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Hydrolysis of sodium acetate trihydrate
Aug 04, 2018

How to ob­tain sodi­um ac­etate

Not only an al­ka­li can be used to ob­tain this salt – an anal­o­gous prod­uct is ob­tained in the re­ac­tion of sodi­um car­bon­ate or bi­car­bon­ate with acetic acid:

CH₃­COOH + NaH­CO₃ = CH₃­COONa + CO₂ + H₂O

This is the most pop­u­lar method for ob­tain­ing the sub­stance known as “slak­ing soda of vine­gar” – in this way pure sodi­um ac­etate can be ob­tained, which crys­tal­lizes when wa­ter evap­o­rates.

Some­times the com­pound is also ob­tained ac­cord­ing to a typ­i­cal meta­bol­ic re­ac­tion:

(CH₃­COO)₂Zn + Na₂S = 2CH₃­COONa + ZnS

Sodi­um ac­etate can be ob­tained in two stages, and in the first one only or­gan­ic sub­stances take part (they do not break down into ions in the so­lu­tion, which means that they are weak elec­trolytes):

C₂H₅OH + CH₃­COOH = CH₃­COOC₂H₅ + H₂O (for­ma­tion of ethyl ac­etate in the pres­ence of sul­fu­ric acid as a de­hy­drat­ing agent)

CH₃­COOC₂H₅ + NaOH = CH₃­COONa + C₂H₅OH (for­ma­tion of sodi­um ac­etate and ethanol un­der the im­pact of an al­ka­li).

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The prop­er­ties of reagents used to ob­tain sodi­um ac­etate de­ter­mine the prop­er­ties of the salt. When sodi­um ac­etate is dis­solved in wa­ter, a so­lu­tion may be ob­tained with an al­ka­line re­ac­tion of the medi­um, as a con­se­quence of hy­drol­y­sis (or solvol­y­sis) – the break­down of a sub­stance on con­tact with wa­ter, with the for­ma­tion of new com­pounds. A de­tailed de­scrip­tion of the process of hy­drol­y­sis of CH₃­COON is re­quired, as this salt has cer­tain spe­cial fea­tures.

How hy­drol­y­sis of sodi­um ac­etate takes place

As sodi­um ac­etate is a sin­gle-base salt, it only hy­drolyzes by one de­gree (it forms one se­ries of com­pounds on hy­drol­y­sis – an al­ka­li and acid). A sub­stance is formed by a strong base (al­ka­li) NaOH and a weak acetic acid CH₃­COOH.

As vine­gar dis­so­ci­ates lit­tle, acid weak­ly breaks down into ions. In a so­lu­tion it ex­ists in the form of a sym­met­ri­cal dimer – a com­pound con­sist­ing of two vine­gar mol­e­cules.

Hy­drol­y­sis takes place with the weak ion – in this case it is the neg­a­tive­ly charged ac­etate ion CH₃­COO⁻.

The cor­rect re­ac­tion equa­tion of the hy­drol­y­sis of sodi­um ac­etate looks as fol­lows:

  • in molec­u­lar form: CH₃­COONa + H₂O = CH₃­COOH + NaOH;

  • in ion­ic form the re­ac­tion can be por­trayed as fol­lows: CH₃­COO⁻ + Na⁺ + H₂O = CH₃­COOH + Na⁺ + OH⁻;

  • in ab­bre­vi­at­ed ion­ic form: CH₃­COO⁻ + H₂O = CH₃­COOH + OH⁻.

In this equa­tion it is clear that an an­ion is hy­drolyzed, and be­cause of the pres­ence of an OH-group in the so­lu­tion its medi­um is al­ka­line.

As potas­si­um and sodi­um hy­drox­ides and salts do not have fun­da­men­tal dif­fer­ences in their prop­er­ties, the process of ob­tain­ing and hy­drol­y­sis of potas­si­um ac­etate (or cal­ci­um ac­etate) is ful­ly anal­o­gous to the one ap­plied for sodi­um salt:

CH₃­COOK + H₂O = CH₃­COOH + KOH;

(CH₃­COO)₂Ca + 2H₂O = 2CH₃­COOH + Ca(OH)₂.

The spe­cial na­ture of sodi­um ac­etate crys­tal hy­drates

Sodi­um ac­etate tri­hy­drates (CH₃­COONa·3H₂O) eas­i­ly move to a flux – gen­tle heat­ing is suf­fi­cient for this. Melt­ing takes place un­usu­al­ly, as the dis­so­lu­tion of salt in wa­ter takes place, which was the com­po­nent part of the struc­ture of the crys­tal hy­drate (if a drop of phe­nolph­thalein is added, it turns crim­son, as when CH₃­COON is dis­solved, an al­ka­line medi­um is cre­at­ed as a con­se­quence of hy­drol­y­sis). On cool­ing, a white ice-like sub­stance is formed (the ex­per­i­ment of “hot ice” is based on this).

Here’s how it works:


At boil­ing tem­per­a­ture of the wa­ter bath, sol­id sodi­um ac­etate with a small ad­di­tion of wa­ter (with­out ex­cess) forms a rich so­lu­tion, which on cool­ing be­comes sat­u­rat­ed, but crys­tal­liza­tion of the sub­stance does not take place, as in the case with oth­er salts. Crys­tals be­gin to set­tle only when a “seed” is put in – sev­er­al crys­tals of sol­id sodi­um ac­etate. If a small amount of wa­ter is not added to the wa­ter be­fore heat­ing, crys­tal­liza­tion im­me­di­ate­ly takes place.

An­hy­drous sodi­um ac­etate also hy­drolyzes in wa­ter. It can be ob­tained by heat­ing at a tem­per­a­ture of around 320 °C in a porce­lain bowl. The salt must be de­hy­drat­ed care­ful­ly, with­out ex­ceed­ing a heat­ing tem­per­a­ture of 324 °C, as the salt may break down (and hy­drol­y­sis will take place ac­cord­ing to a dif­fer­ent equa­tion):

2CH₃­COONa = Na₂­CO₃ + (CH₃)₂CO (ace­tone)

What re­ac­tions take place in a so­lu­tion of sodi­um ac­etate

Usu­al­ly on an anal­y­sis of the be­hav­ior of so­lu­tions of sodi­um ac­etate or potas­si­um ac­etate, only the hy­drol­y­sis of salt is ex­am­ined. But this is not the only re­verse process that takes place in the so­lu­tion – for ex­am­ple, dis­so­ci­a­tion of sub­stances takes place – their break­down into ions in the wa­ter (in hy­drol­y­sisthis is only cal­cu­lat­ed for sodi­um ac­etate dis­solved in wa­ter and the strong sodi­um al­ka­li)

CH₃­COONa = CH₃­COO⁻ + Na⁺

NaOH = Na⁺ + OH⁻

It should also be tak­en into ac­count that to a small de­gree, acetic acid is also sub­ject to dis­so­ci­a­tion (for a so­lu­tion with a mo­lar con­cen­tra­tion of 1 mol/l dis­so­ci­a­tion comes to 0.4%):

CH₃­COOH = H⁺ + CH₃­COO⁻

Sodi­um ac­etate has found wide ap­pli­ca­tion in in­dus­try – this salt is used as a food ad­di­tive, a con­serv­ing agent, a buf­fer so­lu­tion for sta­bi­liza­tion of a re­ac­tion in the pH medi­um (es­pe­cial­ly in bio­chem­istry), and a com­po­nent of chem­i­cal heat­ing pads.



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