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Determination of Free Cyanide in Solution
Dr. A.K. Williams, Ph.D
San Pedro, Costa Rica


Anyway, the examiner will turn on an instrument that produces sound of a certain frequency and ask if you can hear it. If not, he increases the frequency and asks again. When he reaches a point where you can hear the sound, he has reached an "end point". He has "titrated" your hearing response.

Now, we are going to do the same thing to determine how much free cyanide is present in any solution of interest to us. I know someone is going to ask, "free cyanide, is that different from any other kind"? Well yeah, it is. "Free cyanide" is the cyanide in the solution that is not tied up by any metal or other chemical. It is "free" to react with gold or silver. Maybe "free cyanide" could mean the cyanide that the government/do-gooder complex hasn’t managed to track down and throw in jail yet.

Silver does react with cyanide to produce a water-soluble complex. However, if there is no "free cyanide" in solution the complex changes from potassium silver cyanide, KAgCN, to silver cyanide, Ag (CN) 2. The former is soluble (will dissolve) in water, the latter will not. This fact gives us the means to skin this cat too.

We are going on a chemo-trip that will allow you to determine how much cyanide is in any solution that you might run into. If you think you neighbor is dumping cyanide in the storm gutter in front of you house, you can catch him! For this titration you will need a few items from your scientific supply source. You will need a "burette" of either 25 or 50 cc capacity. This should be graduated in 0.1cc increments. This item is simply a long glass or clear plastic tube, accurately graduated, with a stopcock at the bottom so that you can control the flow of liquid from the tube. You will need an Erlenmeyer flask of 250-500 cc capacity. Yeah, you could use some other clear container but everyone really ought to have a couple of these anyway. You really should have 100 or 250 ml graduated cylinder. These are things you should have in the basement anyway. You will find them useful for many purposes. You will need some Silver Nitrate in crystal form. You could make it, but better to buy a little from scientific supply or from local drugstore.

Now, probably the most difficult part, you must weigh out exactly 13.07 grams of Silver Nitrate. This is the critical step because any error you make here will be magnified in your final results. This is your "standard Silver Nitrate solution" that you will be using for a long time. Now you put this in a container and add distilled or de-ionized water to exactly 1 liter or 1000 cc. Put this solution in a tightly stoppered bottle. A dark bottle is best but you can darken the bottle by wrapping it with black electrician’s tape. Don’t forget the bottom. You are doing this because silver nitrate is a little unstable in the presence of light.

Now, we are going to titrate some cyanide solution. I guess you have a cement mixer running with some good concentrates in it and we have to be sure that the cyanide concentration stays about where we want it. Take out a sample and filter it through a coffee filter. Take exactly 100cc of this filtrate and put it in the flask that you bought, or you favorite hi-ball glass if you insist. Fill the burette to above the "zero" mark with the Silver Nitrate solution. Now, carefully turn the stopcock and let the Silver solution run out until the liquid is at the zero level. Now we are ready to get it done.

There is a little art to titrating. You should be able to swish the CN solution in the flask in a circular motion while allowing the Silver Nitrate to slowly drip into it. Sort of like rubbing your stomach and patting your head at the same time. If it seems awkward, not to worry, you ain’t a pro yet. It will work; it’s just that a pro will do it a whole lot faster than you. Now to the guts of the thing. Just like the hearing examiner, you are looking for something to happen, the "end point". In this case the end-point will be when you see a permanent white precipitate (cloud) form. You will probably see a white precipitate form and in a second or two, disappear. This is not what you are looking for but it is an indication that you are getting close, so go careful. You will find a point at which 1 drop of Silver solution will make a cloud that does not dissolve. STOP! This is the end-point that you are looking for.

Now, you simply read off the burette how many ml (cc) of silver solution that it took to get to the end-point. Due to some manipulation that I did for you that you didn’t even know about, the ml of silver solution that you used divided by ten will equal the percent of free cyanide in your cement mixer solution. If it is getting low, add a little more cyanide, etc.

I know that you were thinking about things like "why did this character tell us to weigh out a screwball amount of Silver Nitrate like 13.07 gr.". Well, due to the magic of another science, mathematics, I was able to convert the numbers from molar values into something that would be easier for you to handle. Don’t you admire me for that? Sure you do! Mathematics ain’t Basement Chemistry, but it’s sort of handy to have around sometimes.

Just so you will understand what went on in this titration, I’m going to explain it (I hope) in chemical terms. After all, If someone should stop you on the street and ask "sir, could you explain to me how to titrate cyanide with Silver Nitrate", You would, of course be able to tell him more than he ever wanted to know about the subject. That, by the way, is the secret to being an expert authority on any subject. When someone even mentions a subject, you jump in and tell him or her so much that they run off screaming into the night. Now you are the expert!

If we have a reaction going we would expect to have some gold and silver reacted with cyanide. Might be some other metals reacted also. This is not what we are concerned with. This is cyanide but it has already captured something. We are more concerned with whether or not we have enough cyanide left to get the rest of the precious metals dissolved. Free cyanide! When we titrate the solution, we are adding Silver Nitrate. We know that Silver reacts with cyanide to produce a water-soluble salt like potassium silver cyanide, KagCN. As we titrate the unknown solution with silver we are producing this compound. When the Cyanide is all used up we start to make Silver Cyanide, AgCN. This chemical is not soluble in water and will precipitate as a white cloud. This is our end-point and it indicates the point at which there is no more free cyanide in the solution. The amount of silver needed to reach this end-point tells us exactly how much cyanide was in the solution. Does that make sense to anyone? If not, let me know and I’ll try to explain it some other way

At this point I can just see a whole bunch of you, who would otherwise be watching TV with a beer or two, down in the Basement oxidizing, reducing, titrating, and extracting all sorts of great things. DAMN, I LOVE IT!!!


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