with the iron in haemoglobin in the
blood, so preventing oxygen reaching
the tissues of the body. It is used in
the extraction of precious metals and
in electroplating industries. Aqueous
solutions are alkaline due to salt hy-
drolysis.
sodium dichromate A red crys-
talline solid, Na 2 Cr 2 O 7 .2H 2 O, soluble
in water and insoluble in ethanol. It
is usually known as the dihydrate
(r.d. 2.52), which starts to lose water
above 100°C; the compound decom-
poses above 400°C. It is made by
melting chrome iron ore with lime
and soda ash and acidiÜcation of the
chromate thus formed. Sodium
dichromate is cheaper than the cor-
responding potassium compound but
has the disadvantage of being hygro-
scopic. It is used as a mordant in dye-
ing, as an oxidizing agent in organic
chemistry, and in analytical chem-
istry.
sodium dihydrogenorthophos-
phateSee sodium dihydrogen-
phosphate(v).
sodium dihydrogenphos-
phate(V)(sodium dihydrogen-
orthophosphate)A colourless
crystalline solid, NaH 2 PO 4 , which is
soluble in water and insoluble in al-
cohol, known as the monohydrate
(r.d. 2.04) and the dihydrate (r.d.
1.91). The dihydrate loses one water
molecule at 60°C and the second
molecule of water at 100°C, followed
by decomposition at 204°C. The com-
pound may be prepared by treating
sodium carbonate with an equimolar
quantity of phosphoric acid or by
neutralizing phosphoric acid with
sodium hydroxide. It is used in the
preparation of sodium phosphate
(Na 3 PO 4 ), in baking powders, as a
food additive, and as a constituent of
buffering systems. Both sodium dihy-
drogenphosphate and trisodium
phosphate enriched in^32 P have been
used to study phosphate participa-
tion in metabolic processes.
sodium dioxideSee sodium super-
oxide.
sodium ethanoate (sodium ac-
etate)A colourless crystalline com-
pound, CH 3 COONa, which is known
as the anhydrous salt (r.d. 1.52; m.p.
324 °C) or the trihydrate (r.d. 1.45;
loses water at 58°C). Both forms are
soluble in water and in ethoxy-
ethane, and slightly soluble in
ethanol. The compound may be pre-
pared by the reaction of ethanoic
acid (acetic acid) with sodium carbon-
ate or with sodium hydroxide. Be-
cause it is a salt of a strong base and
a weak acid, sodium ethanoate is
used in buffers for pH control in
many laboratory applications, in
foodstuffs, and in electroplating. It is
also used in dyeing, soaps, pharma-
ceuticals, and in photography.
sodiumÛuorideA crystalline
compound, NaF, soluble in water and
very slightly soluble in ethanol;
cubic; r.d. 2.56; m.p. 993°C; b.p.
1695 °C. It occurs naturally as villiau-
mite and may be prepared by the re-
action of sodium hydroxide or of
sodium carbonate with hydrogen
Ûuoride. The reaction of sodium
Ûuoride with concentrated sulphuric
acid may be used as a source of hy-
drogenÛuoride. The compound is
used in ceramic enamels and as a
preservative agent for fermentation.
It is highly toxic but in very dilute so-
lution (less than 1 part per million) it
is used in theÛuoridation of water
for the prevention of tooth decay on
account of its ability to replace OH
groups with F groups in the material
of dental enamel.sodium formateSee sodium
methanoate.
sodium hexaÛuoraluminateA
colourless monoclinic solid, Na 3 AlF 6 ,489 sodium hexaÛuoraluminate
s