# Cementation -- Reaction Direction (Electrochemical Potential



## Anonymous (Apr 12, 2008)

Dear Guys,

This site is incredible, I wish there was something like this when we started 8 years ago. I just ran into it by mistake; as it happens with the best things in life. Good work. 

Since you provide a wealth of information, I must reciprocate. However I have been thinking with what should be interesting for the group. So after thinking for a while I decided to share one of the things I usually get asked often by many people. 

Why we use some cementing metals instead of others. In chemical terms it boils down to the following reaction

n A(+) + B(0) ---> n A(0) + B(n+)

or perhaps the opposite.

n A(+) + B(0) <n> Na(0) eV= - 2.7
Al(3+) + 3 e(-) ----> Al(0) eV= -1.7
Ti(2+) + 2 e(-) ----> Ti(0) eV= - 2.0
Cr(2+) + 2 e(-) ----> Cr(0) eV= -0.91
Zn(2+) + 2e(-) ----> Zn(0) eV= - 0.76
Cr(3+) + 3 e(-) ----> Cr(0) eV= - 0.74
Fe(2+) + 2e(-) ----> Fe(0) eV= - 0.45
Ni(2+) + 2 e(-) ----> Ni(0) eV= - 0.26
Sn(2+) + 2e(-) ----> Sn(0) eV= - 0.14
Pb(2+) + 2 e(-) ----> Pb(0) eV= -0.13
Fe(3+) + 3 e(-) ----> Fe(0) eV= - 0.04
H(+) + e(-) ----> 1/2 H2 eV= 0.00
Cu(2+) + e(-) ----> Cu(0) eV= 0.34
Rh(2+) + 2 e(-) ----> Rh(0) eV= 0.60
Rh(3+) + 3 e(-) ----> Rh(0) eV= 0.76
Fe(3+) + e(-) ----> Fe(2+) eV= 0.77
Ag(+) + e(-) ----> Ag(0) eV= 0.80
Hg(2+) + 2 e(-) ----> Hg(0) eV= 0.85
Pd(2+) + 2 e(-) ----> Pd(0) eV= 0.95
Pt(2+) + 2 e(-) ----> Pt(0) eV= 1.10
1/2 Cl2 (0) + e(-) ----> Cl(-) eV= 1.4
Au(3+) + e(-) ----> Au(0) eV= 1.5
HClO() + e(-) + H(+) ----> 1/2 Cl2(0) +H2O eV= 1.6



You may ask probably..... and now what????

From this format it is quite difficult to see if one reaction goes forward or backwards. Lets present this same information is another format; but with fewer pairs (Fe, Cu, Ag, Au). In this table we place the highest oxidized estate in the top part of the line and the lowest oxidized state in the lower part. They are arranged with increasing electrochemical potential, the highest potential to the right. As shown in the first picture. 

The reaction will proceed spontaneously if the highest oxidized species is on the upper right hand side of the equation, with the lowest oxidized estate to the lower left hand side. 


For example. (please see the attached diagrams)

The cupreas method for depositing Gold from auric solution is using Ferrous sulfate (FeSO4) which has (Fe(2+)). That yields the following equation

Au(3+) + 3 Fe(2+) ----> Au(0) + 3 Fe(3+). diagram 1

Also we know for a fact, and it has been widely talked about in this forum that Silver can be precipitated with metallic copper.

2 Ag(+) + Cu(0) ----> 2 Ag(0) + Cu(2+). diagram 2

To recuperate Copper form solution we can use metallic iron.

3 Cu(2+) + 2 Fe(0) ----> 3 Cu(0) + 2 Fe(3+) diagram 3

In the old times when gold was obtained by amalgamation, Mercury was dissolved into solution using Nitric acid and later brought down form solution with a copper plate.

Hg(2+) + Cu(0) ----> Hg(0) + Cu(2+) diagram 4


As a last example let me make the HCl - HClO reaction that has a little twist.

HClO + HCl ---> Cl2 + H2O diagram 5

Au(0) + 3 HClO + 4HCl ----> HAuCl4 + 3/2 Cl2

2 Au(0) + 3 Cl2 ---> AuCl3 *


The reaction of Hydrochloric Acid and Sodium hipochlorite is spontaneous, producing in both reactions Chlorine gas. If we look closely, Gold is right in the middle of the reaction pair. Therefore, Gold also reacts with HClO spontaneously, and generates Chlorine gas as well.

Therefore we get a nice mixture, HCl provides the Anion Cl(-), also provides an adequate low PH so that Gold can form the HAuCl4 in solution. HClO reacts with Gold and HCl forming Chlorine gas that also reacts with gold. Most important is that everything is done with Halogen chemistry; VERY CLEAN. Once the reaction is over.... just let it sit overnight with aeration and you get rid of HClO. (There is no nitric acid to get rid of).


Everything looks exciting so far. But there is a second condition. In order for the reaction to proceed, we need to identify the complementary suitable Anion. For example Gold does not form a Nitrate nor a Sulfate, only Chlorides. So in order to Oxidize Gold we need to include the corresponding Anion; In this case Cl(-).

The last thing to consider is the reactivity of the metal to a specific etching agent. For example etching silver with a Chlorine Salt will create AgCl which precipitates and passives the surface and will not allow the reaction to proceed.


The possibilities are endless.


We can gather some interesting information form this graph.

From this table we can see that Gold is the highest electrochemical potential for metals. Therefore, if gold got oxidized in any form, as soon as it got in contact with any other metal, it would turn itself back to metallic form. Also Gold has very few Anions with which it wants to combine.

We can see that Aluminum and Zinc have one of the lowest electrochemical potentials and therefore would imagine that it would drop any metal; actually they do. Zinc is used throughout industry to cement any other metal. But the former has an Aluminum Oxide layer that passives any reaction and prevents this form happening; Anodized aluminum is one of the most weather resistant materials. However if it is placed in a slightly acidic solution it corrodes quite rapidly. (Please be cautions with Aluminum salts, because they are unstable and can produce explosions if mixed with some substances.)

When considering a metal to cement, always look for the lowest cost metals you can get. Many times people use compounds or metals to the left hand side that are probably more expensive that the metal they want to recuperate; perhaps it is the only one they have. For experimental purposes it sounds great , however for commercial purposes.... it is not feasible.


So for any practical purposes I do the following. Here are some recommendations

1) Copper is a very nice metal to work with, its chemistry is simple, their salts are very nice and colored. Therefore it can be used readily to see if it is present. I use it all the time to drop silver. It allows me to see if the final silver has traces of copper or not. It is between the table and drops any Precious metal above it. It can be obtained readily everywhere and it can be recuperated easily. Its detection is very easy, just add NH4OH to a small sample and if it turns dark blue once is alkaline .... you got it.

2) To recuperate my copper and reuse it all the time, I use Iron. Any piece of iron can do it, a food, an iron pipe, hangers.... you name it. Sometimes I use regular iron nails if needed. Iron is the lowest and least expensive of the metals I know and it can be disposed into nature without any concern. Just neutralize your solutions and there they go.

3) Always keep you solutions as simple as possible.

Also, I have seen that many times without knowing, the solutions begin to get contaminated with other metals. In this situation the mixture gets so complicated that you loose track of what is happening. 

I remember one time that a person approached me with a Aqua Regia solution of Gold, Copper, Silver, Platinum and Palladium which they have added too much Urea, and god know what else. The solution cooled and you could see the Urea crystals in the bottom. Then they added sodium bisulfite in excess and nothing precipitated. They came to me for help...... mmmmm.... what to do. We just started form scratch... evaporated to dryness, incinerated everything in the mixture al high temperature, dissolved whatever was left, precipitated with H2S, and start all over again. At the end it also had Iron, Tin and Zinc..... we got a good yield after all, but had a great challenge.

Sometimes, people use Aluminum foil to drop metals. However they do not consider that sometimes, aluminum foil has Lead and Antimony to make it ductile in nice thin sheets. If you use aluminum foil; just verify that it does not have any other metals. Also they use aluminum cans, and tell me they do not work... Remember that sometime aluminum cans have inks and coatings that retards the reaction.

Same with Tin... To make stannous chloride, people buy solder form the hardware store. Just be careful that is pure tin, without lead and antimony. How you know. Just place a small piece of solder in HCl in a test tube and heat it. Once it dissolves there should be no black residues in the bottom. If used as a precipitating agent.... first it is relatively expensive and second it provides contamination in the precipitates if other metals are present.


4) Zinc works wonders, in the whole reaction range. Any excess, can be readily disposed of with HCl and causes little interference. However it may be difficult to obtain in small quantities in industrial form; for example you can only buy a bag of 50 lb. that would last a lifetime for the hobbits. 

5) Stay away form laboratory grade reactants except if you need to make very high grade purity products; they are usually expensive. Try to look for industrial grade reactants. You will find out they are considerably cheaper and produce very acceptable results at a fraction of the cost. You will find out that any valuables can be reprocessed if necessary. Do not waste your money in discarding junk, getting rid of the unwanted things. Once you have a sizable quantity of semi pure valuables. You can reprocess them again.

6) Keep the volume of your solutions as low as possible. 
This will save you quite a bit in reactants and a lot of time during filtration and handling large amounts of liquids. 

7) Before any cementation, ALLWAYS, get rid of any Oxidizing agents (HNO3, HClO). they will redisolve your valuables and you will be confused and waste precious chemicals. A nice reducing agent is Formic Acid (HCOOH). Once placed in hot oxidizing solution (HNO3) forms Carbon monoxide that goes into the air; leaving no anions in solution. VERY CLEAN. Stop when the solution stops bubling. It is harder to get though and maybe more costly, but worth the investment. Check around. Another One is Urea, however the endpoint for neutralization is harder to see; stops fizzing. If you do not have any of these, evaporate twice until almost dryness. Rehydrate with HCL 2% - 5%.

8) Once you have your preliminary tests done, proceed with larger batches. The cost of processing diminishes as the batches increase. However you will need bigger infrastructure. 

9) If you are really planning in going into recycling business, be sure you have sufficient raw material to last you several years.... I mean, sufficient sources, that you will be able to obtain your raw processing materials (scarp, dental pieces, jewelry, etc...) and they will not dry out. You investment in equipment should last you several years.

10) As many have stated in the forum..... SAFETY FIRST. The cost of any valuables you may have is simply not compared with any part of your body or even your life. USE EXTREME PRECAUTIONS ALWAYS.


This is it for my first contribution, any comments.... please put them in this forum.

I travel extensively, so perhaps you will not hear form me often. I will try to answer any question as time allows. 

Cheers

PMdoc

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## Palladium (Apr 12, 2008)

Now that's an explanation.

Dam good post. :wink:


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## Anonymous (Apr 12, 2008)

For some strange reason, this part gets chopped everytime I pase the message... so I am trying to see if sending it separate works. Sorry for this..


Why we use some cementing metals instead of others. In chemical terms it boils down to the following reaction

n A(+) + B(0) ---> n A(0) + B(n+)

or perhaps the opposite.

To answer this question we must look into the Electrochemical potentials of the different metals with their corresponding reactions. Following is a list of some of the most common reactions that are used in precious metal chemistry and electrochemical deposition.
These potentials are taken with Hydrogen to be the reference of 0.0 V (CRC Handbook) and have been placed in increasing electrochemical potential.


Na(+) + e(-) ----> Na(0) eV= - 2.7


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## Scott2357 (Apr 13, 2008)

:shock: :?  Speechless.


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