partsperthousand

Parts per thousand, abbreviated ppt, is a unit of concentration that expresses the ratio of a solute to one thousand parts of the total solution. It is a dimensionless quantity and is commonly used in chemistry, geology, environmental science, and oceanography. The symbol for the per‑thousand scale is the per‑mille sign, ‰, but many texts also write “ppt.” One ppt corresponds to 1/1000, i.e., 0.001 in decimal form. In percentage terms, 1 ppt equals 0.1%.

In practical terms, 1 g of solute per kilogram of solution is equal to 1 ppt (mass-based). For aqueous solutions with density close to that of water, 1 mg of solute per liter corresponds to about 1 ppm, which is 0.001 ppt. Consequently, 1 ppt equals about 1000 ppm, or 1,000,000 ppb (parts per billion).

Usage and examples: ppt is particularly common when expressing salinity in seawater (historical values around 35 ppt, equivalent to about 3.5% salt by mass) and in various geochemical and environmental measurements where concentrations are too large for ppm yet too small for percent. In isotope geochemistry, deviations from a standard are often reported in per mil (‰), which is the same scale as ppt but used for isotopic ratios rather than concentrations.

Measurement and notation: ppt can be defined by mass fractions or, in water measurements, by mass per volume when density is known. Techniques vary by context and may include gravimetric methods, titration, or spectrometric analyses. Care is needed when converting among ppt, ppm, and other concentration units due to density and matrix effects.