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I don't know if why we add Al for the reaction between Nitrate and a base so it would release NH3 gas.

So what I want to know is if what is the role of Al here? And why do we have to add it in excess? Can we do this with every nitrate? ( in my case I came across the reaction of Sodium Nitrate and Sodium hydroxide)

And thank you for your time!

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Hydroxide and nitrate alone do not react with each other.

Their reaction with aluminium is the part of the classical quantitative determination of nitrate content by distillation of produced ammonia and by the back acid titration.

Nitrite + ammonium ions interfere, but the former can be eliminated due their high reactivity, the latter can be determined and/or eliminated by prior distillation without aluminium.

Aluminium acts as a reduction agents, reducing nitrates to ammonia, forming alkali aluminates.

$$\ce{3 NO3-(aq) + 8 Al(s) + 5 OH-(aq) + 18 H2O(l) -> 3 NH3(g) + 8 [Al(OH)4]-(aq)}$$

Generally, aluminium need not to be in an excess. But if there is the desire the all nitrate ions react quantitatively, aluminium and hydroxide must be in excess.

Usually, the Devarda's alloy is used, instead of pure aluminium, containing also copper and zinc.

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    $\begingroup$ Judging from the number of typos, someone started to celebrate the New Year early:) $\endgroup$ – andselisk Dec 29 '19 at 11:14
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    $\begingroup$ Android typing does not require any celebration. I am sober, but Android is not. :-) I make typos all the time. $\endgroup$ – Poutnik Dec 29 '19 at 11:15
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    $\begingroup$ It is worth while mentioning that the reduction of nitrate to ammonia by aluminium corresponds to a huge change of oxidation number. Nitrogen oxidation number passes from +V to -III, which a change of 8, while Aluminium passes from 0 to +III. There are not so many known examples of a change of oxidation number as high as 8. $\endgroup$ – Maurice Dec 29 '19 at 11:26
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    $\begingroup$ @Maurice One of few is the potassium perchlorate reduction to potassium chloride in fireworks. ( +VII -> -I }, or the sulphate reduction to sulphide ( +VI -> -II) $\endgroup$ – Poutnik Dec 29 '19 at 11:36
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    $\begingroup$ @Poutnik. I should have added that I was thinking of reactions in aqueous solution or near room temperature. The reductiosns you are mentioning happen at temperatures higher than 500°C. $\endgroup$ – Maurice Dec 29 '19 at 14:09

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