Why does ferric chloride turn green

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Sign up using Facebook. With the increased emphasis on health and safety at work, we have had to reappraise these and the other chemicals we use. The salt, ferric chloride, has none of the disadvantages associated with the other two acid etching solutions: it does not produce dangerous fumes, is odourless and, though corrosive, is not absorbed through the skin. Ferric chloride solutions in water are strong acidic but in their reaction with copper no significant fumes or gases are produced.

Simply, except in the presence of free oxygen, copper will not react to any significant extent with hydrochloric acid which is the acid produced with ferric chloride solutions. The chemistry of the etching reaction. When ferric chloride is dissolved in water the solution becomes strongly acidic as a result of hydrolysis.

The chemical reactions, in words and formulae are:. Schematic representation of the chemical interactions occurring during etching copper in ferric chloridesolution. The ferric ions will partially combine with the hydroxide ions to form ferric hydroxide, a compound which is only slightly soluble and precipitates from solution as a brown solid.

This precipitation can be regarded as removing hydroxyl ions from the solu-tion, leaving a relative excess of hydro-gen ions and it is this excess that makes the solution acidic. Because of a property of metals known as electro-negativity, copper replace iron from the solution and form a mixture of ferrous and cupric ions. Effectively the copper dissolves without producing any gas;.

Ferrous ions become increasingly stable as the solution becomes more acidic and this helps the copper dis-solution to proceed more easily whilst in basic less acidic solutions the ten-dency is for ferrous ions to convert back to ferric. As more copper dissolves and the solubility limit is exceeded, cupric chloride precipitates from the solution, as anything from a green to a blue solid. This mea-surement equates to a specific gravity of approximately 1.

Purveyors of ferric chloride can use any of these measure-ments and often profess complete igno-rance of any form of the others! At this stage the strongly acidic ferric chloride requires further treatment to remove free acid and to 'condition the solution'. This results in a sludge of ferric hydrox-ide which is allowed to settle before pouring off the liquid and adding the precipitate to 1 litre of the stock solu-tion. Once an etching cycle using ferric chloride has been initiated the third method is probably easier.

The optimum etching conditions usu-ally occur after some etching has been done this reflecting an 'incubation' period for the chemical reaction to 'settle down' but as etching proceeds the solution becomes less effective. The progress of this exhaustion can be oh served by the colour changes - from an initial red brown ferric hydroxide through a turbid muddy brown ferric and ferrous hydroxide when the solution is working at its most effi-cient, and finally to a dark green solu-tion and black precipitate cupric salts in solution and precipitated with the hydroxides.

At this stage the solution is exhausted and requires disposal. This can be effected by the slow addition of either calcium carbonate whiting or sodium carbonate washing soda crystals - until effervescence ceases.

At this point the solution is neutralised and may be disposed of safely. The normal recommendation is to bite copper plates in ferric chloride in-verted or on edge to avoid precipitated ferric hydroxide clogging the etched line, thus halting etching. However, there is little need for this if the plate is bitten face up' and removed from the solution every 20 min then washed in cold running water to remove the pre-cipitate before continuing to bite.

This procedure does not present the hazard of other etchants and has the advantage of checking progress. Extremely deep bites or plates for relief etching are still bitten upside down. On completion of any biting, or if the plate is to stand between bites for any length of time, it should always be cleaned thoroughly with running water, since residual ferric chloride in bitten lines will continue to react.