# Chemicals that form coloured complexes with Pb2+

I’m currently conducting a photo-spectrometric analysis of lead concentration in water and was looking into using Eriochrome Black t as an indicator for Pb2+ (due to the complex forming a reddish colour). Because Eriochrome is relatively expensive, would there be any alternate chemicals that can complex lead in solution of the form Pb(OH)NO3, such that colour is present. This is because the source of lead is from a dissociation reaction of lead (II) nitrate when it is dissolved in water. The resulting lead is complexed.

All the best!

• Any idea of the ball park lead concentration? The dithizone method is what I used long ago, as an undergrad taking Instrumental Analysis, trying to detect lead in Hudson river water. Even after extensively boiling down the water specimens, to pre-concentrate the lead, the method simply was not sensitive enough (or the river was not polluted enough!). Precipitation methods also have detection limits, so 10 ppm should be relatively easy, but 10 ppb is hard.
– Ed V
Mar 31, 2020 at 21:23
• Interesting! I’m performing an adsorption experiment where the initial concentration of solution is 20 mg/L (or equivalently 20 ppm) of dissolved Lead Nitrate. However this is only the initial concentration, and the final solution will have substantially less lead due to uptake by the filtration medium. I was thinking about complexing lead so that I can perform a photospectrometric analysis. Is there a detection limit for this process too?
– Mas
Mar 31, 2020 at 21:53
• That I do not know. My guess is around a ppm or so. Is there any chance you can do a quick and dirty home lead test using the test kits they sell in hardware stores? That might give you a hint about the detection capability you will need.
– Ed V
Mar 31, 2020 at 22:04

In the old book from Hopkin and Williams, Organic Reagents for Mineral Analysis. London (1933), the following information can be found in the Chapter "Diphenylthiocarbazone, or Dithizone".

Diphenylthiocarbazone is a black powder, insoluble in water, but soluble in organic solvants, giving green solutions. Its main application is its reaction with lead. Lead ions produce a red color to its CCl4 solution. To 10 mL Lead aqueous solution, 1 mL KCN 10%, and two drops of ammonia are added. Add 2 mL of a dilute solution of diphenylthiocarbazone in CCl4 (50 mg/L). Stir. A reddish color must appear if the sample contains 0.001 mg Pb. A similar color appears without lead, due to the solubility of the reagent in CCl4. Addition of KCN removes totally any excess of reagent in the aqueous phase.

The only drawback about this method is the necessity to use KCN, which is forbidden today. I have found in the same book a small article about benzidine, where they say that lead solutions give a blue color when mixed with benzidine, if a solution 0,05% benzidine in acetic acid 10% is used.

Lead can also be detected and quantitatively determined by 8-hydroxyquinolein. But the book mentions only the following bibliographic references : V. Marsson, L. Haase, Ueber die Bestimmung des Bleis mit Hilfe von o-Oxychinolein, Chem. Ztg, 52, 993, (1928) V. Hovorka, Sur le dosage du plomb au moyen d'ortho-oxyquinoléine, Coll. Czech. Chem. Comm, 9, 191 (1937)

• I’ve looked into it and I believe using chelators to complex lead would be preferable (and safer), so the 8-hydroxyquinolein would be a viable alternative. Is the chelator strong enough to complex lead despite it being complexed to Pb(OH)NO3?
– Mas
Mar 31, 2020 at 21:40

you add colourless potassium iodide solution (or any other source of iodide ions in solution) to a solution of lead(II) nitrate, a bright yellow precipitate of lead(II) iodide is produced.

More interesting, to a soluble lead ion, add chlorine bleach (NaOCl). This results in a white transient precipitate of Pb(OCl)2, which on mild heating decomposes, leaving behind red, PbO2. To quote a source:

Initially, lead(II) ion reacts with hypochlorite ion to give a white precipitate of lead(II) hypochlorite:

$$\ce{Pb^{2+} (aq) + 2 ClO^{-} (aq) -> Pb(ClO)2 (s) }$$

The lead(II) hypochlorite is then oxidized to lead(IV) oxide. Reaction is slow at room temperature but much faster when the tube is placed in boiling water.

$$\ce{Pb^{2+} (aq) + 2 H2O(l) -> PbO2 (s) + 4 H+(aq) + 2 e- }$$

$$\ce{ClO-(aq) + 2 H+(aq) + 2 e- -> Cl-(aq) + H2O(l) }$$

• What's "$\ce{Pb^2++}$"? Besides, the second URL leads to some sketchy-looking webpage that doesn't seem to cite any reputable sources. Mar 31, 2020 at 11:16
• $PbI_2$ and $PbO_2$ are both insoluble. And Thunderbolt114 would like a soluble compound. Mar 31, 2020 at 12:03
• As Maurice suggested, I believe this analytical method only works if the solute is present in the aqueous phase. In other words, I can’t test on a precipitate. Are you familiar with any coloured complex formed with lead in solution?
– Mas
Mar 31, 2020 at 20:00
• Per Wikipedia on PbI2: "Typically, a solution of PbI2 in an organic solvent, such as dimethylformamide or dimethylsulfoxide, is applied over a titanium dioxide layer by spin coating." Mar 31, 2020 at 23:39
• I have performed (and posted online here sciencemadness.org/talk/viewthread.php?tid=62249#pid429056) the reaction of adding NaOCl (as common Bleach) to Lead acetate. The reaction proceeds as indicated resulting in the red PbO2. Quite interesting! Mar 31, 2020 at 23:48