Sodium and qualitative reactions to it

How to detect sodium

[Deposit Photos]

Sodi­um is in the 1st group and 3rd pe­ri­od of the Pe­ri­od­ic Ta­ble. As a sim­ple sub­stance, it is a soft, mal­leable, sil­very-white met­al. It dis­plays typ­i­cal metal­lic qual­i­ties as a sim­ple sub­stance, but sodi­um ox­ides and hy­drox­ides have pro­nounced ba­sic prop­er­ties. Sodi­um has high chem­i­cal re­ac­tiv­i­ty, and so is stored un­der min­er­al oil or kerosene – the met­al can re­act vi­o­lent­ly with oxy­gen in the air. There are sev­er­al qual­i­ta­tive re­ac­tions to es­tab­lish the pres­ence of sodi­um ions in the com­po­si­tion of its salt.

Sodium bars [Wikimedia]

Ob­tain­ing sodi­um and its chem­i­cal prop­er­ties

Nowa­days, metal­lic sodi­um is ob­tained by the elec­trol­y­sis of a flux of sodi­um chlo­ride, as a re­sult of which the met­al and gaseous chlo­rine forms:

К⁻: Na⁺ + 1e = Na⁰;

A⁺: 2Cl⁻ – 2e = Cl₂⁰.

2Na­Cl = 2Na + Cl₂. Sodi­um has typ­i­cal prop­er­ties of ac­tive met­als.

1. It re­acts with oxy­gen with the for­ma­tion of per­ox­ide:

2Na + O₂ = Na₂O₂

(Na₂O ox­ide forms in trace quan­ti­ties, but it may be ob­tained by the re­ac­tion 2Na + Na₂O₂ = 2Na₂O)

2. It re­acts with non-met­als with for­ma­tion of salts:

2Na + S = Na₂S (sodi­um sul­fide)

2Na + Cl₂ = 2Na­Cl (sodi­um chlo­ride)

[Deposit Photos]

3. It re­acts with di­lut­ed acids with for­ma­tion of salts and hy­dro­gen:

2Na + 2HCl = 2Na­Cl + H₂

(sodi­um chlo­ride and gaseous hy­dro­gen form)

4. 2Na + Zn­Cl₂ = Zn + 2Na­Cl

(sodi­um is lo­cat­ed be­fore zinc in the re­ac­tiv­i­ty se­ries (to the left), and so it can force zinc out of a so­lu­tion of its salt)

Sodi­um also re­acts vi­o­lent­ly with wa­ter with for­ma­tion of an al­ka­li:

2Na + H₂O = 2NaOH + H₂

All sodi­um salts are sol­u­ble in wa­ter, and so sev­er­al re­ac­tions can be car­ried out (they are called qual­i­ta­tive re­ac­tions) to de­tect the pres­ence of Na⁺ ions in a salt so­lu­tion ac­cord­ing to var­i­ous signs (for ex­am­ple, the for­ma­tion of a sed­i­ment or a change of col­or in the so­lu­tion).

Qual­i­ta­tive re­ac­tions to sodi­um ions

The pres­ence of sodi­um in a salt can­not be es­tab­lished vis­ual­ly with­out con­duct­ing ex­per­i­ments. There are sev­er­al meth­ods for es­tab­lish­ing the pres­ence of sodi­um ions in a so­lu­tion.

Col­or­ing the flame of a burn­er

A positive flame test for sodium has a bright yellow color [Wikimedia]

If sev­er­al salt crys­tals con­tain­ing sodi­um cations are plunged into the flame of a burn­er, the flame turns green. A re­ac­tion can also be car­ried out with a salt so­lu­tion. If a met­al (for ex­am­ple plat­inum) wire is soaked in a sodi­um salt so­lu­tion and put into the flame of a burn­er, the col­or of the flame should change to yel­low. It is im­por­tant that the salt is pu­ri­fied from mix­tures as much as pos­si­ble – cations of oth­er met­als (for ex­am­ple potas­si­um or lithi­um) may “dis­guise” the col­or of the flame.

Re­ac­tion with uranyl zinc ac­etate Zn(UO₂)₃(CH₃СOО)₈

Sodi­um salts re­acts as fol­lows with uranyl zinc ac­etate (or zinc diox­ourane(VI) ac­etate) Zn(UO₂)₃(CH₃СOО)₈ in the pres­ence of acetic acid:

NaCl + Zn(UO₂)₃(CH₃­COO)₈ + СН₃СООН + 9Н₂О = NaZn(UO₂)₃(CH₃­COO)₉·9Н₂О + HCl

In ion­ic form:

Na⁺ + Zn(UO₂)₃(CH₃СOО)₈ + CH₃­COO⁻ + 9H₂O = NaZn(UO₂)₃(CH₃СOО)₉·9H₂O

The re­ac­tion is mi­cro­crys­talline – to de­tect the ef­fect, it should be car­ried out on a mi­cro­scope slide (if the mix­ture is heat­ed on the glass at the mo­ment the re­ac­tion takes place, the sen­si­tiv­i­ty of the re­ac­tion in­creas­es). A yel­low crys­talline sed­i­ment forms which is in­sol­u­ble in acetic acid. Click here for safe ex­per­i­ments with sodi­um com­pounds.

This re­ac­tion can be con­sid­ered spe­cif­ic for sodi­um ions (it makes it pos­si­ble to es­tab­lish the pres­ence or ab­sence of sodi­um cations in a so­lu­tion more pre­cise­ly (com­pared with a flame), as oth­er cations of al­ka­line met­als do not dis­guise the re­sult).

The re­sult may be dis­tort­ed by the pres­ence of am­mo­nia cations NH₄⁺ or potas­si­um cations (K⁺), only if their quan­ti­ty ex­ceeds the quan­ti­ty of sodi­um ions by 20 times. The pres­ence of cations of heavy met­als may also in­ter­fere – for ex­am­ple, Fe³⁺, Bi³⁺, Hg²⁺ etc.

Im­pact of potas­si­um an­ti­monate on sodi­um ions

With the im­pact of potas­si­um an­ti­monate KH₂S­bO₄ on dis­solved ta­ble salt, the fol­low­ing re­ac­tion takes place:

NaCl + КН₂S­bО₄ = NaН₂S­bО₄ + КCl

The white crys­talline sed­i­ment NaН₂S­bО₄ forms (its for­ma­tion can be ac­cel­er­at­ed by cool­ing the so­lu­tion or rub­bing a glass rod against the wall of the ves­sel). The re­ac­tion is car­ried out in a neu­tral or weak al­ka­line medi­um, in the ab­sence of cations of oth­er groups and am­mo­nia ions.

A re­ac­tion may also be car­ried out with potas­si­um an­ti­monate(V):

NaCl +K[Sb(OH)₆] = Na[Sb(OH)₆] + KCl

The white crys­talline sed­i­ment Na[Sb(OH)₆] forms, which is sol­u­ble in al­ka­lis. The pres­ence of am­mo­nia ions pre­vents car­ry­ing out a qual­i­ta­tive de­tec­tion of sodi­um, as the white amor­phic sed­i­ment of meta-an­ti­mon­ic acid HS­bO₃.

Sodi­um salts are wide­ly used in in­dus­try – for ex­am­ple, phys­i­o­log­i­cal so­lu­tions are pre­pared from sodi­um chlo­ride. NaCl is also used in cook­ing and mu­nic­i­pal ser­vices – for ex­am­ple, an­tifreeze agents are pre­pared from sodi­um chlo­ride. Some sol­u­ble sodi­um salts are used as dry­ing agents in or­gan­ic syn­the­sis.