Sciencemadness Discussion Board

prussic acid preparation ?

slyder - 17-9-2005 at 02:09

hello all ! :)

i was recently get about 300g of 98% of pure potassium cyanide (KCN) from fluka

now i would like to know if someone know what i need to do to and what is synthesis for preparation of prussic acid(hydrocyanic acid HCN) ?

is sufficient if i pour H2so4 on KCN ? is posible to obtain hydrocyanic acid in this case ?
by mine knowladge if pour h2so4 on KCN it produce only HCN gas but it produce also hydrocyanic acid (hcn) liquid in this case ?

i would like to please you if you can gave me accurate procedure for preparation of hydocyanic acid HCN ?

sorry for my bad english!

thank you very much !


unionised - 17-9-2005 at 03:21

If you need to ask questions like that you don't know enough to be playing with cyanides. Cyanide poisoning isn't a nice way to die and you are quite likely to find that out the hard way.

slyder - 17-9-2005 at 03:43

lol haha i do not planing to kill oneself LOL! i just want to make prussic acid only for fun !

yet anyone who can help me out with preparation of prussic acid ?

>don't double post please

[Edited on 17-9-2005 by Ramiel]

Ramiel - 17-9-2005 at 03:59

If you add sulphuric acid to your KCN, you will more than likely produce enough HCN gas to kill yourself. Please don't 'play' with these kinds of chemicals.

If you die, I don't want your blood on my hands.

garage chemist - 17-9-2005 at 05:12

Actually, with conc. H2SO4 you won't get HCN because conc. H2SO4 instantly oxidises the formed HCN to CO and N2. Concentrated H2SO4 is rather an oxidiser than an acid.

Only dilute H2SO4 or HCl works.

Tell me one thing- what do you want the HCN for? There are some interesting preparations using it, that's sure (like sodium cyanoborohydride from sodium borohydride) but without efficient fume hood and gas mask, you'll die.

What kind of equipment do you have?

Mr. Wizard - 17-9-2005 at 06:39

Asking how to prepare HCN is like a kid with a box of matches and a bucket of gasoline asking how to start a fire. Nobody wants to be responsible for the probable death to follow. If you have to ask you shouldn't be doing it.

Fleaker - 17-9-2005 at 08:10

As others mentioned, if you have to ask, then you should not attempt it with something like cyanide. It's a quick, painful way to die and if you don't have the proper equipment and knowledge, you're setting yourself up for it.

At least read through this so you better know the dangers:

Interesting that it can be a liquid at 78F (26C). I did not know that.

[Edited on 17-9-2005 by Fleaker]

Taaie-Neuskoek - 17-9-2005 at 08:46

Besides the injury one could do to itself, think of the people around you. Even with proper ventilation I would not play with the stuff in these quantity's. It is lighter than air, but if the neighbour's child is just passing by, you cán do serious damage.

At a chemical company a couple of aquantices of me work for, there are only a few people allowed to work with HCN, and also only a couple of safety guys who are allowed to go in there if something bad happens... please look out, and use only very proper and good glassware when you handle it, a single breakage will most likely kill you.

[Edited on 18-9-2005 by Taaie-Neuskoek]

mick - 17-9-2005 at 09:27

Just to add a bit more info from ACS prudent practice in the lab.


Substance Hydrogen cyanide (Hydrocyanic acid; prussic acid) CAS 74-90-8
Formula HCN

Physical Properties
Colorless or pale blue liquid or gas
bp 26 °C, mp -13 °C
Miscible in water in all proportions

Bitter almond odor detectable at 1 to 5 ppm; however, 20 to 60% of the population are reported to be unable to detect the odor of HCN
Vapor Pressure 750 mmHg at 25 °C
Flash Point -18 °C
Autoignition Temperature 538 °C

Toxicity Data
Approx LD oral (rat) 10 mg/kg
Approx LD skin-1500 mg/kg (rabbit)
LC50 inhal (rat) 63 ppm (40 min)
PEL (OSHA) 10 ppm (11 mg/m3)—skin
TLV-TWA (ACGIH) Ceiling 10 ppm (11 mg/m3)—skin

Major Hazards

High acute toxicity; inhalation, ingestion, or skin contact may be rapidly fatal.


The acute toxicity of hydrogen cyanide is high, and exposure by inhalation, ingestion, or eye or skin contact can be rapidly fatal. Symptoms observed at low levels of exposure (e.g., inhalation of 18 to 36 ppm for several hours) include weakness, headache, confusion, nausea, and vomiting. Inhalation of 270 ppm can cause immediate death, and 100 to 200 ppm can be fatal in 30 to 60 min. Aqueous solutions of HCN are readily absorbed through the skin and eyes, and absorption of 50 mg can be fatal. In humans, ingestion of 50 to 100 mg of HCN can be fatal. Because there is wide variation in the ability of different individuals to detect the odor of HCN, this substance is regarded as having poor warning properties.

Effects of chronic exposure to hydrogen cyanide are nonspecific and rare.

Flammability and Explosibility

Hydrogen cyanide is a highly flammable liquid. Liquid HCN contains a stabilizer (usually phosphoric acid), and old samples may explode if the acid stabilizer is not maintained at a sufficient concentration.

Reactivity and Incompatibility

HCN can polymerize explosively if heated above 50 °C or in the presence of trace amounts of alkali.

Storage and Handling

Because of its high acute toxicity, hydrogen cyanide should be handled using the ''basic prudent practices" of Chapter 5.C, supplemented by the additional precautions for work with compounds of high toxicity (Chapter 5.D) and flammability (Chapter 5.F). In particular, work with HCN should be conducted in a fume hood to prevent exposure by inhalation, and splash goggles and impermeable gloves should be worn at all times to prevent eye and skin contact. Never work alone with hydrogen cyanide. HCN should be used only in areas free of ignition sources. Containers of HCN should be protected from physical damage and stored in areas separate from ignition sources and other materials. Hydrogen cyanide should not be stored for extended periods (>90 days) unless testing confirms product quality.


In the event of skin contact, immediately wash with soap and water and remove contaminated clothing. In case of eye contact, promptly wash with copious amounts of water for 15 min (lifting upper and lower lids occasionally) and obtain medical attention. If hydrogen cyanide is ingested, obtain medical attention immediately. If HCN is inhaled, move the person to fresh air and seek medical attention at once. Specific medical procedures for treating cyanide exposure are available but usually must be administered by properly trained personnel. Consult your environmental safety office or its equivalent before beginning work with hydrogen cyanide.
In the event of a spill, remove all ignition sources. Cleanup should be conducted wearing appropriate chemical-resistant clothing and respiratory protection.


Excess hydrogen cyanide and waste material containing this substance should be placed in an appropriate container, clearly labeled, and handled according to your institution's waste disposal guidelines. For more information on disposal procedures, see Chapter 7 of this volume.

This is my stuff now, neutralisation is usually with hypochlorite (bleach) to cyanate or ferrous salts. The old cyanide antidotes, amyl nitrite or some iron injection + thiosulphate are not generally recommended now since if you have not got cyanide poisoning they can kill you. The one thing that was recommended is have an oxygen cylinder handy and some one who knows how to use it.


Polverone - 17-9-2005 at 13:03

Go to the <A HREF="">Library</A> and get the Handbook of Preparative Inorganic Chemistry. It has a proper laboratory preparation of HCN from alkali cyanides.

praseodym - 18-9-2005 at 01:38

The most convenient way, I think, would be to distill potassium ferrocyanide with dilute sulphuric acid to get a solution of hydrocyanic acid.

Nicodem - 18-9-2005 at 02:09

Originally posted by slyder
lol haha i do not planing to kill oneself LOL! i just want to make prussic acid only for fun !

I would seriously question the intentions of someone who writes an insane statement like that without any further explanation except that it is not meant to be used for a suicide.
I wander what kind of fun someone can attain from having some hydrogen cyanide. Since when is killing such fun? (OK, I guess there are many people who consider killing as fun, but I do not support this attitude) I apologize in advance if I somehow failed to see any other ‘fun’ use of HCN. I guess you could use it as a perfume for that half of the population that can smell it.

praseodym - 18-9-2005 at 02:32

Haha, I agree with you, Nicodem. Even if he doesn't kill himself, small doses can already cause pains in the head, giddiness and nausea, perhaps accompanied by paralysis of respiration and of the spinal cord.

woelen - 18-9-2005 at 09:50

@slyder: Please explain what equipment you have and how you think you assure that you do not any harm to yourself and people around you.

How much of the gas do you want to make? And, if you make it, how do you think you want to store it?

Think twice (or trice) before you proceed with this. The best thing you can do with your 300 g of KCN otherwise may be to put it in a safe container, out of reach of anyone and first educate yourself about the risks of this, before even an attempt is made to open that container again.

mick - 2-10-2005 at 09:42

Potasium cyanide will hydrolize to HCN in the presence of moist air (I think CO2 is enough to do it).

Cloner - 9-11-2005 at 13:43

Related to the cyanide and hydrogen cyanide, I was coming to this forum to discuss antidotes, and find this more or less recent thread.

It's not like I am very afraid of chemicals but the T+ symbol on this one should be taken seriously. I once caught a little whiff of it, instantly causing a panic attack. In the seconds that followed it felt like cold water flowed into my brains from the arteries, which is probably due to oxygen depleted blood. After that I got dizzy, but I got over it quickly. This was only a little bit of the stuff. Think of the vapor from a plating solution of potassium cyanide at PH 12 and RT.

somehow, the effect of hydrogen cyanide scares me irrationally, that i would rather not breathe even a little bit that I am sure won't harm me. This damn shit also penetrates the skin by the way, thus a solution of cyanide on the skin is a bad idea as well. found that out the hard way too. lastly, an open container of cyanide catches water and carbon dioxide from the air, releasing the deadly gas, so it's also a storage hazard.

About the topic of antidotes: I found out from wikipedia that the following antidotes are used: respiratory amyl nitrite and intravenous sodium nitrite, 4-dimethylaminophenyl, and hydroxy-b12 (like vitamin b12 but hydroxy instead of cyano). they are followed by intravenous sodium thiosulphate.

What interests me now is whether the antidotes are a way to immunize oneself to the unpleasant effects of hydrogen cyanide beforehand, and to what dose these medicines are still helpful. How to best prepare an intravenous dosis of sodium thiosulphate (intravenous potassium would be a very very bad idea I think?)? How much to use? I suppose the eye shower water could be used as a solvet since it is basically water with some salts in it that make it osmotically biologically compatible.

And the last point: disposal of cyanides. The obvious would be to dilute, put in a drop funnel and slowly add hydrochloric acid while letting the fume get out in the outside. But this is crude. Even though i read in this thread that sulfuric acid would probably destroy cyanide, I'd rather not try that out, but how about other oxidizing reagents like hydrogen peroxide?///

[Edited on 9-11-2005 by Cloner]

[Edited on 9-11-2005 by Cloner]

Theoretic - 9-11-2005 at 14:29

I think H2O2 oxidizes cyanides to cyanates, which are harmless.

neutrino - 9-11-2005 at 20:13

Industrially, Caro's acid (peroxymonosulfuric acid, H<sub>2</sub>SO<sub>5</sub>;) is used for safely destroying used cyanide. It efficiently oxidizes cyanide to harmless cyanate.

BromicAcid - 9-11-2005 at 20:39

Wha... wha... wha.... what about just bleach, that is what is syonomous in my mind with cyanide destruction, works like a charm and cheap too (Like Mick said further up thread). Even the MSDS from J.T.Baker for potassium cyanide lists that as a method to control spills of the product
Collect material and place in a closed container for recovery or disposal. Do not flush to sewer! Decontaminate liquid or solid residues in spill area with sodium or calcium hypochlorite solution.
As for one of the other things mentioned here by Cloner about immunization against cyanide to some degree, one of the books in my collection titled "Assorted Nasties" has this to say on cyanide production
It is a good idea to have a supply of cyanide antidote ready in case you are affected by the fumes. These are sodium thiosulfate, a common photographic chemical, and amyl nitrite, a heart drug. Butyl nitrite is availible in some "sex" shops as a room odorant, and may be substituted for the amyl nitrite. A few hours before beginning production of the actual cyanide take a capsule containing 500 mg of sodium thiosulfate. It is comparatively non-toxic but may cause a small bout of "egg burps".


If you feel yourself developing the symptoms of cyanide poisoning, break an ampoule of amyl nitrite and inhale its contents. Seek medical attention. The foregoing does not mean that with the antidotes you can inhale fumes with impunity, only you probably will not die from it. Even a sub-lethal does of cyanide is extremely unpleasent.
It should be noted that the author is not describing working with hydrogen cyanide, just the preparation of cyanide by the method involving charcoal, scrap iron, etc. Where some HCN may be inadvertantly generated somehow. Anyway, I remember reading something awhile back where people who were exposed to sub-lethal doses of HCN experienced a much greater incidence of cancer years down the road, kind of an interesting side note.

[Edited on 11/10/2005 by BromicAcid]

Cloner - 10-11-2005 at 05:30

hmmmm... is the cancer by chance related to the use of intravenous nitrites? I find that a horse medicine, much rather would I use the hydroxy vitamin b12. According to wikipedia, this is the common cyanide medicine in France at least, even though it is probably somewhat expensive.

Because I use it for plating and it works like a charm, I would like to keep the cyanide, but be prepared for accidents.

hydrogen cyanide being formed by any process involving cyanides. Also, touching the gas gets me the cyanide feeling, so even a full face mask is only limited protection.

[Edited on 10-11-2005 by Cloner]

[Edited on 10-11-2005 by Cloner]

Douchermann - 10-11-2005 at 06:45

Seriously, what do you plan to do with HCN, its the same stuff they use in gas chambers in prisons to kill people. If you do plan to make it, keep the acid cool while you add the KCN so that it doesn't produce the gas. It is an exothermic reaction.

unionised - 10-11-2005 at 12:49

Oral thiosulphate as a prophylactic for defending against cyanide.
That will work provided that thiosulphate is stable in the strongly acid conditions in the stomach.
Oh Dear!
If you are going to work with HCN please don't take advice that high school chemistry should rule out as bull.
The use of bleach as a means to destroy cyanide works but be sure to keep the pH under control.