Exercise 100. Colloidal Platinum

Colloidal platinum may be prepared very easily by arcing under distilled water between platinum electrodes (Fig. 24). The current is taken from an ordinary 110-volt lighting circuit through an ammeter and an adjustable resistance in series or simply through a known 

Try the action of an electrolyte on colloids by adding a salt solution. The platinum should precipitate after a time. 

To a solution of hydrogen peroxide, add some of the colloidal platinum solution. The catalytic action of the finely divided platinum is plainly evidenced by the evolution of oxygen. 

The poisoning of a catalyst may be shown by adding some hydrogen sulfide solution to the hydrogen peroxide before the colloidal platinum is introduced. No decomposition of the peroxide is observed in this case, since the platinum has been poisoned by the presence of the hydrogen sulfide. This method is applicable to most of the metals below hydrogen in the electrochemical series. 

Many colloids are highly colored, and the question often arises as to whether the color of a given liquid is due to a small amount of a colloid or to a much larger amount of some dissolved substance a very simple experiment in dialysis will often serve to decide the matter. 

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