The topic at hand is making a soluble rhodium salt from 99,95 pure rhodium sponge. Emphatically, this post is not intended as a how-to and is not something to conduct by unqualified individuals or the casual hobbyist/professional refiner. Chlorine gas is corrosive, poisonous, and a potent oxidizer when hot-- all appropriate safety considerations should be followed. It has been prepared for reference purposes only, and the general amusement of my friends here on the goldrefiningforum!
Introduction
Rhodium is very difficult to put into solution, and virtually impossible to do so when in a bulk form such as wire, sheet, or plates. Even a 10% Rh addition to platinum (i.e. in thermocouple wire, Types S,B, R etc.) significantly increases the dissolution time from hours to days under atmospheric conditions. Many methods for refining rhodium involve melting with other metals (Zn, Cu, Mn etc.) and leaching away the matrix to provide a rhodium black with the result being that the higher the surface area of the rhodium black allows for faster solubilization.
It is possible to microwave pressure digest rhodium sponge (or other alloys), but results are often sub-optimal.
Lixiviants for rhodium are fairly limited:
Melts can be made with any acidic sulfate salt to generate the red-brown rhodium sulfate as previously described by freechemist. This reaction proceeds faster when chlorine is produced in situ with the addition of NaCl.
The goal is to make Na3RhCl6, a highly soluble rhodium salt that gives a burgundy wine colored solution, even at low concentrations. To do this expediently and with a high first past conversion, acid-insoluble RhCl3 is formed as a dark brown powder. This is then mixed with considerable excess of sodium chloride, and rechlorinated to make the soluble salt.
Reactions governing the system are as follows:
1. Rh(s) + 3/2 Cl2 (g) --> RhCl3 (s), 500-650*C, excess Cl2
102.91 209.27
2. RhCl3 (s) + 3 NaCl (s) --> Na3RhCl6 (s), 500* C, excess Cl2
58.44 278.24
Chlorine generation:
Instead of lugging out a chlorine cylinder, it's easier to make it in the fume hood chemically.
C3N3O3Cl3 (s) + 3 HCl (aq) --> 3 Cl2 (g) + C3N3O3H3 (s)
232 g/mol 36.46 g/mol 70.91
Trichloroisocyanuric acid (TCCA) is used as the oxidizer and acts upon 4 M HCl to cleanly generate chlorine; no heating is required to drive the reaction as the production and removal of a gas favors the products; stirring the slurry helps. The 4 M HCl is slowly dripped from an equal pressure addition funnel into the TCCA slurry such that a smooth rate of evolution is produced as produced at the bubbler. The joints are greased with Krytox 240 AC to tolerate the chlorine, and the connections are made with FEP tubing. The gaseous chlorine so produced is ran through a column of 18 M sulfuric acid to remove any HCl from the gas stream. Excess Cl2 is scrubbed with two traps (always have an empty trap in line to prevent suck-back into the line. It would be disastrous to have that happen). The end trap is 8 M NaOH which will yield hypochlorite and other oxo-chlorine compounds. Quartz wool is stuffed into the gas output end of the gas wash bottle to prevent any H2SO4 mist from entering the tube.
Rhodium (III) chloride
Next, pure rhodium sponge is weighed out and loaded into a clean quartz tube (Photos 1,2,3). It is important not to touch the quartz with one's bare hands as salts from human sweat will eventually cloud it from devitrification. This tube is packed with quartz wool on the output end. This is done for several reasons: first, in the event that a large surge of chlorine forms, it would be bothersome to have to remove blown out Rh from the trap, second, to look for the deposit of RhCl3 indicating that it has started vapor transport and the reaction is finished. Overall setup in Photo 4.
In the small test hood:
One can see in Photo 5 the production of RhCl3 and various subchlorides. Chlorine was used to displace air from the tube at lower temperatures (the rate of reaction is minimal at less than 400 C). The reaction depends upon diffusion of hot chlorine into the rhodium sponge, so periodically rotating the tube (caution! the glass will not appear hot in well-lit conditions!) helps complete the chlorination. Note the progression from left to right.
In the next installment, we'll show sodium hexachlororhodate being made by intimately grinding the RhCl3 with NaCl and re-chlorinating.
And yes, that hood is dirty :-/
Introduction
Rhodium is very difficult to put into solution, and virtually impossible to do so when in a bulk form such as wire, sheet, or plates. Even a 10% Rh addition to platinum (i.e. in thermocouple wire, Types S,B, R etc.) significantly increases the dissolution time from hours to days under atmospheric conditions. Many methods for refining rhodium involve melting with other metals (Zn, Cu, Mn etc.) and leaching away the matrix to provide a rhodium black with the result being that the higher the surface area of the rhodium black allows for faster solubilization.
It is possible to microwave pressure digest rhodium sponge (or other alloys), but results are often sub-optimal.
Lixiviants for rhodium are fairly limited:
- Aqua regia, boiling
Hydrobromic acid, boiling
HCl/Cl2, 120*C, pressure
18 M + sulfuric acid; reflux, preferably under pressure and/or admixed with a Cl2 source
Melts can be made with any acidic sulfate salt to generate the red-brown rhodium sulfate as previously described by freechemist. This reaction proceeds faster when chlorine is produced in situ with the addition of NaCl.
The goal is to make Na3RhCl6, a highly soluble rhodium salt that gives a burgundy wine colored solution, even at low concentrations. To do this expediently and with a high first past conversion, acid-insoluble RhCl3 is formed as a dark brown powder. This is then mixed with considerable excess of sodium chloride, and rechlorinated to make the soluble salt.
Reactions governing the system are as follows:
1. Rh(s) + 3/2 Cl2 (g) --> RhCl3 (s), 500-650*C, excess Cl2
102.91 209.27
2. RhCl3 (s) + 3 NaCl (s) --> Na3RhCl6 (s), 500* C, excess Cl2
58.44 278.24
Chlorine generation:
Instead of lugging out a chlorine cylinder, it's easier to make it in the fume hood chemically.
C3N3O3Cl3 (s) + 3 HCl (aq) --> 3 Cl2 (g) + C3N3O3H3 (s)
232 g/mol 36.46 g/mol 70.91
Trichloroisocyanuric acid (TCCA) is used as the oxidizer and acts upon 4 M HCl to cleanly generate chlorine; no heating is required to drive the reaction as the production and removal of a gas favors the products; stirring the slurry helps. The 4 M HCl is slowly dripped from an equal pressure addition funnel into the TCCA slurry such that a smooth rate of evolution is produced as produced at the bubbler. The joints are greased with Krytox 240 AC to tolerate the chlorine, and the connections are made with FEP tubing. The gaseous chlorine so produced is ran through a column of 18 M sulfuric acid to remove any HCl from the gas stream. Excess Cl2 is scrubbed with two traps (always have an empty trap in line to prevent suck-back into the line. It would be disastrous to have that happen). The end trap is 8 M NaOH which will yield hypochlorite and other oxo-chlorine compounds. Quartz wool is stuffed into the gas output end of the gas wash bottle to prevent any H2SO4 mist from entering the tube.
Rhodium (III) chloride
Next, pure rhodium sponge is weighed out and loaded into a clean quartz tube (Photos 1,2,3). It is important not to touch the quartz with one's bare hands as salts from human sweat will eventually cloud it from devitrification. This tube is packed with quartz wool on the output end. This is done for several reasons: first, in the event that a large surge of chlorine forms, it would be bothersome to have to remove blown out Rh from the trap, second, to look for the deposit of RhCl3 indicating that it has started vapor transport and the reaction is finished. Overall setup in Photo 4.
In the small test hood:
One can see in Photo 5 the production of RhCl3 and various subchlorides. Chlorine was used to displace air from the tube at lower temperatures (the rate of reaction is minimal at less than 400 C). The reaction depends upon diffusion of hot chlorine into the rhodium sponge, so periodically rotating the tube (caution! the glass will not appear hot in well-lit conditions!) helps complete the chlorination. Note the progression from left to right.
In the next installment, we'll show sodium hexachlororhodate being made by intimately grinding the RhCl3 with NaCl and re-chlorinating.
And yes, that hood is dirty :-/