the hydrogencarbonate, which make
the mixture rise.baking sodaSee sodium hydro-
gencarbonate.balanceAn accurate weighing de-
vice. The simple beam balance con-
sists of two pans suspended from a
centrally pivoted beam. Known
masses are placed on one pan and
the substance or body to be weighed
is placed in the other. When the
beam is exactly horizontal the two
masses are equal. An accurate labora-
tory balance weighs to the nearest
hundredth of a milligram. Specially
designed balances can be accurate to
a millionth of a milligram. More
modern substitution balances use the
substitution principle. In this cali-
brated weights are removed from the
single lever arm to bring the single
pan suspended from it into equilib-
rium with aÜxed counter weight.
The substitution balance is more ac-
curate than the two-pan device and
enables weighing to be carried out
more rapidly. In automatic electronic
balances, mass is determined not by
mechanical deÛection but by elec-
tronically controlled compensation
of an electric force. A scanner moni-
tors the displacement of the pan
support generating a current propor-
tional to the displacement. This cur-
rentÛows through a coil forcing the
pan support to return to its original
position by means of a magnetic
force. The signal generated enables
the mass to be read from a digital
display. The mass of the empty con-
tainer can be stored in the balance’s
computer memory and automatically
deducted from the mass of the con-
tainer plus its contents.Balmer seriesSee hydrogen spec-
trum.
banana bondInformal name for
the type of electron-deÜcient three-centre bond holding the B–H–B
bridges in *boranes and similar com-
pounds.band spectrumSee spectrum.band theorySee energy bands.
barA c.g.s. unit of pressure equal
to 10^6 dynes per square centimetre
or 10^5 pascals (approximately
750 mmHg or 0.987 atmosphere).
The millibar(100 Pa) is commonly
used in meteorology.
Barbier–Wieland degradation
The stepwise degradation of a car-
boxylic acid to the next lower homo-
logue. First the ester is converted
into a tertiary alcohol using a Grig-
nard reagent (PhMgX) and acid (HX):
RCH 2 COOCH 3 →RCH 2 C(OH)Ph 2.
The secondary alcohol is then dehy-
drated using ethanoic anhydride
(CH 3 COOCOCH 3 ) to give an alkene:
RCH 2 C(OH)Ph 2 →RCH=CPh 2.
The alkene is oxidized with chromic
acid:
RCH=CPh 2 →RCOOH + Ph 2 CO.
The result is conversion of an acid
RCH 2 COOH to the lower acid
RCOOH.barbiturateAny one of a group of
drugs derived from barbituric acid
that have a depressant effect on the
central nervous system. Barbiturates
were originally used as sedatives and
sleeping pills but their clinical use is
now limited due to their toxic side-
effects; prolonged use can lead to ad-
diction. SpeciÜc barbiturates in
clinical use include phenobarbitone,
for treating epilepsy, and methohexi-
tane sodium, used as an anaesthetic.
Barfoed’s test A biochemical test
to detect monosaccharide (reducing)
sugars in solution, devised by the
Swedish physician Christen Barfoed
(1815–99). Barfoed’s reagent, a mix-
ture of ethanoic (acetic) acid and cop-baking soda 56b