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Three Ways to Prepare Nitric Acid

Contents

  • Introduction
  • Safety
  • Single Displacement Synthesis
  • Double displacement Nitric Acid Synthesis
  • Reduction of Acidified Nitrate Salts

Nitric acid 3D model

3D molecular model of nitric acid
3D molecular model of nitric acid

Introduction

This article will be concerned with three ways to prepare nitric acid. Nitric acid is a valuable chemical reagent for the laboratory. It is a toxic and corrosive acid as well as being a strong oxidizer as well, in some cases dangerously so. It has many laboratory and industrial uses which include, metallurgy, testing for precious metals, analyzing mineral samples, metal cleaning and treating, making fertilzers, and preparation of explosives. Undoubtingly, the last use is what concerns authorities the most and for this reason, it is difficult to obtain this reagent. If you do find a source, buy the time you consider the intial cost of the chemical and shipping costs, it can be an expensive endeavor sourcing nitric acid. In this site, I will describe three main methods to synthesize this valuable acid only in quantities you need. In many cases, ingredients that are readily available are used and the only real expense is the all glass distillation apparatus.

Corrosive hazard placard
Corrosive hazard placard

Safety

Before we get started, I need to cover safety. During any preparative chemistry session , prudent laboratory safety measures must be practiced. This is especially true whenever handling concentrated acids and toxic vapors. In the preparation of nitric acid, you will encounter both of these serious hazards. Nitrogen dioxide or the brown vapors are insidiously poisonous and you may inhale a lethal concentration without realzing until hours later. It will get into your lungs and cause the gradulal buildup of fluids until you drown. If the nitrogen dioxide exposure does not kill you it can cause permanent damage like the COPD breathing disorder. This is not something for the amateur to explore. It is advisable to practice other, less hazardous, chemical synthesis before performing these techniques.

Also, don't forget, you will also be dealing with sulfuric and hydrochloric acid. Concentrated sulfuric acid causes third degree burns in three seconds and faster when hot. Hydrochloric is slightly less corrosive but is poisonous and has toxic fumes. This synthesis must be performed outdoors or in a well workign fume hood.

Three Types Of Nitric Acid

In the scheme of lab prepared nitric acid, there are generally three types of nitric acid. These distinct types of nitric acid are defined by concentration and conditions of preparation. First is constant boiling nitric acid of 68% concentration with a specific gravity of 1.42 . This is the acid of commerce and is the easiest to handle and transport. It is called constant boiling because, at this concentration, water and nitric acid form a binary azeotrope that always boils at the same temperature. In layman's terms, the 68% nitric acid can be boiled at atmospheric pressure and the vapors condensed unchanged.

The next two types are the fuming nitric acids. They are called so because of the copious choking fumes these more acids give off. Not only are these more concentrated forms of nitric acid but also much more reactive. They are white fuming and red fuming nitric acids. Both are about 95+% and have a specific gravity of 1.52. Nitric acid of this strength has more properties of a strong oxidant instead of acid properties. White fuming nitric acid is made from distilling alkali metal nitrates or constant boiling nitric acid with concentrated sulfuric acid at reduced pressure. Distillation of such mixtures at atmospheric pressure generates red fuming nitric acid. This is nitric acid with dissolved brownish red nitrogen dioxide giving the acid its characteristic reddish brown tint.

Single displacement synthesis

The simplist and most used preparation of nitric acid in the lab is by adding concentrated sulfuric acid to a dry nitrate salt, usually, potassium or sodium nitrate. This mixture is heated to melt the pasty mixture and distill the nitric acid. The nitric acid that is collected is usually 95% or higher concentration and a strongly fuming liquid. If vacuum distillation was used, the acid will be clear and be white fuming nitric acid. If the distillation was carried out at atmospheric pressure then a yellow acid is obtained due to some decompositon of the acid into nitrogen dioxide when remain dissolved. This is referred to red fuming nitric acid and this is the best nitric acid for oxidizing non metals such as iodine, antimony, etc. What is interesting to note is that when a little water is added to the red fuming acid it becomes clear. This is alright considering nitric acid that used for making silver nitrate can be somewhat dilute. And if preparing aqua regia, the nitrogen oxides will increase the activity of the acid for processing scrap gold.

For some, obtaining sulfuric acid can be quite difficult if not impossible, An alternative that will work equally well is concentrated phosphoric acid. The role of phosphoric acid is the same. A heat stable nonvolitile acid that displaces the nitric acid from a nitrate. Phosphoric acid can be found in the diluted form in rust removal and rust conversion products. It can be concentrated by boiling.

Single Displacement Nitric Acid Sythesis

Distillation of nitric acid setup
Distillation of nitric acid setup

Distillation Glassware

Double displacement reaction

If you do not want to prepare white or red fuming nitric acid, or just want more efficient use of your sulfuric acid, then this approach might be best. It has recently come to my attention in youtube that 68% nitric acid cna be prepared from calcium nitrate fertilzer and sulfuric acid and a little water. You mix the calcium nitrate with the water to dissolve as much as possible then add the concentrated sulfuric acid with constant stirring. This makes a thick pasty mixture which is distilled in an all glass apparatus. First water is collected in the receiving flash containing small amounts of nitric acid. Then at 121 degrees Celsius, the receiving flask is switched to collect the constant boiling 68% nitric acid. The video below describes the procedure in detail.

Easy Nitric acid

Reduction of acidified nitrate salts

Now I present a less efficient but functional method of nitric acid preparation. This method entails dissolving a nitrate salt in water and adding an acid. This acidified nitrate is then reacted with copper scraps to generate the deadly nitrogen dioxide gas which is led through water or a hydrogen peroxide solution. The hydrogen peroxide solution yields better acid due the fact it oxidizes any nitrous acid formed in water solution to nitric acid increasing yields. Ideally, the receiving solution should be in a slender tall cyclinder and surrounded by a freezing mixture to maximize gas absorption. It should be noted the acid obtained is very dilute ,and if equipment is lacking, may be contaminated with muriatic acid or metal salts from the intitial reaction.

The real plus with this method is sulfuric acid is not necessary and cheaper and more available muriatic acid can be used. It should be noted that sulfuric acid is available as a drain opener and it most cases it will suffice for nitric acid preparation. Another plus is that the copper scrap used can be recovered from the copper chloride/sulfate created with cheaper aluminum or iron scrap and reused to reduce more nitrate. Consult the Nurdrage video below for an exhaustive review of this process

Nurdrage Nitric acid guide video

Nitric Acid Synthesis Technique

What method have you used to prepare nitric acid?

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