treatment systems. Brackish or saline water may be unsuitable for agriculture, hydroponics, and aquaculture. Nutrient loading may compromise the health of a water body and impact its use as a potable water source.

TDS is commonly determined by gravimetry, chemical analysis, or conductivity.

Gravimetry – The gravimetry protocol requires that a volume of filtered sample be evaporated to dryness at about 100°C, then dried to a constant weight at 180°C.  The increase in dish weight represents the total dissolved solids per the filtered sample volume.

Chemical analysis – The chemical analysis protocol requires that the sample be measured for major ions (such as sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, and fluoride) and other parameters, such as nitrate and alkalinity. The results are used to calculate TDS.

Conductivity – The conductivity protocol requires only a conductivity measurement to be made. This measurement is multiplied by a factor following the simple formula, i.e., TDS = k EC (in 25°C) whereby k is a function of type of water being measured, and EC is the conductivity.

Of the three common TDS measurement protocols, only conductivity is suited for field testing and continuous monitoring. In addition, it’s a much quicker and simpler measurement, which requires little training for good results.

Using an Orion Star portable conductivity meters or a Versa Star Pro or Orion Star A bench meter combined with an Orion conductivity probe it’s simple to obtain a measurement of a sample's estimated TDS value in mg/L.  The meter automatically reads the conductivity and multiplies by the selected TDS factor.  Data can be automatically or manually stored in the meter memory for later download.