Swede
Hazard to Others
Posts: 491
Registered: 4-9-2008
Member Is Offline
Mood: No Mood
|
|
Replication of Pb "test strips"
Background: Probably 95% of my posts are in the "Technochemistry" group where I have been exploring the creation of Lead Dioxide anodes for some
time. It is a complex effort, but one that is beginning to bear fruit. Lead dioxide is one of a very few anodes suitable for the creation of
perchlorates electrochemically.
Deposition of lead dioxide invariably involves large cells that use lead nitrate by the kilogram; a nasty and toxic brew. Concerned that lead nitrate
could potentially mist, and travel, I purchased a commercial "Lead Detection" kit that was not cheap for what I got... 2 postage stamp sized heavy
paper strips that are wetted with water, the test surface wiped, and any crimson color indicates lead contamination. I later found this system is
based upon sodium rhodizonate dihydrate, in an acid environment. It is specific for lead, and rejects other metals. I decided to attempt to recreate
these strips in my own lab.
Within the strips themselves are acidic salts that create the necessary environment to "develop" the color created by the sodium rhodizonate. Wetting
the strips dissolves the acid salt. I decided to try four common acid salts: Sulfamic, tartaric, ascorbic, and boric acid.
These strips were created by simply immersing the filter paper sections in four test tubes full of the acidic salt, dissolved in water. They were
placed on the screen, and allowed to dry. The concentrations were fairly low.
The next step was to introduce the sodium rhodizonate into the strips, and again, allow them to dry. Law enforcement uses this test to check for Pb
residue in gunshot holes, but it is all mixed and done on the spot, using spray bottles and the like, as the rhodizonate, once wetted, has a limited
shelf life. I wanted something that would potentially keep.
A stock solution of sodium rhodizonate was created using a small portion weighing 0.023 grams in 30 ml distilled water. This solution was brilliant
orange. A portion of the acidified strips were quickly dipped in the solution, set aside to dry, and the rhodizonate solution was further diluted.
Additional strips were created, and ultimately, I had 12 samples, differing in the acid type, and in the concentration of the rhodizonate indicator.
To test, I created a 1% solution of lead nitrate. I then cut out a "control" strip of the commercial product, and added a drop of the lead nitrate
solution to the control. I was shocked when there was NO color change. I took a second control strip, wetted it first, then added the lead nitrate.
It turned a brilliant crimson, indicating that the strip had to be wetted first. Very odd... I would have thought the aqueous lead nitrate would do
both steps simultaneously - wetting AND adding Pb. Armed with this knowledge, further tests were run by first wetting the strip with plain water,
then wiping down my PbO2 plating rig.
Of the four acid salts, the best performing was tartaric, then sulfamic. Ascorbic and boric acids both did not work well. The intensity of the color
was proportional to the strength of the rhodizonate indicator, with the 0.023g/30ml being more than adequate. This particular salt is not
inexpensive, but a little goes a very long way.
Further tests will involve tartaric and sulfamic acids, and checking the shelf-life of these replicated test strips. In all cases, the color tends to
fade with time, and in the above picture, the sulfamic acid tests, just to the right of the control, were brighter initially. While my strips worked
OK, none of them matched the commercial product.
Why bother? Mostly just for fun, but creeping lead nitrate salts can create a real hazard in a lab, and it is convenient to be able to test a surface
with a simple "wiping" action for Pb contamination.
|
|
|
Fluorite
Hazard to Others
Posts: 132
Registered: 26-12-2018
Location: United Arab Emirates
Member Is Offline
|
|
Great! This is so useful and the test is so sensitive, do you think this can test lead in drinking water? Also! have you tried testing a used car
battery electrolyte? I know that lead sulfide and phosphate are so insoluble and they're less toxic, can rhodizonates test something very insoluble?
Ugh lead is perfect for electrolysis and it's similar to silver 
I wish we could make a heavy metals vaccine?
|
|
|
Sulaiman
International Hazard
Posts: 3558
Registered: 8-2-2015
Location: 3rd rock from the sun
Member Is Offline
|
|
Rather than diy test papers of potentially unknown sensitivity after an extended period of storage,
I'd consider doing it similarly to your description of how the police do it.
Not as convenient as test papers but probably worth the little extra effort just to be sure.
In either case... stay safe.
CAUTION : Hobby Chemist, not Professional or even Amateur
|
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
