Synthesis of Lead(II) Nitrate

Introduction

Lead(II) Nitrate is a soluble compound of Lead, and so, a reliable source of Pb²⁺ ions. It is also an easy and interesting compound to make. I’m going to use it, in a future post, in an attempt to produce anhydrous Nitrogen Oxide gas (NO₂/N₂O₄).

Reagents

• Source of Lead (I used fishing weights);
• Nitric Acid 60% (any concentration above 30% should work with the procedure given);
• Distilled Water (Tap water has been used without any visual problems);

Dangers

Lead compounds are generally toxic. However, because Lead(II) Nitrate is soluble in water, which means it easily gets absorbed through the skin, it is especially toxic. Any direct contact with the substance must be avoided. The main danger, if contact occurs (if you spill one drop don’t freak out), is general Lead Poisoning. Wear Gloves when handling it.

Toxic Nitrogen Dioxide gas is going to be produced. The entire procedure must be carried out in a fume hood or in a very well ventilated area.

• The source of Lead doesn’t pose any significant risk (unless you throw it at someone’s head);
• 60% Nitric Acid is extremely corrosive and releases toxic vapors. It must be used outside or in a Fume Hood. Wear gloves and goggles.

Procedure

Start by weighting about 40g of Lead. I could only get 38,9g because I’m using a fishing weight. This is fine, though.

Now prepare a solution of about 30% Nitric Acid. I added 81g of 60% Nitric Acid to 100g of water (final concentration of about 27%).

Note: During heating, very small droplets of solution start “jumping” around. You might not see this without strong lighting but it is there. These very small droplets will end up all around your work space. That may be fine with low toxicity solutions, but we’re producing soluble Lead ions. That’s why I’m using a Florence Flask instead of a regular Beaker. Its shape doesn’t allow these tiny droplets to escape. If you must use a Beaker, cover it with a watch glass or a similar item.

Add the Lead to the Nitric Acid solution.

A slow reaction will start to take place. Now you can leave that for a day or two or you can make it faster (2/3 hours) by gently heating it. Anyway, remember that toxic Nitrogen Dioxide gas is going to be produced. I decided to heat it.

Two minutes later, Nitrogen Dioxide could already be seen.

Here are two close-ups, ten minutes after starting the heating:

After about 2,5 hours of heating almost all of the Lead had dissolved. You can’t see it, though, because there is a huge precipitate of Lead(II) Nitrate:

To dissolve most of the Lead(II) Nitrate, 100mL of water are added to the solution (you may add more if necessary). Some unknown precipitate (probably Tin Dioxide from Tin in the fishing weight) and metal leftovers still remain in suspension. Here is a picture of said “crap”:

Now set up for Vacuum Filtration and filter the solution. This filtration just serves to remove all the solid contamination:

Collect your filtered solution. It shouldn’t have any precipitate. Add to it 50mL of Nitric Acid. A precipitate of Lead(II) Nitrate immediately appears.

Set up your Vacuum Filtration apparatus once more and filter the solution. You will get a mass of tiny white crystals.

Note: You will only get tiny shiny crystals. The big ones you see in the picture appeared because I grew them, using the general technique for crystal growth (I may do a post on this if anyone is interested). Pictures of these crystals are in the bottom of this post.

Wash your crystals with 20ml of Nitric Acid, twice. Then, keep the vacuum pump going for about half an hour to dry the crystals. Once they’re dried, you’re done. Here’s my final product (42g):

Theory

Regular acids can’t dissolve Lead. However, because Nitric Acid acts as an oxidizing agent, it is able to dissolve Lead. Depending on the concentration of Nitric Acid, one of the following reactions occurs:

1. Pb(s) + 4HNO₃(aq) –> Pb(NO₃)₂(aq) + 2H₂O(l) + 2NO₂(g)
2. 3Pb(s) + 8HNO₃(aq) –> 3Pb(NO₃)₂(aq) + 4H₂O(l) + 2NO(g)

Reaction two tends to dominate when there is high concentration of Nitric Acid. On the contrary, reaction one tends to dominate when there is low concentration of Nitric Acid. In my experience, reaction one predominates in the conditions of this experiment.

You might be asking why I use ~30% Nitric Acid instead of just sticking with the 60% Nitric Acid. Well, empirically, I’ve found that 30% Nitric Acid dissolves Lead faster. If the concentration of Nitric Acid is too high, an immediate precipitate of Lead(II) Nitrate forms. In order to get oxidized, Lead must go into solution in the form of Lead(II) ions. With too much concentration of Nitric Acid, this process (Lead going into solution) is much slower because of very low solubility.

Molar Masses

Lead: 207,21 g/mol

Nitric Acid: 63,01 g/mol

Lead(II) Nitrate: 331,2 g/mol

Considering reaction one, 207,21 grams of Lead react with 252,04 grams of Nitric Acid. Scaling down by a factor of “0,193”, we roughly get 40 grams of Lead reacting with 48,6 grams of Nitric Acid. Considering 60% Nitric Acid, we get 81 grams of Nitric Acid (the value used).

General Information:

• At room temperature Lead and Nitric Acid react slowly. Heating the reaction vessel increases dramatically increases reaction rate.
• Lead(II) Nitrate is very soluble in water. However, it is insoluble in the presence of Nitric Acid (Common-ion effect). Thus, once Nitric Acid is added a precipitate of Lead(II) Nitrate appears.
• Lead(II) Nitrate tends to decompose into Basic Lead Nitrates if left in an environment with pH 7 or higher. That’s why I didn’t wash mine with water. Only Nitric Acid. Doing so keeps the pH low during storage.

My final product weighted 42g. The theoretical mass for this procedure is 62g. This represents a yield of 68%. The expected yield for this sort of straightforward reaction is above 90%. This is the yield you should expect. The reason why my yield was lower than normal is because I lost some during the crystallization process to form those larger crystals.

Disposal

This reaction generates toxic waste that must be disposed of properly. Check your city/state/country laws to see if there’s any specific regulations for Lead Compounds.

The goal of decontamination is to produce insoluble compounds of Lead.

I recommend the following:

• The solution itself and the filter paper leftovers must be neutralized. There will be a large quantity of Nitric Acid in solution. Thus, if you use Sodium Bicarbonate too much foam is going to be produced. It may even spill from the flask. Instead, neutralize with Sodium Hydroxide. After the acid is neutralized, Sodium Hydroxide will start to react with the Lead(II) Nitrate in solution to form insoluble Basic Nitrates and, after, Lead(II) Hydroxide, all of them insoluble.
• All the glassware that contained or came in contact with Lead(II) Nitrate should be neutralized with Sodium Bicarbonate. Just add a few spoons of Bicarbonate to the glassware and wash away. Sodium Bicarbonate reacts to form Lead Carbonate species, which are insoluble.

Crystals

Final Notes

As always, hope you find this useful. Please comment and leave some feedback! If you have any doubts or questions feel free to comment.