How to Remove Blood Stains

Blood stains can be very hard to remove, particularly if they’ve been there a while, particularly if they are on natural fibres, and most particularly if they have been exposed to heat.

But really, we don’t have to remove the stain, we just have to change the colour.

Blood is an iron-based stain. It is an organometallic complex involving iron bound to polypeptide chains.

Iron itself (in solution) is not intensely coloured, nor are the peptides. But when put together, it takes on an intense red colour. As it happens iron also forms intensely coloured complexes with a number of other species, but that’s another story.

The colour of anything is a complex result of the way that visible light interacts with it. In particular, the electronic energy levels of the molecule in question determine the wavelengths at which the molecule absorbs light, thus determining its colour.

So to change the colour, often only very minor changes in the electronic structure are required. An example is litmus paper – red in acid, and blue in alkali, and there are dozens of examples of chemicals that change colour when the pH is changed. Perhaps the most widely used is phenolphthalein, which is intensely purple in alkali solutions, and completely clear in acids.

But back to blood. The simplest and quickest way to change the colour is to treat it with a chemical that binds more strongly to the iron than the peptide groups.

Such as citric acid.

The other day my toddler daughter cut her finger andn while she was crawling around on the carpet she left blood stains all over it.

I sprinkled some citric acid on it, wet it, and instantly the colour began to fade. A bit os swabbing and it was all gone.

Although it’s not as readily available as it should be, citric acid is used to disinfect baby’s bottles, and since it only takes a few grains to do the job, it’s a cheap fix.

The other way to get blood off is with enzyme-based products, but they do it by a different mechanism and that’s a story for another day.