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blogfast25
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Conversion of potassium dichromate to ammonium dichromate
In the UK, ammonium dichromate is now all but outlawed from what I understand. Potassium dichromate is likely to follow shortly but at least that
compound is not beyond the synthesis capability of many a home chemist.
Ideas on how to convert K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> into (NH<sub>4</sub> <sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> would be
welcome.
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AJKOER
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Blogfast:
Here is a suggested preparation route based on my research. First prepare CrO<sub>3</sub>, the anyhydride of Chromic acid, whose
preparation per this source (http://chromium.atomistry.com/chromium_trioxide.html ) is described as follows:
"Chromium Trioxide, Chromic Anhydride, CrO<sub>3</sub>, was first obtained by Unverdorben by the interaction of chromyl fluoride and
water. It is usually prepared by the action of concentrated sulphuric acid on solutions of a chromate, usually of potassium dichromate. Zettnow
recommends that 300 grams of potassium dichromate be warmed until dissolved with 500 c.c. of water after the addition of 420 c.c. of concentrated
sulphuric acid; after standing for ten to twelve hours, the liquid is poured away from the potassium hydrogen sulphate crystals that have separated,
heated to 80° to 90° C., and 150 c.c. of concentrated sulphuric acid added, then water, a few drops at a time, until the chromium trioxide crystals
which separate at first are just redissolved. Crystals are allowed to deposit during twelve hours, and subsequently, after concentration, they are
collected upon a platinum, asbestos, or pumice-stone filter, and washed with pure nitric acid of density 1.46. The crystals are then mixed with a
little concentrated nitric acid, and dried upon a porous plate, the process being repeated until the product is quite free from potassium sulphate and
sulphuric acid. The potassium sulphate may also be removed as potassium alum by addition of aluminium sulphate, and the sulphuric acid separated by
fusion. The crystals are freed from nitric acid by warming, preferably in a current of dry air in a tube at 60° to 80° C. Other methods of formation
are by the action of concentrated sulphuric acid upon lead chromate; from barium and strontium chromates by the action of sulphuric acid or nitric
acid; by the action of chlorine, or of hydrochloric acid upon silver chromate."
Next, per this link http://chromium.atomistry.com/ammonium_chromate.html , to quote:
"Ammonium Chromate, (NH<sub>4</sub><sub>2</sub>CrO<sub>4</sub>, is formed when a solution of chromic acid is neutralised by ammonia, and the mixture
evaporated. It yields golden-yellow needles (monoclinic prisms):
a:b:c = 1.9603:1:1.2226; β = 115° 13',
of density 1.886, and soluble in water. On heating or on exposure to air they decompose, evolving ammonia, and form the dichromate:
2(NH<sub>4</sub><sub>2</sub>CrO<sub>4</sub> =
(NH<sub>4</sub><sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 2NH<sub>3</sub> + H<sub>2</sub>O"
Hope this helps.
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[Edited on 2.8.13 by bfesser]
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unionised
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That would be one approach.
A less suicidally hazardous way would be to make Na<sub>2</sub>CrO<sub>7</sub> instead of the K salt, dissolve it in water,
add ammonium chloride, let the ammonium salt crystallise and then clean it up by recrystalisation.
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[Edited on 2.8.13 by bfesser]
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blogfast25
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Yes, NaCl is much more soluble at 0 C than (NH<sub>4</sub><sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>. A few recrystallisations would do it.
AJ: dichromate synth from Cr(III) and a real base like NaOH or KOH is much simpler than via CrO<sub>3</sub>.
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[Edited on 2.8.13 by bfesser]
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AJKOER
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I agree with the comments as CrO<sub>3</sub> is properly described as highly toxic, corrosive, and carcinogenic although remains more
popular in older textbooks being a powerful oxidizer (readers be wary).
However, assuming one has to start with the Potassium salt due to availability, my concern (perhaps unfounded, just based on experience working with
NH<sub>4</sub> salts) centers around the tendency of ammonia to readily form double salts (Potassium ammonium...), hence the more direct
(and unsafe) approach employing Chromic acid. Having access to a Sodium salt would obviate this argument.
---------------------------------------------------------------------
My personal manner to prepare (still unsafe) is to add aqueous H<sub>2</sub>C<sub>2</sub>O<sub>4</sub> to nearly
any available dichromate salt, filter out the insoluble oxalate or, more safe, just pour out the clear solution into an excess of aqueous ammonia. Let
evaporate and expose to air and/or gently heat to form ammonium dichromate.
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[Edited on 2.8.13 by bfesser]
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S.C. Wack
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CrO<sub>3</sub> is relatively benign IME. It's no benzidine.
An abstract in JCS 88ii, 707 (1905) kind of suggests that either salt is OK:
Potassium or sodium dichromate (1 mol.) is heated in aqueous solution with ammonium chloride (2 mols.) until the liquid assumes a deep garnet red
colour. After evaporating to half its bulk, the solution deposits in one or two days deep garnet-red needle shaped crystals of ammonium
dichromate,
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[Edited on 2.8.13 by bfesser]
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12AX7
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Outside chance the K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> - (NH<sub>3</sub><sub>2</sub>SO<sub>4</sub> system might work.
K<sub>2</sub>SO<sub>4</sub> is lower solubility than the other three possibilities, but it's not much less than
K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>, so it may be a challenge to crystallize (for example, it may exhibit a
eutectic sort of behavior as the solvent evaporates).
Doing it at high pH (chromate) should be successful, as K<sub>2</sub>CrO<sub>4</sub> is quite soluble.
Tim
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[Edited on 2.8.13 by bfesser]
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Bezaleel
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No K2Cr2O7 crystallising from solution
Maybe this is a good moment to share an experience in my attempt to synthesize CrO<sub>3</sub>.
CrCl<sub>3</sub> soluiton was treated with NaOH solution, and the precipitate vacuum filtered, washed once and vacuum filtered again
(coarse fritte, as huge size as available, to reduce filtering time).
Cr(OH)<sub>3</sub> lumps were put in a beaker and bleach added through a filter (to keep as much residue out as possible, bleach was at
least 1 year old). Soda lye solution was added, and the mixture stirred on a magnetic stirrer untill all the Cr(OH)<sub>3</sub> had
dissolved. Liquid temperature was about 60 C. A strongly coloured yellow solution of sodium chromate was obtained.
The solution was left to evaporate on the hotplate (liquid temperature was about 40 C) until half the volume had been obtained (bleach used was around
5% solution). Next the pH was lowered with HCl solution to around 3. A colour shift towards orange was observed near pH = 6. Take care here, to put
some cottonwool in the beaker while allowing the HCl solution to drip in drip by drip under stirring (don't heat!!!) Excess chlorine and
CO<sub>2</sub> absorbed from the air in the previous steps were released; cottonwool is imperative to absorb aerosols containing
hexavalent chromium. (Keep your environment clean, and the air in particular.) Note that there is a considerable heat-up in this step (40 to 50 C
temperature raise observed).
Finally a concentrated solution of KCl was added to the solution, expecting
K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> to crystallise out. This did NOT happen. Instead, after continuing
evaporation at 40 C liquid temperature, NaCl and/or KCl crystallised out in square crystals.
If you look up the respective solubilities of Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>,
K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>, NaCl, KCl, NaClO<sub>3</sub>, KClO<sub>3</sub>,
Na<sub>2</sub>CO<sub>3</sub>, and K<sub>2</sub>CO<sub>3</sub>, the (molar) solubility of
K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> is by far the lowest of all these (liquid temperature is around 25 C).
Why the K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> doesn’t crystallise out is a mystery to me.
The final step I wanted to do was to add H<sub>2</sub>SO<sub>4</sub> to the crystallised
K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>.
Further steps
This solution has been standing around for half a week now, but nothing has changed. I think I will add NaOH until the yellow colour reappears, and
then add BaCl<sub>2</sub> solution to it. BaCrO<sub>4</sub> is as insoluble as BaSO<sub>4</sub>, so this should
work. In order to get conversion of Ba(OH)<sub>2</sub> to the chromate, stirring the heated solution may be a good idea.
I wonder whether I may obtain a solution of CrO<sub>3</sub> when adding a stoichiometrc amount of
H<sub>2</sub>SO<sub>4</sub> to the BaCrO<sub>4</sub>. A solution of CrO<sub>3</sub> will do in my
case.
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[Edited on 2.8.13 by bfesser]
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plante1999
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Finally something still very easily available in Canada that is outlawed in other country, don't get me wrong, gennerally stuff you can't get easily
in Canada are available throughout the world. Anyone with 150 CAD can buy 2.5 kg of one or other dichromate, and be shipped. When I say everyone, it
include minors...
I can only think to one way from K dichromate to NH<sub>4</sub> dichromate. Make K chromate with KOH and add a soluble solution of
strontium salt. Filter the strontium chromate. Boil a solution of ammonium bisulphate with the strontium chromate and filter while hot to get the
dichromate crystals . However I can think fast to a way to turn chromite ore/Cr<sub>2</sub>O<sub>3</sub> into
NH<sub>4</sub> dichromate.
Melt calcium/barium nitrate/sodium (barium works better) at high heat and add the chromium source througly melt for an hour and dissolve the solubles,
decant and add ammonium salt of your choice (not sulphate) and boil till crystals form (ammonia gas will escape) . Put it the cold to crystalise the
dichromate.
Reference: My personal work.
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[Edited on 2.8.13 by bfesser]
I never asked for this.
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blogfast25
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@Tim: at high pH that's gonna stink!
@Bezaleel: highly unusual, indeed! But I think adding a concentrated solution of KCl was a mistake. Just acidifying with HCl would yield the
dichromate, without an ocean of KCl... Your bleach also turns to chloride, you just seem to have far too much Cl- in there!
@Plante: hmmm... not sure I'd want to get barium involved here: it'll be a job and a half to fully recover it. I think Cr source + NaOH +
Na(ClO<sub>3</sub>,NO<sub>3</sub> + heat, leaching + HCl +
NH<sub>4</sub>Cl might stand a good chance though, because acc. wiki's solubility table
Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> is highly soluble even cold.
[Edited on 2-8-2013 by blogfast25]
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[Edited on 2.8.13 by bfesser]
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plante1999
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No need for NaOH. Only bare nitrate digest the chromium oxide. It directly make dichromate when done this way.
I never asked for this.
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Fantasma4500
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i know a guy in the UK who sells ''lead chromate'' i then realised that it wasnt chromate, but DICHROMATE
i think it was 6£ for 200g
looks dangerously alot like bacon powder (=
i believe converting it into the lead salt could perhaps be a good idea as most lead salts arent very soluble, PbCl<sub>2</sub> (if
considering NH<sub>4</sub>Cl) is 0.67/100 mL @ 20°C
(NH<sub>4</sub><sub>2</sub>CO<sub>3</sub> could
be used for this then..
Pb(Cr<sub>2</sub>O<sub>7</sub><sub>2</sub> +
(NH<sub>4</sub><sub>2</sub>CO<sub>3</sub> >
PbCO<sub>3</sub> (s) + NH<sub>4</sub>Cr<sub>2</sub>O<sub>7</sub>
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[Edited on 2.8.13 by bfesser]
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blogfast25
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No, it's very likely to be lead chromate (PbCrO4), which is highly insoluble and a slightly surplus-to-requirements yellow pigment.
Dichromates exist in equilibrium with chromates:
Cr2O72- + H2O < === > 2 CrO42- + 2 H+
This equilibrium shifts to the right in alkaline conditions.
But because PbCrO4 is so insoluble, adding Pb2+ to a dichromate also pulls the equilibrium to the right by precipitation of
PbCrO4.
[Edited on 2-8-2013 by blogfast25]
Without any alkali?
Here's quite a decent page about chromates from Cr(III)/alkali/nitrate or chlorate fusion:
http://webpages.charter.net/dawill/tmoranwms/Chem_Chromate.h...
... although I don't see the need for KCl if you use KOH instead of NaOH.
Would 'bare nitrate' not be quite wasteful in nitrate? Any textbook mentions alkali + oxidiser although at high temp/forcing conditions air seems to
work too.
[Edited on 2-8-2013 by blogfast25]
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sequential posts]
[Edited on 2.8.13 by bfesser]
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woelen
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Heavy metal dichromates usually are much more soluble than heavy metal chromates.
Using BaCrO4 one can indeed make solutions of CrO3. I once tried adding H2SO4 to a suspension of BaCrO4 in water. This produces an orange solution and
a white suspension of BaSO4.
You have an equilibrium:
BaCrO4 <--> Ba(2+) + CrO4(2-)
2CrO4(2-) + 2H(+) <-->>> Cr2O7(2-) + H2O (mostly to the right)
This equilibrium is driven to the right by means of the following equlibrium:
Ba(2+) + SO4(2-) ---> BaSO4
I never tried with PbCrO4. I am afraid that PbCrO4 is so insoluble that this does not convert to dichromate on addition of H2SO4, but I never tried
personally.
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plante1999
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It depend if you value nitrate. Dichromate can be more valuable depending on the case.
2 KNO<sub>3</sub> + Cr<sub>2</sub>O<sub>3</sub> -->
K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 2 NO
Dirrecly dichromate, and my personnal work prove this too.
<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: rxn. eqn.
cleanup]
[Edited on 3.8.13 by bfesser]
I never asked for this.
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12AX7
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As for synthesis, I find simply cooking chrome oxide with a chlorate suffices. Base is unnecessary, and if you use a slight excess of KClO3, a source
of potassium is not required (of course). Some HCl, Cl2 and O2 is released in the reaction; perform it outdoors. Obviously, if you use NaClO3, you
get the soluble sodium salt, which is more amenable to further reactions, like precipitating the ammonium salt.
My experience is with chlorates, but I'm sure nitrates work just as well. Anyone with Cr2O3 and NaNO3 could give it a shot and see if it works.
Tim
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bfesser
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Thread Split
2-8-2013 at 21:16 |
plastics
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Fairly straightforward synthesis of CrO3 ('Preparation 62'). Tried it myself. Remember not to use standard filter paper to isolate the
crystals!
http://www.sciencemadness.org/talk/viewthread.php?tid=6116
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blogfast25
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<del>@bfesser: what does 'split thread' mean? Split into what and where?</del>
@plante and 12AX7: that's really interesting. The only snag is that chlorates and nitrates will become increasingly difficult to obtain in the EU, due
to more chemophobic legislation being brought on.
@12AX7: the link I brought up higher up is one of your pages, is it not? Yet it prescribes alkali?
[Edited on 3-8-2013 by blogfast25]
<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: del,
off-topic question answered <a href="viewthread.php?tid=25472">elsewhere</a>]
[Edited on 3.8.13 by bfesser]
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plante1999
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Blogfast, I have tried a 1/1 mix of sodium/calcium hydroxide, chromium source and air, it take a while, but eventually all dissolve.
I never asked for this.
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bfesser
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Thread Split
3-8-2013 at 08:18 |
blogfast25
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Quote: Originally posted by plante1999  | | Blogfast, I have tried a 1/1 mix of sodium/calcium hydroxide, chromium source and air, it take a while, but eventually all dissolve.
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What's the idea with the slaked lime? Higher melt temperature?
1/1 molar or 1/1 w/w?
[Edited on 3-8-2013 by blogfast25]
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bfesser
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Thread Split
3-8-2013 at 08:50 |
plante1999
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by plante1999 | | Blogfast, I have tried a 1/1 mix of sodium/calcium hydroxide, chromium source and air, it take a while, but eventually all dissolve.
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What's the idea with the slaked lime? Higher melt temperature?
1/1 molar or 1/1 w/w?
[Edited on 3-8-2013 by blogfast25] |
1/1 molar, to cheapen the cost and increase Oxygen absorption, in industry I believe pure calcium oxide is used because they have very high temp
furnace.
I never asked for this.
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bfesser
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Thread Split
3-8-2013 at 09:53 |
blogfast25
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Any idea how the lime increases oxygen absorption? Or any references that confirm that?
[Edited on 3-8-2013 by blogfast25]
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plante1999
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Peroxide formation maybe?
I will try to find a reference.
I never asked for this.
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blogfast25
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These roastings of chromite/chrome sources with alkali in air are all done in rotary kilns: that points to very long residence times of the
reagent mixture in the furnace.
One industrial process I found uses Na2CO3 (ash), Chromite (Fe/Cr oxidic ore) and Dolomite (presumably a carrier - increases specific surface area of
the mix) in a cement kiln. After roasting the 'klinker' is then ground and water leached for NaCr2O4. Acidification with CO2 under pressure and
cooling drops out bicar and leaves a Na2Cr2O7 solution.
[Edited on 3-8-2013 by blogfast25]
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Fantasma4500
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| Quote: Originally posted by blogfast25 |
No, it's very likely to be lead chromate (PbCrO4), which is highly insoluble and a slightly surplus-to-requirements yellow pigment.
Dichromates exist in equilibrium with chromates:
Cr2O72- + H2O < === > 2 CrO42- + 2 H+
This equilibrium shifts to the right in alkaline conditions.
But because PbCrO4 is so insoluble, adding Pb2+ to a dichromate also pulls the equilibrium to the right by precipitation of
PbCrO4.
[Edited on 2.8.13 by bfesser] |
mmm well i cant say anything else but that its pure orange, in which shocked me abit when i looked up pictures of lead chromate, in which is yellow
alike any other chromate
but what i have 200g of is orange
i dont see how you can miscolour lead dichromate, or well lead chromate, or why
im quite sure it is lead dichromate, he doesnt always really give many fucks about the titles he smacks on the bags he sells with chemicals and
heatsealing
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