Why does molasses remove rust?

Answer 1

This answer only refers to sugarcane molasses. Molasses from sugar beets reportedly tastes so foul to humans that it is only used as an additive to animal feed and as a feedstock for ethanol production. Other types of "molasses" are made from various fruits and vegetables.

It makes sense that sugarcane molasses removes rust, which is Fe3O4. Thus, the iron in rust is in the +3 oxidation state, also denoted as iron(III) or Fe(III).

Rust is rather difficult to dissolve because the primary iron oxides hematite and magnetite have zero solubility in water practically speaking, and many of the common iron(III) salts are practically insoluble in water. On the other hand, iron(II) salts (not oxides) tend to be very soluble in water, and chelated iron(III), iron(II/III), and especially iron(II) complexes are usually extremely water soluble, but beware, the solubility of complexed iron may fall precipitously at low and high pH values.

There are three reasons that most grades of sugarcane molasses will usually dissolve rust: 1) Molasses contains a significant amount of copper(II), and Cu(II) will reduce Fe(III) to Fe(II) leaving elemental copper behind as particulate matter of very small size. 2) Molasses contains a significant amount of glucose, which is a reducing sugar that can reduce iron(III) to iron(II), giving the iron a greater propensity toward water solubility. On the average, only about 67% of sugar in sugarcane is sucrose. Virtually all the sugar in sugar beets is sucrose. 3) The final, but most important, reason that molasses dissolves rust and other iron oxides is the chelating agents it contains. The noun "chelant" comes from the Latin word "chele" which means claw. Chelating agents are organic molecules that contain at least two atoms that chemically bind to metals as if the metal atom or ion were being grabbed by a claw; the binding sites are by necessity always in close proximity to one another, and that aids in keeping the metal more tightly bound.

The major amino acid chelating agents in molasses are aspartic acid (tridentate - contains three binding sites), glutamic acid (tridentate), alanine (bidentate), and about two or more percent of valine (bidentate), serine (bi- or tridentate), and glycine (bidentate). Non-amino acid chelants in molasses are the very strong chelants citric acid (tridentate) and malic acid (tri- or bidentate), and other good chelants such as lactate and polyols.

The most common bonding atoms in chelants are nitrogen, oxygen, sulfur, phosphorus, and in certain instances, carbon. Chelants, or chelating agents, bind the strongest to transition metals since those metals possess partially filled d orbitals that are very important in forming bonds to the atoms mentioned above. The strongest ligands, i.e., the ions or molecules bound to the metal, are capable of forming chemical bonds between one of their empty d orbitals and a filled d orbital on the metal. This is in addition to a sigma bond between a ligand's lone electron pair (Every binding atom has at least one lone pair) and possibly even another pi bond between empty, half-filled, or filled p orbital on a ligand atom and a p or d orbital on the metal.

Answer 2

Molasses is an excellent rust remover, but it needs to be watered down, both for economy and ease of handling. The usual formula is 1 part molasses and 9 parts water.