Simultaneous plating of two different metals

A. A number of alloys are successfully plated, Adele, including brass--which is an alloy of a very noble metal (copper) and a very active one (zinc). So, alloy plating is very doable in principle.

The first thing you want to look into is the "Nernst Equation". From this you will see that it is sometimes impractical to plate an alloy by simply varying concentrations and temperatures. One metal will sometimes be so much more noble than the other that there is no way to reduce its concentration far enough to deposit an alloy. What often needs to be done is the more noble metal needs to be complexed, chelated, or in some way tied up so that its potential in the Nernst equation approximates that of the more active metal. You presently don't know the deposition potentials, but you may have to find them.

Platinum and palladium, however, are both noble precious metals, so it may be possible to get the 50/50 ratio simply by reducing the concentration of the platinum relative to the palladium.

A. Yes, you are right. We have to chelate the noble metal stuff. We have a problem here; as the Pt is in the form of chloroplatinic acid (H2PtCL6), ptCl6~2 is a negative atom group, not a simple metal ion pt++ or pt+4, so it is hard to chelate the atom group into complex. So the problem is how can turn the group into simple ions to be chelated. Or can the negative atom group be chelated into a complex? I am doing the same, how can we prepare the platinum to have it chelated?

A. Hi Adele
I admire your courage! You have picked on two metals which are difficult to plate and then wish to combine them.
To give an idea of the size of the problem; you are unlikely to hit on a winning formulation without many trial runs - the supply houses frequently take months or even years. Thus you will use a lot of very expensive Pt and Pd salts so an essential requirement is an economic means of refining them. That alone is a major reason why such work can only be practically carried out by the specialists. You also need rather good analysis facilities and means of evaluating the deposit - XRF as a minimum. It is not a project for someone fairly new to electroplating.
I suggest that you contact the experts. The Institute of Metal Finishing (link on this site or http://www.uk-finishing.org.uk ) and a major supplier like Metalor who may have a commercial process already available.

A. Norbert, I am not sure why you want the cell potential of an alloy. Alloy electrochemistry is very complicated and susceptible to significant variations resulting from localised microcell reactions that can cause changes in the polarisation of the bulk electrode. The important feature in depositing alloys is the redox potential of the ionic - to - metal species. Admittedly the Nernst equation can be used to calculate the reduction potential of a half cell, but it will not be relevant to an alloy, as the highest reduction potential of the metals usd in the alloy will be the preferred reduction reaction. That is, if you have a solution of copper and zinc, the metal that will be reduced first is copper. If you want to codeposit copper and zinc, you need to create complexes and make it less favourable for copper to be deposited and/or more favourable for zinc to be deposited.