Total Dissolved Solids

Introduction

Solids are found in streams in two forms, suspended and dissolved. Suspended solids include silt, stirred-up bottom sediment, decaying plant matter, or sewage-treatment effluent. Suspended solids will not pass through a filter, whereas dissolved solids will. Dissolved solids in freshwater samples include soluble salts that yield ions such as sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), bicarbonate (HCO₃^–), sulfate (SO₄^2– ), or chloride (Cl^– ). Total dissolved solids, or TDS, can be determined by evaporating a pre-filtered sample to dryness, and then finding the mass of the dry residue per liter of sample. A second method uses a Vernier Conductivity Probe to determine the ability of the dissolved salts and their resulting ions in an unfiltered sample to conduct an electrical current. The conductivity is then converted to TDS. Either of these methods yields a TDS value in units of mg/L.

The TDS concentration in a body of water is affected by many different factors. A high concentration of dissolved ions is not, by itself, an indication that a stream is polluted or unhealthy. It is normal for streams to dissolve and accumulate fairly high concentrations of ions from the minerals in the rocks and soils over which they flow. If these deposits contain salts (sodium chloride or potassium chloride) or limestone (calcium carbonate), then significant concentrations of Na⁺, K⁺, Cl^– will result, as well as hard-water ions, such as Ca²⁺ and HCO₃^– from limestone.

Objectives

• Measure the total dissolved solids in a water sample from a stream or lake using a Conductivity Probe.