Sciencemadness Discussion Board

Pattinson's paint help!

caveman - 11-10-2013 at 19:47

Pattinson's lead paint is said to be made by dissolving lead chloride in hot water and then adding Calcium oxide in just enough quantity to
remove half the chlorine. How will I know when I've added enough CaO? Thanks

Vargouille - 11-10-2013 at 19:55

Remove half the chlorine? How do you expect to do that with CaO?

BromicAcid - 11-10-2013 at 20:11

A quick search yielded this page with somewhat more information:

The proportion of lime also should be exactly calculated for neutralizing half the chlorine of the lead chloride, and the precipitated basic salt should contain equal atoms of chloride and oxide of lead.

So the you want to add calcium oxide to maintain a 2:1 (lead chloride to calcium oxide) molar ratio. In that way half of the chlorine will be available to be taken up by the calcium oxide which I am assuming will just stay in solution as the chloride.

<b>If for example you had 500 grams of lead chloride</b>...

500 grams / 278.10 (g/mol) = 1.80 mol lead chloride

1.8 mol lead chloride x (1mol calcium oxide / 2 mol lead chloride) = 0.90 mol of calcium oxide

0.90 mol calcium oxide x 56.08 (g/mol) = <b>50.4 grams of calcium oxide would be needed</b>

Note the key principles, rapid mixing, complete dissolution of the lead chloride (takes lots of water), done at boiling (can result in flash boiling due to nucleation on precipitation).

Should be lots of fun. Don't get lead poisoning.

Edit: Assumed PbCl<sub>2</sub>, though not specified due to the bucket chemistry nature of the prep as well as it being carried out in aqueous medium.

[Edited on 10/12/2013 by BromicAcid]

blogfast25 - 12-10-2013 at 06:31

Note that the 'Chest of Books' reference speaks of 'lime water', a saturated solution of Ca(OH)2 in water (about 1.5 g of Ca(OH)2 per litre, Wiki). So no actual 'quick lime' (CaO) needs to be used. 'Slaked lime' (Ca(OH)2) is easy to obtain.

With poor solubilities of both reagents, large amounts of solutions will have to be handled for relatively low amounts of the end product. And you end up with large amounts of low concentration CaCl2 solution, possibly contaminated with trace amounts of lead. Hmmm, there must be a better way...

[Edited on 12-10-2013 by blogfast25]

caveman - 13-10-2013 at 11:34

Thank you all so much! :) Special thanks to Bromic acid and Blogfast 25 for the indepth analysis. Will enter you both in my last will and testament! Caveman.