Introduction

If you usually use ferric chloride as an oxidizing agent in the process of etching PCBs, sooner or later you may get your clothes stained. These brown-colored stains are difficult to clean off.

To remove the stains we are going to use an acid as an agent to remove ferrum chloride ions by transforming them into another sustance that is both colorless and easy to remove from the fabrics.

Here’s a simple way of doing this, suggested by my dad (my personal guru when it comes to chemical matters). It truly has good results.

Theory

When the ferric chloride (FeCl₃) dissolves in water, it hidrolizes, moves the PH of the dissolution and turns into a corrosive liquid acid of a dark-brown color.

In electronics we often use it to etch the copper surface of the PCB, due to its power as an oxidizer. The ferric chloride transforms the copper in cupric chloride:

FeCl₃ + Cu → FeCl₂ + CuCl

FeCl₃ + CuCl → FeCl₂ + CuCl₂

The oxalic acid (H₂C₂O₄) is an organic acid, and thus relatively powerful. It is commonly used to remove stains and whiten fabrics.

When it reacts with the ferrum, it transforms into a salt (iron oxalate). This salt is both colorless and easy to remove:

2FeCl₃ + 3H₂C₂O₄ → (C₂O₄)₃Fe₂ + 6HCl

You can buy the oxalic acid in any drugstore, and even in some pharmacies. Also in some veterinary shops where apiculture products are sold. It is a cheap and easy-to-find  product. Commercially, it can be found as a white dust.

You must be careful when you manipulate it, since it is poisonous and in big dosis it can be lethal. It must not be mixed with other acids, for it may transform into carbon dioxide, which is toxic and odorless.

Procedure

The most common concentration used is between 5% and 10%. For 100 ml of stain-remover you will need:

• 100 ml of distilled water. Be careful not to use tap water, or you will have calcium oxalate as a result.

• 10 gr of oxalic acid (dust)

Even though the concentration of the dissolution is not a crucial factor, a preparation diluted as this facilitates the application, making the distribution of the acid more even in the fabrics and preventing damage in the most delicate ones.

This preparation must be kept out of sunlight.

To employ it, you simply pour as much as you need over the stain, until its color fades.

It may seem to you that the stain spreads and becomes a kind of yellowish halo. Don’t fright. It will disappear after the first wash.

Immediately afterwards, wash the zone with water and neutral soap.

If any hint of the stain remains, repeat all the procedure: it may be that the quantity of dissolution poured was not enough.

You can observe the step-by-step process in the following photos:

This method is also useful to remove ferric oxide stains from fabrics.

Translation by Mavi.

Referencias:

[1]-http://es.wikipedia.org/wiki/Ácido_oxálico