Dictionary of Chemistry [6th Ed.]

piperidineA saturated heterocyclic

compound having a nitrogen atom in

a six-membered ring, C 5 H 11 N; r.d.

0.86; m.p. –7°C; b.p. 106°C. The struc-

ture is present in many alkaloids

piperidine 420

p

N

H

2 CH

2 CH

C

H 2

CH 2

CH 2

Piperidine

pipetteA graduated tube used for

transferring measured volumes of

liquid or, sometimes, gases.

Pirani gaugeAn instrument used

to measure low pressures (1–10–4

torr; 100–0.01 Pa). It consists of an

electrically heatedÜlament, which is

exposed to the gas whose pressure is

to be measured. The extent to which

heat is conducted away from theÜla-

ment depends on the gas pressure,

which thus controls its equilibrium

temperature. Since the resistance of

theÜlament is dependent on its tem-

perature, the pressure is related to

the resistance of theÜlament. The

Ülament is arranged to be part of a

Wheatstone bridge circuit and the

pressure is read from a microamme-

ter calibrated in pressure units. As

the effect depends on the thermal

conductivity of the gas, the calibra-

tion has to be made each time the

pressure of a different gas is meas-

ured.

pirsonniteA mineral consisting

of a hydrated mixed carbonate of

sodium and calcium, Na 2 CO 3 .CaCO 3.

2H 2 O.

pitchA black or dark-brown residue

resulting from the distillation of coal

tar, wood tar, or petroleum (bitu-

men). The term is also sometimes

used for the naturally occurring pe-

troleum residue (asphalt). Pitch is

used as a binding agent (e.g. in road

tars), for waterprooÜng (e.g. in

rooÜng felts), and as a fuel.

pitchblendeSee uraninite.

pKvalue A measure of the

strength of an acid on a logarithmic

scale. The pK value is given by

log 10 (1/Ka), where Kais the acid disso-

ciation constant. pK values are often

used to compare the strengths of dif-

ferent acids.

Planck, Max Karl Ernst Ludwig

(1858–1947) German physicist, who

became a professor at Berlin Univer-

sity in 1892. Here he formulated the

*quantum theory, which had its

basis in a paper of 1900. One of the

most important scientiÜc discoveries

of the century, this work earned him

the 1918 Nobel Prize for physics.

Planck constantSymbol h. The

fundamental constant equal to the

ratio of the energy of a quantum of

energy to its frequency. It has the

value 6.626 0755(40) × 10 –34J s. It is

named after Max Planck. In quan-

tum-mechanical calculations the ra-

tionalized Planck constant (or Dirac

constant) 
= h/2π= 1.054 589 ×

10 –34Js is frequently used.

plane-polarized lightSee polar-

ization of light.

plaster of ParisThe hemihydrate

of *calcium sulphate, 2CaSO 4 .H 2 O,

prepared by heating the mineral gyp-

sum. When ground to aÜne powder

and mixed with water, plaster of

Paris sets hard, forming interlocking

crystals of gypsum. The setting re-

sults in an increase in volume and so

the plasterÜts tightly into a mould. It

is used in pottery making, as a cast

for setting broken bones, and as a

constituent of the plaster used in the

building industry.

plasticizerA substance added to a