No Lions And Witches—The Pantry Leads To Chemistry

When I’m in the kitchen, it’s hard to keep my mind off pure substances, even if I’m just getting items from the pantry. I picked up some MAGIC baking powder, looked at the label and wondered why sodium bicarbonate(baking soda) is the last ingredient, meaning there’s less of it than anything else in the container. The pantry door once again became like the wardrobe in C.S. Lewis’ novel, but instead of carrying me into a fantasy world of lions and witches, it brought me into the more interesting realm of chemistry.

Unless there’s an acidic ingredient like lemon in baked goods, there won’t be anything to neutralize baking soda(NaHCO3) to produce the needed carbon dioxide. So MAGIC also has an acidulant, in this case monocalcium phosphate (Ca(H2PO4)2), which has more aliases than a rap star–the IUPAC name being the best: calcium dihydrogenphosphate.

But why provide more acidulant than baking soda? The dihydrogen phosphate ion (H2PO4) ion reacts with baking soda’s hydrogen carbonate ion(HCO3) in a one to one ratio.

H2PO4+ HCO3–> HPO42-+ H2O + CO2

Dissociating to provide two moles of H2PO4 , a mole of Ca(H2PO4)2  reacts with a pair of baking soda moles. But doubling the latter’s molar mass of 84 still amounts to less than the acidulant’s molar mass of 234 grams per mole, so for every gram of baking soda, you need more Ca(H2PO4)2, 1.39 to be exact.  Finally, the primary ingredient is corn starch, which may also have the role of acting as a filler to accommodate teaspoon measurements but is definitely there to absorb moisture. Otherwise there would some reaction in the packaging stage or on the shelves, leading to a premature release of carbon dioxide.

Another surprise is the presence of calcium chloride(CaCl2) in most canned tomatoes. People from the Food Network believe the additive leads to lumpier sauces, and they advise cooks not to use tomatoes from cans. But one of the 3 brands in our pantry, Italpasta, is free of calcium chloride. What’s more interesting is the strong industrial link between the ubiquitous calcium chloride and baking soda and soda ash(Na2CO3), a connection that is however vanishing, thanks to the less costly means of producing soda ash directly from the mineral trona

(Na3CO3HCO3.2H2O).

Exactly 150 years ago, in 1867,  using a method he had developed a few years earlier, the Belgian chemist Ernest Solway founded a company in order to produce sodium carbonate(soda ash).

The compound is used mostly in glass-making to lower the melting point of silica, but it finds its way into many other consumer products. The method, which only consumes table salt and limestone, is brilliant in that it creates little waste. It reuses two intermediate products, carbon dioxide(CO2) and ammonia(NH3), and creates not only soda ash but our firming agent for tomatoes.

The overall reaction, as is often the case in both natural and industrial processes, is very deceiving:

2 NaCl(aq) + CaCO3(s) –> Na2CO3(s) + CaCl2(aq) .

It’s as if you would bring in the groceries, place them on the counter, walk away from the kitchen during the preparation of the meal, return just in time when the cooked meal is on the table and conclude:

                                                       groceries  –> lasagna.

You would not be acknowledging any of the cooking process. Calcium carbonate is sparingly soluble at neutral pH’s. Adding a sodium chloride solution to it would yield insignificant amounts of products. But the Solway method begins by treating brine with NH3 gas to generate ammonium chloride(NH4Cl). In Solway towers, carbon dioxide is then injected to yield baking soda. Next heat is used to drive off carbon dioxide from the baking soda to yield soda ash and regenerating CO2, which is used again in the towers.The ammonium chloride meanwhile reacts with limewater(Ca(OH)2), releasing ammonia gas that is kept to re-initiate the cycle. The alkaline solution is produced by cooking calcium carbonate, which releases lime and which creates more carbon dioxide for the in-between reaction. Our calcium chloride is the byproduct of the step that releases ammonia.

If an industry is only interested in making calcium chloride it can also rely on the direct action of 36% HCl (hydrochloric acid) on calcium carbonate. The reaction is 2 HCl + CaCO3 –> CaCl2 + H2O + CO2, and the recovered carbon dioxide is used to make soft drinks. A cheaper method that however produces a less pure product relies on the purification of natural brine water. First magnesium ions are precipitated out with the addition of limewater.The water is slowly evaporated which forces out sodium chloride solid, leaving behind the more soluble calcium chloride.

The greater solubility of CaCl2 is part of the reason it is a valuable additive to street salt to melt ice in colder climates. Although CaCl2 is about three times as expensive as NaCl, its calcium ion does not harm plants like sodium ion does, and it melts ice at much lower temperatures than rock salt(NaCl)’s -10 oC limit. The reason why calcium chloride is a firming agent for tomatoes is one of the reasons it makes regular salt more effective at melting ice.

It’s because CaCl2 is hygroscopic–it easily attracts and holds on to water.

Compared to table salt, calcium chloride tastes much saltier, but it cannot be used as a substitute. Ca2+ plays an important role in cell signalling, and cells are sensitive to high levels of the ion. Not surprisingly the lethal dose that will kill 50% of mice is only 1940 mg CaCl2 /kg of body weight as opposed to 4000 mg/kg for table salt. But the concentrations of calcium chloride in canned tomatoes is nowhere near toxic levels. It’s not only approved in the United States but in Europe, which is usually more strict with additives.
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