Copper sulfate is surely one of the most recognisable chemicals that exist, reminding many people of school laboratories and childhood chemistry sets. Its habit of forming beautiful crystals of the deepest blue led it to be known in the olden days as Blue Vitriol (vitrum being the Latin word for glass), and in 2008 led an artist named Roger Hiorns to fill an entire council flat with a solution of it. Upon draining the flat, every surface was
“covered in a thick layer of glistening, knife-sharp copper sulfate crystals – creating an angular cave that was at once alluring, sensuous and needlingly dangerous.”
1) For many years, copper sulfate was used as a herbicide, fungicide and pesticide. A mixture of slaked lime (calcium hydroxide) and copper sulfate is known as Bordeaux Mixture, and when sprayed over grapes (and other fruit) it acts to prevent the germination of mildew. The name arose from its discovery in Bordeaux – as the mixture is both visible (due to the blue of the copper sulfate), and very bitter (due to the lime), this mixture was sprayed over the vines nearest the road to prevent passers-by eating the grapes. Professor Pierre-Marie-Alexis Millardet noted that these vines were unaffected by outbreaks of mildew, and this led to the development of the mixture as a fungicide:
“Arriving at the Chateau Beaucaillon I questioned the manager, Mr. Ernst David, who told me that it was the custom in Medoc at the turning of the grape to cover the leaves with verdigris or sulfate of copper mixed with lime in order to keep away marauders; they, seeing the leaves covered with coppery spots, were not bold enough to taste the fruits hidden beneath for fear that they had been contaminated with the same matter. I called the attention of Mr. David to the fact of preservation of leaves, which question was discussed and I made him share the hope, which this observation raised in me, of finding in the salts of copper the basis of the treatment of mildew”
It is perhaps worth noting that the effect of copper sulfate as a fungicide (in treating rust in wheat) had in fact been noted many years earlier by Benedict Prévost in Montaubon, though it was seemingly not widely used. A similar mix of copper sulfate and sodium carbonate is called Burgundy Mixture, and a mixture of copper sulfate and ammonium carbonate is called Cheshunt Compound. All of these mixtures are much less widely used than they used to be. Copper compounds are extremely toxic to aquatic life and invertebrates (M. T. Horne and W. A . Dunson, Arch. Environ. Contam. Toxicol., 1995, 29, 500-505) and do not biodegrade, and so their use as a general fungicide is being reduced, but it is estimated that around 200 000 tons of copper sulfate is produced globally every year and 75% is still used in agriculture (The Copper Development Association Inc.)
Organic is a word that has gained different meanings depending upon where it is used. To chemists, organic compounds are generally those based upon carbon, and a compound like copper sulfate, which contains no carbon at all, is classified as inorganic (hence the categorisation of this blog post). To the general public though, organic has largely become synonymous with natural – organic food is grown without the use of ‘chemicals’ – so it would probably be a surprise to many that organic farmers were allowed to use Bordeaux mixture as a fungicide. As has been pointed out though, copper sulfate is neither safe nor organic (in either sense of the word) (B. Dixon, The Lancet Infectious Diseases, 2004, 594), and its use is a historical anomaly.
2) The internal structure of blue vitriol contains not just copper sulfate, but also water; its proper name is copper(II) sulfate pentahydrate, and the pentahydrate bit means that there are five water molecules present per copper atom. The colour is created by the copper ion absorbing red light and leaving just the blue for us to see. It only does this in the presence of the water molecules, and, if a sample is heated, then these water molecules will evaporate and the colour will eventually become white (W. E . Garner and M. G. Tanner, J. Chem. Soc. 1930, 47-57).
The figure below shows how the atoms are arranged in solid copper sulfate; a crystal of copper sulfate contains gazillions of these units, stretching off as far as the eye can see,* in three dimensions. The copper atom in the middle (brown) is next to six oxygen atoms (red). Hydrogen is white, so you can see that there are four OH2 units around the middle of the copper atom. OH2 is of course water, so these are the water molecules that are required for the blue colour. Sulphur is yellow, so the SO4 entities top and bottom are sulfate ions (G. E. Bacon and N. A. Curry, Proc. Royal. Soc. Lond. A 1962, 266, 95-108).
The four oxygen atoms about each sulphur atom are arranged in a tetrahedron, the same shape as a pyramid-shaped teabag. The six oxygen atoms around the copper atom are in an octahedron – one at the top, one at bottom, and four in a square around the middle. Chemistry is full of tetrahedra and octahedra, because these are very efficient ways of arranging one spherical thing about another (and atoms are essentially spherical). This was appreciated as far back as ancient Greece; the tetrahedron and octahedron are two of the Platonic solids, named after the philosopher who theorised that the classical elements (earth, air, fire and water) were made from these shapes.
Copper sulfate occurs in nature as the mineral chalcanthite. Because copper sulfate is very water-soluble, this mineral tends to occur in drier parts of the world (it would dissolve in the rain), and samples need to kept away from humidity to prevent their decomposition. Indeed, in 1751 it was written that
“There are many who pretend to have seen blue Vitriol native and in its proper form; but they have deceived themselves. There are indeed sometimes found Crystals of blue Vitriol on the Stones that lie about the Edges of the Springs where the Cement Waters issue out of the Earth; but these are only Crystals form’d by the Evaporation of that Water by the Sun’s heat”
Or, in other words, that it is easy to grow crystals of copper sulfate from solution, but difficult to find otherwise. This is reflected in the fact that it is manufactured rather than mined, generally by the reaction of copper (recovered from non-ferrous ores) with sulphuric acid; or, to put it another way
“Copper in Abundance acted upon by the native sulphureous Acid, and as in that State the Metal is capable of Solution in Water it has dissolved and taken up in its Passage, and comes out fraught with it, and ready on evaporation to shew it in the form of Vitriol.”
Copper sulfate is used to test blood for anaemia (R. A. Phillips et al., J Biol Chem., 1950, 183, 305-330), by putting a drop of blood into a solution of copper sulfate of known concentration. The copper sulfate solution has a known density, and seeing whether the drop of blood rises or falls gives an indication of its density – normal blood should sink to the bottom of the solution, but anaemic blood is lighter than normal due to a lack of haemoglobin, and either floats or rises.
This method takes advantage of the fact that copper ions react with blood; as the droplet of blood enters the copper sulfate solution, the copper ions cause the proteins on the surface of the droplet to coagulate and form a shell around the drop, which for a few seconds prevents it from dissolving into the solution – this contrasts with what happens normally, where blood simply diffuses through water (think Jaws!). This coagulation reaction between copper and blood is useful in some other situations too: in 1751 John Hill wrote that solutions of copper sulfate were
“an excellent Styptic, and particularly serviceable in Haemorrhages of the Nose”
and to this day it is still used in veterinary styptic powders (a styptic is something that can be used to stop bleeding).
The blood test works because of a reaction between copper ions and a component of the blood called albumin. Albumins are proteins that are globular in shape, and this shape bestows water-solubility upon the protein – all the bits that interact nicely with the water (hydrophilic is the scientific term) are on the outside, and all the bits that don’t like water (hydrophobic) are tucked away on the inside. The copper ion destroys this structure and makes the proteins become insoluble – they precipitate, which has the effect of making the blood coagulate.
3) The same principle also finds use in the preparation of meringues; it has long been known that using a copper bowl makes it easier to make meringues. Meringues are made by beating together egg whites and sugar, and can be tricky to get right – the secret is in beating them just enough to stiffen them, and not so much that they become lumpy and collapse.
Egg whites are basically a mixture of albumins and water, and when you beat them you make the albumin proteins unravel. When they unravel, they can trap air and water between them, and essentially you end up with a protein mesh containing bubbles of air and water – these bubbles give meringues their fluffiness. However, if you overbeat the mixture then you force the air and water out of the mesh and the proteins start sticking to each other instead of the air and sugar. This gives lumpy meringues.
The use of a copper bowl has been held to produce better meringues. The theory is that copper reacts with one of the proteins, called conalbumin, in the egg whites – the biological function of conalbumin is to sequester metallic impurities in the egg, so it is particularly suited to reaction with the copper. The structure of conalbumin is due in part to a kind of bond called a disulphide bond; there are many of these, and they act like the spokes of a wheel, holding different parts of the protein together at the right distance (represented in purple in the above diagram). The copper breaks these bonds (G. K. Oster, Nature, 1971, 234, 153-154) and sticks to the sulphur atoms, allowing the protein to be stretched out to make the fluffy mesh. Not only does the copper do this, but because the albumins are stuck to the copper they can’t stick to each other, preventing lumpiness.
Contributors: Chris Adams (research, ideas, structures and words); Jenny Slaughter (editing); Ian Nichols (photographs).
We would like to thank Roger Hiorns and “la fattina”, for allowing use of their photographs under Creative Commons.
Our thanks as well to Don Hayes Butchers for providing some of our materials.
*Well, once the crystal structure has been determined.