AB's As and Bs: Acid–Base Chemistry vs. Good Eats
/I am a big fan of Alton Brown. He's funny and smart, and he doesn't (usually) do things just because "it's always been done that way." Why buy a pizza stone when you can use a quarry tile? Why have a kitchen full of unitaskers? Why not explain fermentation and leavening with burping sock puppets1?
In 14 seasons of Good Eats, Alton Brown covered a wide range of culinary and food science topics. Everything from the best way to deep fry a turkey,2 to how the ratio between amylose and amylopectin3 create differences in rice varieties. And though things are occasionally (over)simplified, they're also mostly right, like the facts about fat in "Fry Hard,4" ice crystal formation in "Melondrama,5" and protein denaturation in "The Egg-Files VII: Meringue.6"
Problematic Chemistry
There's one thing he doesn't get right, though, and it bothers me every time the show comes on reruns: the explanation of acids and bases in "Pretzel Logic.7"
Here's the explanation as written in the companion cookbook (emphasis mine):
Pretzels, pH and You: Of all the chemical concepts a cook must grasp in order to move up the ladder of culinary enlightenment, pH—that is, the concentration of hydrogen ions in the solution—is one of the toughest. This is especially true for cooks who, like myself, slept through the entirety of high school.
In a nutshell, if a solution has an equal number of positively and negatively charged hydrogen atoms, or "ions," it is said to be "neutral," which means it has a pH of 7, which by the way, is the pH of distilled water.
As the number of positively charged atoms increases, the pH number goes down, and the solution becomes more acidic. Most of your common kitchen ingredients fall along this end of the scale, from milk at 6, to black coffee at 5, orange juice at 4, most vinegars at around 3, lemon juice at around 2, stomach acid at around 1, and below that, well, you're talking about battery acid, which is never good eats.
On the other end of the scale, from 8 to 14, are solutions higher in negatively charged hydrogen atoms. These are called "alkalis" or "bases," and there aren't too many of them in the kitchen environment. Sea water is slightly alkaline, as are egg whites, which usually have a pH of 8. Baking soda scores a 9, antacid tablets about a 10. Ammonia 11 to 11.5, bleach somewhere around 12, and then we get up to NaOH, a.k.a. lye, at 13, which is so caustic that every bottle features a funny little drawing of a skull and crossbones—and that does not mean that Johnny Depp gets a percentage of each bottle sold. It means that this stuff is very poisonous. As any Fight Club fan can tell you, lye is a critical ingredient in soap making. But what you may not know is that lye, oddly enough, is also critical to the production of great pretzels.
Oh, AB! You wound me. Some of my students are watching your show, and you are teaching them crappy chemistry. You may have slept through high school, but I'd hope that whoever taught you about protein denaturation or crystallization was staying awake. Perhaps they could have proofread your notes on acids and bases.
Let's set things straight. There's a mix of true and false, here.
Positively and negatively charged atoms are called ions.
True.
If a solution has an equal number of positively and negatively charged hydrogen atoms, it is neutral.
False.
I don't know what you would call it, but a mixture of H+ and H–, called "hydrogen ions" and "hydride ions," respectively, strikes me as dangerous, no matter the ratio. There are many acid–base systems, but the one most familiar to non-chemists is the Arrhenius system.8 Even if you've never heard of Arrhenius, you'd recognize Arrhenius acids as acids (e.g. coffee, orange juice, vinegar) and Arrhenius bases as bases (e.g. baking soda, bleach, lye). Arrhenius acids and bases are defined by the ratio of hydrogen ions (we're still talking about just the positive ones, i.e. H+) to hydroxide ions. Hydroxide (as in, "sodium hydroxide," the chemical name for lye) is not an atom with a charge; it's two atoms with a charge: OH–. Also, that negative charge is mainly on the O, not the H.9
The general rule (as all of my Gen. Chem. students should know!) is that Arrhenius acids are H+ donors, and Arrhenius bases are OH– donors. Vinegar (a.k.a. acetic acid or CH3CO2H) in water donates H+, so it is an acid. Lye (our friend NaOH) in water donates OH–, so it is a base. If you have equal concentrations, the H+ ions and the OH– ions match up and cancel out each other (that's the neutralization part). If the concentrations are unequal, whatever is left over "wins." Extra OH–? The solution is basic. Extra H+? The solution is acidic. How basic or how acidic depends on how many ions are left over (and some other stuff, but I'm covering the simple version for this post).
Water, H2O, is connected H–O–H (not H–H–O). Pure water all on its lonesome will sometimes split apart into H+ and OH–, but there will be equal concentrations of the two ions, so the solution is neutral.
As the number of positively charged atoms increases, the pH number goes down.
Mostly true.
If those positively charged atoms are hydrogens and they increase relative to the total number of atoms, then yes, the pH goes down. If it's positively charged something else,e.g. Na+, the pH doesn't change. If the number hydrogen ions and the volume of solution increases, the pH doesn't change. It's not the number that matters, it's the concentration (i.e. the number in a given volume).
pH is a scale to measure how acidic/basic a solution is, using Arrhenius's definitions. I like to think of pH like a mathematical function. You may have called functions "f(x)" at some point in high school or college.10 f(x) had some definition, like f(x) = x2. It meant that there was some process that modified x and f(x) told you what the result was. pH is a special kind of function, called a p-function. The f(x)-style expression for a p-function is:
p(x) = –log(x)
So pH would be the negative logarithm of H. In this case, H is the concentration of H+, or [H+]. For the solution you get after mixing two things together, the pH is calculated based on the amount of H+ left over. There is a corresponding p-function for the concentration of OH–, too. It, unsurprisingly, is called pOH.
If you have extra OH– after combining two solutions, you can calculate the pH from the pOH. The two values (pH and pOH) add up to 14.00. So if you have a pOH of 6, you have a pH of 8.
As I said earlier, water is neutral and it splits into hydrogen11 and hydroxide molecules. Take a whole lot of those molecules splitting into ions, say 6.022×1023 of them—or 18 mL,12 and you'll find that 1.084×1015 will have split apart. That's a concentration of 10–7 M. To take a logarithm (in base ten), you can grab the power to which the ten is raised, i.e. log(10–7) = –7. So the negative logarithm is just 7. This is why neutral things have a pH of 7. It's the pH of pure water.
If you have a concentration higher than 10–7 M, it would have a less negative superscript. Remember that 10–7 means 0.0000001 M. A higher concentration might be 0.0001 M, or 10–4 M in scientific notation. The negative logarithm of 10–4 is 4, so as the concentration of H+ goes up from 10–7 M to 10–4 M, the pH goes down from 7 to 4.
Solutions higher in negatively charged hydrogen atoms are called "alkalis" or "bases."
False.
As I said before, negatively charged hydrogen atoms are called "hydride ions." An "alkali" or "(Arrhenius) base" is a solution higher in negatively charged hydroxide ions, or OH–. They're also lower in H+ ions.
If you go from an H+ concentration of 10–7 M down to 10–9 M (i.e. 0.0000001 M to 0.000000001 M), the pH goes up from 7 to 9. There's less H+ (more OH–), so it's basic.
Lye is so caustic that every bottle has a skull and crossbones label.
Mostly true.
"Caustic" means it causes chemical burns or corrosion. Poisonous means it'll make you sick (typically when ingested). Sure, you might be sick after chemical burns, but that's not really the same thing as getting poisoned. Caustics may merit the skull and crossbones label anyway, as it's often used for lethal dangers in general.
The skull and crossbones means that something is poisonous.
Solutions to the Acid–Base Problem
So here's the text as I wish it were written (my changes in bold):
Pretzels, pH and You: Of all the chemical concepts a cook must grasp in order to move up the ladder of culinary enlightenment, pH—that is, the concentration of hydrogen ions in the solution—is one of the toughest. This is especially true for cooks who, like myself, slept through the entirety of high school.
In a nutshell, water molecules can come apart in two pieces: positively charged hydrogen atoms, or "ions," written H+, and negatively charged hydroxide ions, written OH–. If a solution has an equal concentration of hydrogen ions and hydroxide ions, it is said to be "neutral," which means it has a pH of 7, which by the way, is the pH of distilled water.
As the concentration of positively charged hydrogen ions increases, the pH number goes down, and the solution becomes more acidic. Most of your common kitchen ingredients fall along this end of the scale, from milk at 6, to black coffee at 5, orange juice at 4, most vinegars at around 3, lemon juice at around 2, stomach acid at around 1, and below that, well, you're talking about battery acid, which is never good eats.
On the other end of the scale, from 8 to 14, are solutions higher in negatively charged hydroxide ions. These are called "alkalis" or "bases," and there aren't too many of them in the kitchen environment. Sea water is slightly alkaline, as are egg whites, which usually have a pH of 8. Baking soda scores a 9, antacid tablets about a 10. Ammonia 11 to 11.5, bleach somewhere around 12, and then we get up to NaOH, a.k.a. lye, at 13, which is so caustic that every bottle features a funny little drawing of a skull and crossbones—and that does not mean that Johnny Depp gets a percentage of each bottle sold. It means that this stuff is very poisonous. As any Fight Club fan can tell you, lye is a critical ingredient in soap making. But what you may not know is that lye, oddly enough, is also critical to the production of great pretzels.
I can only wish that some future edition of the cookbook will correct the errors. There's so much good science (and scientific thinking) in these books and in the TV series, that I can still recommend them with confidence. I just wish Mr. Brown had gotten this bit of basic chemistry right.
The Books
Good Eats: The Early Years, Good Eats 2: The Middle Years and Good Eats 3: The Later Years by Alton Brown, published by Stewart, Tabori & Chang.
The Show
Good Eats on the Food Network
Good Eats on Hulu
1: "Dr. Strangeloaf" Good Eats S8 E19
2: "Fry Turkey Fry" Good Eats S10 E13
3: "Do the Rice Thing" Good Eats S8 E22; amylose and amylopectin show up at about 2:40 in the linked video.
4: "Fry Hard" Good Eats S2 E9
5: "Melondrama" Good Eats S8 E14
6: "The Egg-Files VII: Merginue" Good Eats S14 E13
7: Good Eats S11 E2; also on pages 50–51 of Good Eats 3: The Later Years, if you're following along at home.
8: Runners-up for students of chemistry are Brønsted–Lowry and Lewis, but there are others.
9: As the name suggests, the "OH" part of "NaOH" is the "hydroxide" in "sodium hydroxide." When it comes to names, chemists are pretty systematic, folks.
10: As I learned it, you pronounce "f(x)" as "eff of x."
11: You may have learned about "hydronium" ions, or H3O+. It's true that the hydrogen ions don't just hang out alone, but hydronium isn't fully accurate either. It's handy for figuring out reactions, though, so we chemists use it frequently. I'm using H+ here because I think it makes a clearer connection to the calculation of pH.
12: Yep, a mole (6.022×1023) is a really big number for groups of really small things. If you're not used to thinking in milliliters, that's about 3 1/2 tablespoons of water. Atoms are really small. Also, every time I write M I mean "moles per liter," so there are whatever number of moles times 6.022×1023 molecules in each liter of solution.