nanometer- Abbreviated "nm", a unit of length equal to one thousandth of a micrometer. Often used to express the wavelength of ultraviolet light and the colors of visible light in colorimetric analytical procedures.

negative charge - The electrical charge on an electrode or ion in solution, due to the presence of an excess of electrons. (See electron, anion.)

negative head - A condition of negative pressure or partial vacuum.

negative pressure - A pressure below that of the surrounding atomspheric pressure at a specific point; a partial vacuum.

nephelometric turbidity unit- An arbitrary unit of measuring the turbidity in water by the light scattering effect of fine suspended particles in a light beam (contrast to "Jackson Turbidity Unit").

neutral - In electrical systems, the term used to indicate neither an excess nor a lack of electrons; a condition of balance between positive and negative charges. In chemistry, the term used to indicate a balance between acids and bases; the neutral point on the pH scale is 7.0, indicating the presence of equal numbers of free hydrogen (acidic) and hydroxide (basic) ions.

neutralization - The addition of either an acid or a base to a solution as required to produce a neutral solution. The use of alkaline or basic materials to neutralize the acidity of some waters is a common practice in water conditioning.

neutralizer - A common designation for alkaline materials such as calcite (calcium carbonate) or magnesia (magnesium oxide) used in the neutralization of acid waters.

noncarbonate hardness - Water hardness due to the presence of compounds such as calcium and magnesium chlorides, sulfates or nitrates; the excess of total hardness over total alkalinity.

normal solution - A solution containing a gram equivalent weight of a substance in one liter of solution. (See equivalent weight.)

NTU - Abbreviation for "Nephelometric Turbidity Unit".