pH. A measure of the acidity or alkalinity of a solution. A value of seven is neutral; low
numbers are acid, large numbers are alkaline. Strictly speaking, pH is the negative
logarithm of the hydrogen-ion concentration.
Photometer. An instrument that measures the intensity of light or the degree of light
Photosynthesis. The synthesis of complex organic materials, especially
carbonhydrates, from carbon dioxide, water, and inorganic salts with sunlight as the
source of energy and with the aid of a catalyst such as chlorophyll.
Picocurie (pCi). That quantify of radioactive material producing 2.22 nuclear
transformations per minute.
Piezometer. An instrument for measuring pressure head in a conduit, tank, soil, etc. It
usually consists of a small pipe or the tube tapped into the side of the container,
connected with a manometer pressure gage, mercury or water column, or other device
for indicating pressure head.
Piezometric Surface. An imaginary surface that everywhere coincides with the static
level of the water in an aquifer. If at any given place the water from different depths in
the aquifer rises to different levels, the aquifer has more than one piezometeric surface.
Pitometer. A device that uses the principle of the pitot tube for determining the velocity
of flowing water in closed conduits, usually flowing under pressure.
Pitot Tube. A device for measuring the velocity of flowing water, using the velocity
head of the stream as an index of velocity. It consists essentially of an orifice held to
point upstream, connected with a tube in which the rise of water due to velocity head
may be observed and measured. It may also consist or two orifices or openings, one
facing upstream, one downstream, each connected to tubes (or water columns), in
which case the difference in height, or the water differential, in the two columns is an
index of velocity.
Pollution. The addition of sewage, industrial wastes, or other harmful or objectionable
material to water. A general term that does not necessarily signify the presence of
Potable. Water which does not contain any objectionable substances or pollution, and
is satisfactory for human consumption.
Precipitate. To separate a substance, in the solid form, from a solution. The substance
in solid form which has been separated out.