[Catalysm]

Hi all!

I have a problem with making an etchant for electrochemical etching of gold tips. The etchant I have to use (according to the paper) is 3M KCl in 1% HClO4 (perchloric acid). I want this etchant in a 80mL beaker.

This is the way I did it (which may be wrong ofcourse):

The molar mass of KCl is 74.5513 g mol−1. So I need 17,97g for a 3M concentration in 80mL water. Then I add 1,14mL of 70% perchloric acid. This gives 0,8mL perchloric acid in the solution.

The problem I get, is that I keep getting a sort of suspension. I tried stirring it automatically with a stirbar for an hour or so, but it doesn't help.
I also looked up the solubility of KCl in water, but that seems to be no problem.

Has anybody got an idea of what I am doing wrong?

[Wastrel]

What is the paper, are you sure it doesn't say NaCl?  Potassium perchlorate is quite poorly soluable.

I don't quite follow your method, I'd use 17.89g of KCl, add most of the water, add the 1.14g acid and then dilute to 80ml volume.  You could also make a stock acid solution at a lower concentration to measure by volume.  Are you letting the concentrated acid touch the bare KCl?

[Catalysm]

What is the paper, are you sure it doesn't say NaCl?  Potassium perchlorate is quite poorly soluable.

Well, to be quite honest. It says NaCl indeed. I asked someone at my university and he said that it doesn't matter for the positive ions, because the chloride is the active part in etching. And he is right about that part.
But (!), I think he didn't realise that the perchlorate is messing things up. He just thought about the salt-part (KCl in stead of NaCl).

Potassium Perchlorate is soluble for 1.5g /100mL I see. So that figures. 

I don't quite follow your method, I'd use 17.89g of KCl, add most of the water, add the 1.14g acid and then dilute to 80ml volume.  You could also make a stock acid solution at a lower concentration to measure by volume.  Are you letting the concentrated acid touch the bare KCl?

I did:
1. Make a 80mL 3M KCl solution
2. Add the perchloric acid (1.14mL & 70%)

But the main problem is the potassium with the perchlorate. It's really stupid that this issue didn't cross my mind. I'm a physics-student, so not quite into chemical mixtures. I only need this to create sharp stm-tips.

I'm going to get some SodiumChloride then. I'll let you know! And thanks a lot for your message!

[Wastrel]

If this is an analytical grade reagent job it's probably best to make the extra effort.  Unless you have an 80ml volumetric flask I'd make 100ml of solution.  70% perchloric acid will be weight for weight so take a beaker put on an electronic scale and press tare, add 1.43g of the acid.  Now add distilled water until the total mass of solution is 100g.  This is now a 1% solution.  Now put a 100ml volumetric flask on the scale, press tare and add sodium chloride till you reach 17.53g.  Add the 1% acid solution in stages and swirl until it dissolves.  When completely dissolved continue adding acid solution to the 100ml mark, discard remaining 1% solution.

This should be the correct method.  Hopefully someone will say if I've made a mistake.  Good luck with the STM and do let us know how it goes, I'm more than a little envious if that's what you get to play with.

[Catalysm]

Thanks for the additional comment on this, Wastrel! I appreciate that.

I'm looking for a reliable, reproducable and easy method to create gold wire SPM-tips for a machines called 'ReactorSTM' and 'ReactorAFM'. The machines do in-situ imaging of catalytic-processes at 600K and 5bar (several gas compositions). Up till now, they use PtIr wires and they cut it mechanically. The apex-radius of the tips are then ~30-40nm.
The method I'm trying at the moment claims a mean radius of ~15nm and have it done within 10minutes. The gold tips will last longer in the machines than the PtIr ones.

I'll let you know when I made the solution and got some sharp tips tomorrow after SEM imaging them.

[Catalysm]

Well, I got a nice transparent solution. But the etching still needs some improvements. The tips is by far not smooth. Apex radius is for 40% of the tips lower than 50nm. So that seems to be quite nice.
I still have to experiment with the parameters. And maybe another etchant and/or another method.