guys, we are going on a little excursion into the world of
titration. What is "titration"? Well, its a method
of finding out something that you want to know by setting up an
experiment in graded steps so that we can know when a certain
event takes place. For example, If you are near deaf, like me, and
go for a hearing test one the things that they test for is the
frequency range that your ears respond to. A person with normal
hearing can detect frequencies of about 20 to 20,000 cycles/sec.
As we get older we usually start to lose our high frequency
acuity. When I got out of the Navy, after being around aircraft at
high-power turn-up, for 4 yrs, my hearing acuity was about 20-
14,000 cps. Now, I suspect its about 500-10,000 cps.
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.
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 hasnt managed to track down and
throw in jail yet.
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.
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.
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
electricians tape. Dont forget the bottom. You are
doing this because silver nitrate is a little unstable in the
presence of light.
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.
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 aint a pro yet. It will work; its
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.
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 didnt 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.
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. Dont
you admire me for that? Sure you do! Mathematics aint
Basement Chemistry, but its sort of handy to have around
you will understand what went on in this titration, Im 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!
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 Ill
try to explain it some other way
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!!!