Notes on Experiments with OTC 57% HI(aq)
Feeling a little stupid at constantly re-inventing the wheel, when so many members know exactly how to do this (because they actually studied chemistry in school), SWIA investigated hydriodic acid.
Checking the criminal code, SWIA did not find HI to be regulated in this country. In the land of the maple leaf peso, hydriodic is not illegal, at least according to a cursory check of the on-line criminal code. (Any information to the contrary would be greatly appreciated. Neither SWIA nor I desire to break any laws.)
SWIA used TFSE and checked Rhodium’s chemistry page for information and methods. TFSE revealed mass confusion. Several members declared the Rhodium Chemistry write-up to be bullshit (but for all the wrong reasons). One bee, SuperAssman, posted a full page of his best chemistry on phosphates declaring that the write-up on the Rhodium Chemistry page was not physically possible--he also stated that terbium, of all people, needed a chem lesson. Then SuperAssman
Post 171036 (missing)
(SuperAssman: "Re: Hydriodic Acid", Stimulants) boastfully concluded by betting both his gonads that HI could not be made from NaI or KI with H3PO4. (Boy, is he gonna feel funny walking around without any balls).
In the face of such disparate views on the Hive about a seemingly simple inorganic reaction, SWIA went to the library. A review of the Kirk-Othmer and chem texts showed that the assertions of several other bees that HI could be made from reacting either HCl or H2SO4 with either KI or NaI to be absolutely wrong. HI cannot be made by reacting KI or NaI with either HCl or H2SO4, which proves Rhodium correct as usual, since he has posted this fact several times, patiently trying to fend off the flood waters of misinformation.
There are only five practical ways to make hydriodic acid, as far as SWIA could determine:
1. React H2 and I2 under pressure and over catalyst at high temperature. (Complicated and expensive engineering. I2 not exactly OTC in these perilous times.)
2. React I2, red P, and water. (Definitely not OTC. Sort of defeats the purpose. However, this is how it’s done in sunny California by cooks in multiple 22L flasks and on a large scale, according to review of news articles.)
3. Pass a gas current of H2S through an aqueous slurry of I2. Filter elemental S. Drive off residual H2S with heat. Distill hydriodic acid. (Second runner up as a possibility, but I2 not OTC and H2S quite deadly. Hydrogen sulfide gas numbs the senses after a minute or so. Then a person no longer smells it. It is just as deadly as hydrogen cyanide(HCN) or arsine (H3As). The H2S generator would have to be perfectly engineered and monitors installed to detect leaks.)
4. React BaI with H2SO4. Forms BaSO4 and HI. (This would be the easiest reaction, except for fact that BaI is rare, expensive, and poisonous.)
5. React KI or NaI with H3PO4, per Rhodium chemistry page. (OTC ingredients, simply reaction, seems perfect, why do so many bees insist it won’t work?)
What to do in the face of such confusion on Hive and such uncertain choices?
SWIA began experimenting.
After a few days the situation became clear(er). The write-up on Rhodium’s page is more-or-less correct inasmuch as HI can be made from KI and H3PO4, but it leaves a lot out, which makes SWIA wonder whether the guy actually did it.
Here’s how to make 57% HI(aq) from OTC ingredients with at least 55% yield mol/mol. Improvements on yield are expected as experience grows. Improvements will be posted.
KI (potassium iodide) can be purchased OTC. It is a heavy white powder. It is currently being sold online as anti--radiation prophylaxis at exorbitant prices. Check it out, but don’t buy it as pills--too expensive. Use the yellow pages and you will find it in abundant supply. SWIA obtained (USP) KI for $us 36.75/kg retail, which isn’t too much of a mark-up over its world market price of $us 25/kg. Don’t PM me for sources, I personally don’t know anything about this, just what SWIA tells me.
H3PO4 (Ortho-phosphoric acid) can be purchased as “pH Down” from hydroponics stores. Technical grade is 75%. A clear syrupy liquid. Below is a chart showing b.p. of H3PO4 at various grades. A simple way to determine the concentration of your H3PO4 is to measure the boiling point. SWIA obtained 75% H3PO4 for $15/gallon, which is high retail. Bulk will be less. The guys at the store had no clue as to the concentration of the “pH Down”, so SWIA boiled it and discovered it was 75%. DO NOT use the tile cleaner containing phosphoric acid as was recommended in several posts--this tile cleaner is only 20% H3PO4 and 25% surfactants (soap) and other additives--it will NOT work.
Per Cent Boiling Point
H3PO4 ºC
10 100.2
20 100.8
30 101.8
50 108.0
75 135.0
85 158.0
105 **1
115 **2
**1 --A true boiling point can not be given, since, in the liquid state, 105% phosphoric acid reverts to a mixture of ortho-phosphoric acid and condensed phosphoric acids. At 325ºC, the vapor pressure is 760mm.
**2 --At 532ºC the vapor pressure is 760mm.
The write-up on Rhodium’s chemistry page infers the following reaction mechanism:
KI(s) + H3PO4(s) => HI(g) + KH2PO4(s)
However, this does not actually happen in SWIA’s experience. SWIA does not know what the reaction mechanism is, perhaps the formal chemists could help out. But the one inferred on the Rhodium page is definitely not it. Besides, crystalline H3PO4 is not easy to find. And if liquid H3PO4 was used by the person writing the synth, then his (her) write-up is dead wrong on several key points.
As best SWIA can fathom, the initial reaction mechanism is like this:
KI(s) + 2H3PO4(aq) + delta t(65ºC) => HI(aq) + KH2PO4•H3PO4 (aq)
Some kind of reaction is readily observed. Upon heating, the mixture quickly turns the characteristic brown color of hydriodic acid and this acid will boil over at 125-127ºC and can be condensed and recovered. According to the above reaction (and subsequent reactions--see next paragraph), from 1 to 2 moles of H3PO4 will react with 1 mole of KI to produce 1 mole of HI. However, this does not occur--at least SWIA has not worked out the ratios and conditions to make it occur. There must be other reactions unknown to SWIA. The initial brown acid is not even close to being a stoichiometric amount. Once the initial brown acid has been boiled off, other reactions must take place. Hydriodic acid continues to be generated at higher temperature in the flask, although it still comes over at 127ºC, even though the temperature of the clear goo bubbling in the bottom of the flask is over 200ºC. The further formation of acid may be due to dehydration of ortho-phosphoric acid and formation of tri-potassium phosphate, according to this equation which SWIA cannot balance because the waters of hydration of tri-potassium phosphate vary from 2 to 12:
KI +(?)KH2PO4•2H2O+ delta t => HI•H2O + (?)K3PO4•(2 to 12?)H2O
(I know this equation is not balanced--don’t flame it--this is just SWIA’s idea of what might be going on)
Metaphosphates probably form--these are polymers--long chain or ring molecules--also known as condensed phosphates. Phosphate reactions can be complex and much of what happens to ortho-phosphoric acid at higher temperature as it dehydrates and polymerizes is a few steps beyond SWIA’s level of understanding.
Why SWIA cannot achieve 100% recovery of HI is a mystery, since there are H+ ions in abundance with H3PO4. The first H+ is highly active. The second less, the third even less, but they all should react with KI to form HI. Unless iodine is attaching itself to a phosphate molecule somehow, it should all come over as HI, either as hydriodic acid or as HI gas. When SWIA figures it out, he’ll let everybody know. Of course, if a wise bee would tell him, much better, less work.
Contrary to what is stated in the write-up on Rhodium’s page, HI gas is not produced in abundance by reacting H3PO4 with KI. Even reacting KI with 105% phosphoric acid yields little gas. This was proven by experimentation. 105% H3PO4 was obtained by boiling 75% acid to 186ºC and calculating weight loss as water loss (close enough). Ortho-phosphoric acid cannot be boiled to dryness. It polymerizes... Anyway--only about 10% of the iodine comes over as hydrogen iodide--the bulk is hydriodic acid.
In the write-up on Rhodium’s page you are instructed to connect a hose to a flask containing the ingredients, heat up the ingredients to a balmy 60ºC and dip the other end of the hose (connected to an aquarium bubbler) into a flask filled with water and prepare to recover HI gas in the water, weighing the solution until a 57% HI solution (hydriodic acid) has been formed. SWIA finds this hard to believe, given that liquid phosphoric acid (even boiled to 105%) gives mostly hydriodic acid, not hydrogen iodide. Anhydrous phosphoric acid (crystalline powder) is not easy to find, whereas liquid phosphoric acid is plentiful and OTC. SWIA wonders what the conversion rate of KI to HI was using the Rhodium page write-up method, which we have to assume used powder H3PO4.
SWIA’s conducted several experiments--they go like this (qualitative description, not quantitative--that will come later):
KI powder and 75% H3PO4(aq) were combined in a erhlenmeyer flask placed on a hotplate. A still head and condenser were attached to the flask. A receiver was attached to the condenser. After the receiver, a trap was connected; after the trap, a second receiver was attached, which was filled with dH2O to capture any HI gas. All under a fume hood.
The contents of the flask were heated. At 50ºC there was an obvious and immediate reaction. HI could be seen to form, which reacted with the water portion of the 75% ortho-phosphoric acid to form hydriodic acid. Upon further heating, the contents of the flask boiled. The distillate that came over at 125-127ºC was collected. This was 57% hydriodic acid. No HI gas came over while the brown acid was boiling.
[Hydriodic acid is formed from the combination of hydrogen iodide--HI(g)--and water. At a concentration of 57% it forms a constant boiling mixture that will boil at 125-127ºC at sea level. Dilute hydriodic acid boils at lower temperature. A small portion of dilute acid will come over until the temperature at the still head stabilizes at 127ºC. The dilute acid can be further concentrated by reboiling, but there will always be a fraction that is less than 57% and which comes over at a lower temperature. SWIA imagines that the dilute acid could be added to subsequent reactions of KI and H3PO4. Recycled so to speak, and thus no HI(aq) would be lost. Anyway, not much dilute acid is formed. Pretty quickly the temperature at the still head will reach 127º and stay there. The fraction that comes over at 127ºC can be collected as 57% hydriodic acid. Density 1.7. A dark brown fuming liquid.]
Now the curious part begins. Continuing heating, all the brown liquid will boil over and be collected. Nowhere near the theoretical yield of hydriodic acid. What remains in the flask is a viscous clear syrup that continues boiling. Amazingly, from that clear syrup, yellow-brown hydriodic acid will continue to condense and come over. “Amazingly”, because supposedly all the H2O has been boiled off, so in fact the water portion of the hydriodic acid must be coming from the dehydration of H3PO4. A certain amount of HI gas is formed, but not a significant amount. However, there is a tendency toward suck-back, so the stop-cock and trap are important.
Even after the contents of the flask are boiled to near “dryness”, the total amount of hydriodic acid collected (condensed 57% HI•H20 and dilute HI(aq) from the bubbler) is about 60% of theoretical. (The contents of the flask are never solid while heated, but remain syrupy. Upon cooling, a hard, dense white cake forms in the bottom of the flask. This cake is extremely hard and difficult to remove from the flask. It is not very soluble in water.) K3PO4? Or a polymerized metaphosphate combined with the tri-potassium phosphate in a complex lattice? Anybody know?
If the contents of the flask are not boiled to “dryness” a different type of residue forms, more waxy, almost greasy. Both of these residues coincide with descriptions of different types of condensed phosphates, according to the degree of dehydration.
Here’s the million dollar question: Where is the rest of the iodine? Not in the receivers, where SWIA would like it to be, that’s for sure. Does it combine with phosphate? K2HPO4 (perhaps formed by the continued reaction of KI and KH2PO4) is basic, does HI combine with it?
When SWIA solves the mystery of the missing iodine and gets 95%+ yield of HI, a detailed write-up will follow. But for now, even though yield is not at maximum, SWIA can affirm that OTC HI(aq) can be produced from KI and H3PO4 with at least 60% conversion through simple heating and condensing.
SuperAssman owes terbium an apology along with those two gonads. Ouch, that’s gonna hurt. The gonads, I mean.
SWIA would appreciate the comments of informed members. Where is the remaining iodine?
Regards
Argox