Physical and chemical properties of sodium bisulfate

Sodi­um bisul­fate NaH­SO₄ is the prod­uct of the in­com­plete neu­tral­iza­tion of sul­fu­ric acid by sodi­um hy­drox­ide. It has the for­mu­la NaH­SO₄. There are sev­er­al com­mon names of the salt–for ex­am­ple sodi­um bisul­fate or sodi­um hy­dro­gen sul­fate.

Phys­i­cal prop­er­ties of sodi­um bisul­fate

Sodi­um bisul­fate is a crys­talline sub­stance which forms the crys­talline hy­drate NaH­SO₄·H₂O (a crys­talline hy­drate is a sub­stance which con­tains wa­ter in the struc­ture of its crys­tals). In nor­mal con­di­tions, NaH­SO₄ con­sti­tutes col­or­less hy­dro­scop­ic (ca­pa­ble of ab­sorb­ing wa­ter) crys­tals. The melt­ing point of the crys­talline hy­drate is 58.5 °C (137.3 °F).

Ob­tain­ing sodi­um bisul­fate

There are sev­er­al meth­ods for ob­tain­ing the acid salt NaH­SO₄

1. By the neu­tral­iza­tion re­ac­tion–the re­ac­tion of an acid and a base with the for­ma­tion of salt and wa­ter. Sodi­um bisul­fate may be ob­tained in the re­ac­tion of sul­fu­ric acid and sodi­um hy­drox­ide, but the sub­stances must be tak­en in a cer­tain ra­tio: the acid should be in abun­dance in re­la­tion to the base. The for­ma­tion of acid salts is an ex­am­ple of the in­com­plete neu­tral­iza­tion of acid:

H₂­SO₄ + NaOH = NaH­SO₄ + H₂O

(the re­ac­tion is car­ried out with con­cen­trat­ed sul­fu­ric acid, the reagents re­act in the ra­tio of 1:1 with the for­ma­tion of the acid salt sodi­um bisul­fate).

In the re­ac­tion of 1 mole of H₂­SO₄ and 2 moles of NaOH sodi­um sul­fate Na₂­SO₄ forms:

H₂­SO₄ + 2NaOH = Na₂­SO₄ + 2H₂O

(the pro­tons of sul­fu­ric acid are com­plete­ly sub­sti­tut­ed to the sodi­um ions– com­plete neu­tral­iza­tion takes place, the prod­uct is a nor­mal salt)

2. By the re­ac­tion be­tween an acid ox­ide and an al­ka­li (if the ox­ide is tak­en in abun­dance, an acid salt may be ob­tained):

NaOH + SO₃ = NaH­SO₄

If NaOH is tak­en in abun­dance in car­ry­ing out this re­ac­tion, the re­sult will be the nor­mal salt Na₂­SO₄.

3. In the re­ac­tion of an acid and a nor­mal salt:

Na₂­SO₄ + H₂­SO₄ = 2NaH­SO₄.

Some acid salts (for ex­am­ple NaH­CO₃, formed by the weak acid H₂­CO₃ and the strong base NaOH) are ob­tained by hy­drol­y­sis (the re­ac­tion of a sub­stance with wa­ter, in which the ini­tial sub­stance breaks down with the for­ma­tion of new com­pounds). Sodi­um bisul­fate can­not be ob­tained from sul­fate by hy­drol­y­sis, as this salt is formed by the strong base NaOH and the strong acid H₂­SO₄–Na₂­SO₄ does not re­act with wa­ter, and only dis­solves in it.

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Chem­i­cal prop­er­ties of sodi­um bisul­fate

Many re­ac­tions with acid salts al­low nor­mal salts to be ob­tained from them. The chem­i­cal prop­er­ties of sodi­um bisul­fate are the fol­low­ing.

Re­ac­tion with hy­drox­ides

NaH­SO₄ + NaOH = Na₂­SO₄ + H₂O

(adding an al­ka­li to an acid salt makes it pos­si­ble to ob­tain the cor­re­spond­ing nor­mal salt, in this case sodi­um sul­fate Na₂­SO₄)

If a dif­fer­ent al­ka­li is tak­en, the for­ma­tion of two salts is pos­si­ble:

2NaH­SO₄ + 2KOH = Na₂­SO₄ + K₂­SO₄ + 2H₂O

(with mild heat­ing to 42 °C or 107.6 °F)

The re­ac­tion may pro­ceed fur­ther with the for­ma­tion of the dou­ble salt sodi­um-potas­si­um sul­fate):

Na₂­SO₄ + K₂­SO₄ = 2K­Na­SO₄

(the reagents must be present in a ra­tio of 2:1 by mass re­spec­tive­ly)

Re­ac­tion with salts

NaH­SO₄ + NaCl = Na₂­SO₄ + HCl

(re­ac­tion takes place with sin­ter­ing – heat­ing up to 450-800 °C (841-1472 °F) is re­quired, hy­dro­gen chlo­ride HCl is re­leased in gaseous form)

Re­ac­tion with ox­ides

2NaH­SO₄ + CuO = Cu­SO₄ + Na₂­SO₄ + H₂O

(re­ac­tion takes place with heat­ing up to 450 °C(842 °F)

Heat­ing

2NaH­SO₄ = Na₂S₂O₇ + H₂O

(with heat­ing up to 250 °C (482 °F) sodi­um bisul­fate changes to py­ro­sul­fate Na₂S₂O₇)

Sodi­um bisul­fate has found ap­pli­ca­tion in in­dus­try–for ex­am­ple, it is used in the man­u­fac­ture of non-fer­rous met­als, to change poor­ly sol­u­ble com­pounds into sol­u­ble sul­fates, and to in­crease the acid­i­ty of wa­ter in swim­ming pools. As a food ad­di­tive, sodi­um bisul­fate has the code E514.