There are two ways to measure an acid. They are by titration for total titratable acid and by measuring pH. One of the hardest things for me to understand was that they do not measure the same thing and do not correlate with one another. In my mind, if they both measure acid, one should be able to convert one to the other, but it just doesn't work that way.

To understand this, I present an analogous situation. We ask some people how big they are. One person says 5 feet 7 inches. Another person says 155 pounds. A third person says 6 feet tall. A fourth says 160 lbs. I now ask you which person is the biggest? We can't assume that the taller person weighs more than the shorter one, and likewise we can't assume the heavier person is taller than the lighter. Further more there is nothing at all to relate the size of the people who gave their height to those who gave their weights. And so it is with pH and titration. They both measure how "big" an acid is but each refers to something different.

Before talking about acid measurement it would be helpful to know just what an acid is. Most discussions in this area begin with the basic atomic structure. (protons, electrons, neutrons, etc.) I'm going to skip that part- read the first chapter of any chemistry book if you're interested.

Acids are molecules, and they must be made up of at least two atoms. At least one of those atoms must be hydrogen. This hydrogen ion is the magic part of the acid, has a positive charge, and is often written [H+].

The other part of the acid molecule could be as small as a single atom or consist of a large molecule with many atoms. For now we will call this part of the acid "the glob". It is technically called the anion and has a negative charge. The [H+] part is called the cation. The glob is the part of the acid that gives the acid its name, like malic, tartaric, acetic, or nitric. These names depend on exactly which atoms are in the glob and how they are hooked or bonded together. The glob may have more than one magic [H+] ion and an acid may have hydrogen atoms in it's glob.

When acid molecules are present in aqueous solutions such as water, wine or beer, they can dissociate. That means that the glob and its [H+] ion(s) separate from each other. The [H+] ion is highly reactive. Think of it as a loaded gun with an itchy trigger. This [H+] is what makes acids so special. It can react with many things and so that is why it is so important to us. When we talk of measuring acids, we are really talking about measuring something about the [H+] ion.

The two basic types of acids are organic acids and inorganic acids. Inorganic acids are considered strong acids because they dissociate completely. That is to say all of their hydrogen ions make the split from their respective glob. This in many ways makes them simpler, but unfortunately, they are not the ones we have to deal with.

Organic acids are found in living things like plants and our favorite beverages. The glob part consists mostly of carbon, hydrogen, and an occasional other atom. Some contain very large numbers of atoms.

Here's where it starts getting fun. Organic acids do not dissociate completely, so when we put them in solution we get some globs still hanging on to their [H+] ions as well as those which have split apart. If we had only one acid in a water solution we could make a graph and show strangely shaped curves relating how they dissociate with varying concentrations and what pH was associated with various positions along the curve. We can even do this with those acids which have more than one [H+], and these acids will have some globs with two H+, some with one, and some with none, each having different dissociation curves. In our beverages we don't know exactly which acids we have nor do we know how much of each one. What we will do is lump them all together (as they are in solution). We then take some measurements and relate them to important characteristics in our brew.

Titration measures the total [H+] ion concentration -- the ones floating around in solution, plus the ones still held on to by their globs. The pH measures only the ones that are dissociated and floating around in the solution. Titration measures how many COULD react. The pH measures how many are READY to react.

Before further discussion on acids I want to talk briefly on bases and water. Bases are roughly the opposite of acids. Instead of giving up, or dissociating with a positively charged hydrogen ion, [H+], they dissociate with a hydroxyl ion, [OH-], which has a negative charge. The "glob part" of a base would then have a positive charge.

Pure water, which is made up of two hydrogen atom and one oxygen atom, really isn't pure water. Part of the water dissociates. When it dissociates, one water molecule makes one molecule of [H+] and one molecule of [OH-]

H2O <=> [H+] + [OH-]

Only a very very small part of the water does this: Ten to the minus seven moles per liter. And now you say what the #@$% is that? Take a deep breath and trust me It's not as hard as all the Chemical texts make it.

Short diversion: I don't know how to write Ten to the minus seven in numbers so the e-mail system will format correctly so I'll make up a sort of code. when you see a double "**" it means to the power of. So one times ten to the minus seven will be written: 1x10**-7. I think I have seen similar adaptation elsewhere. So 1x10**-7 moles per liter of water dissociate according to the chemical formula :

H2O <=> [H+] + [OH-]

Imagine we wanted to "make" water by going to the bulk bins at the alchemical grocery store and getting the appropriate amounts of each ingredient. If we wanted to make a dozen molecules of water we need a dozen H+ and a dozen OH-molecules. If we wanted to make a Million molecules of water we need a million H+ and a million OH-.molecules. If we wanted to make 6.02x10**23 molecules of water we need 6.02x10**23 H+ and 6.02x10**23 OH- molecules. Now this HUUUUUUUUUUUUGE number is kind if special, and it called Avogadro's number. We could write this 0.602x10**24, (note the change in the decimal point and the power factor) because that would be 0.602 times 1 million, times 1 million, times 1 million, times 1 million. Avogadro's number really isn't all that important, other than it just happens to be how many hydrogen ions it takes to make one gram. It is also how many in a mole.

Now, let's suppose we get one gram of H+ ions. That is the same as one mole and has 6.02 x 10**23 molecules in it. As I said before, we need to add the same number of OH- molecules, so we need 6.02 x 10**23 molecules of OH-. We can't just measure out a gram of OH-. Obviously, The OH- molecule weighs more than just the H- because it has an atom of hydrogen AND an atom of oxygen.

Chemist derived a solution to this problem by using the atomic weights, which relates everything back to hydrogen. Hydrogen has an atomic weight of one. Thus gram molecular weight, is the molecular weight of a substance measured out in grams. (We could have used pound molecular weights or ton molecular weights but we don't Like how internationally politically correct would that be!)

So here is how it all works: Find out what the atomic weights are for all the atoms in your equation.

H = 1 O=16

Add up what each molecule weighs

H = 1 OH = 16 + 1 = 17 H2O = H x 2 +16 = 18

The above means there are one mole or AG's number molecules in:

1 gram of H

17 grams of OH

18 grams of H2O

so 1 gram of H +17 grams of OH =18 grams of H2O

Or 1 mole of H + 1 mole of OH = 1 mole H2O

This will make our water with no shortage or extras.

Concentrations:

Now with molarity we want to know how many moles of something there is in a liter of the stuff it is dissolved in. For pH we want to know how many moles H+ there are in water or our brew, be it wine or beer or something else. We usually don't concern ourselves with the OH- of bases for several reasons.

1) First of all with beer, wine and other beverages, we are always dealing with acids, not bases.

2) With the H+ ion, one mole per liter is one gram per liter. It is much simpler than the OH- ion where one mole per liter is 17 grams per liter. With the H+ ion, we can almost forget about the concept of molarity and just think in terms of grams per liter.

2) We can calculate the OH- concentration if we know H+ concentration.

But DON'T forget we are dealing with molarity with HO- ions.

Now back to pure water: H2O <=> [H+] + [OH-]

If we add an acid, it dissociates adding extra H+ ions to the pot. A new equilibrium must be developed. Of these extra H+ the acid has added, Some stay around as extra H+ but some combine with the OH- making water. This equilibrium is predictable in that the product of the H+ concentration and OH- concentration (both expressed in molarity) is always 1X10**-14

Pure water has equal parts H+ and OH-, and thus is considered neutral. The concentrations are 1X10**-7 H+ and 1X10**-7 OH- therefore, 1X10**-7 times 1X10**-7 is 1X10**-14

The actual pH number is calculated from the H+ concentration. It is -(log[H+]) Here is where you can do one of two things: Freak out! OR get a calculator and enter 1X10**-7 This is the same as .0000001. Push the LOG button. The number should be -7. Then multiply by -1 to get the pH: 7 Then say to yourself "Neat! Now what?"

We can make a chart like this:

Note that each time we change one pH unit, our [H+] concentration changes by 10. For example the [H+] concentration at pH =3 is 1000 times that at pH = 6 It is important to realize that the STEPS get BIGGER as we move away from neutral for both [H+] and [OH-] concentration. That is another reason pH and titration do not agree. Note also that the smaller the pH number the higher the acidity . With titrations, bigger numbers mean more acid. The pH of some common substances:, and process mid points:

Sulfuric, N, & Battery Acid	0.3
Hydrochloric (Stomach) Acid	0.8
Limes & Lemons	2.0
Vinegar	2.8-3.0
Coca-Cola	3
Jelly	2.5-3.5
Wines	2.8-3.8
Photo engraving	3.2 (process midpoint)
Pickling	3.5 (process midpoint)
Oranges	3.0-4.0
Acid Rain & Tomato Juice	4.0
Beer	4.0-5.0
Cottage cheese, Boric acid & Black Coffee	5
Nickel plating	5.8 (process midpoint)
Milk	6.0
Corn	6.2
Pure water	7
Swimming pool water	7.2
Blood, Human	7.3-7.5
Egg white & Sea water	8.0
Sodium bicarbonate	8.4
Borax	9.2
Lime-soda softening	9.4 (process midpoint)
Milk of Magnesia & Great Salt Lake	10
Brass plating	11.2 (process midpoint)
Ammonia, N	11.6
Photographic Developer	12
Lime,saturated	12.4
Bleach	12.6
Copper plating	12.8 (process midpoint)
Bottle washing (process midpoint)	13.05
Oven Cleaner	13.8
Drain-O & Sodium Hydroxide	14.0

Why pH is so important? The pH is generally measured in one of two ways, pH strips or a pH meter.

The paper strips have pH sensitive dies on them that change color when a particular pH is reach. The strips are compared to a color chart and a relative pH can be determined. These strips can be inaccurate if the solution being tested chemically reactive with the indicator or if the solution is strongly colored.

A pH meter is the best way to determine the pH. There are two parts to a pH meter. The first is a specialized voltage meter designed to read directly in pH. The second is the probe. The probe of a good meter will have three parts that are usually combined into a single unit called a combination probe. The three parts are the glass electrode, the reference probe and a thermometer. The glass electrode is made from a special glass sensitive to H+ ions and surprisingly they are all hand blown. These electrodes vary from unit to unit and therefore must be calibrated.

Calibration requires two buffer solutions: For acids pH 4 and pH 7, For bases pH 7 and pH 10. Buffers are solutions that resist change to pH, so they make good standards to calibrate to. Probes should be calibrated frequently; temperature as well as aging can account for differences in electrodes and let's not forget that they're hand blown. My meter automatically adjusts for temperature. On some meters it must be done manually.

There are some cheep hand held meters for around $30 that are calibrated at the factory. The one I tested against mine was off over an entire pH unit.

Once the meter is calibrated, put the probe in a small sample of the solution, wait for it to stabilize and take the reading. Do not dip the probe in the entire batch as probes have a tip that leaks solution into the sample. Throw the sample away afterwards.

Acid titration or titratable acid or total titratable acidity or total acid, are all, as I understand it , names for the same thing. I think it is easiest to understand titratable acid if we first talk about how to do a titration.

You can purchase kits (which include instructions.) from home wine making stores. They will contain:

Sodium Hydroxide Solution. (0.2 N NaOH)

An indicator (Usually phenolphthalein.)

A syringe for measuring a 15 ml sample

A second smaller syringe for measuring NaOH

A vessel for doing the test in (sometimes.)

It is best for the moment to think of the sample as being clear or a white wine. The test is made as follows.

15 ml of the sample (That's the wine being tested) is measured with the larger syringe. Some syringes are calibrated in cubic centimeters (cc.) instead of milliliters. They are essentially the same thing. This sample is placed in a small clear container. It is very important to be able to clearly see the color of the liquid through the container.

Place 2 or 3 drops of the color indicator solution in the sample and give it a swirl to mix it in. Measure a quantity of NaOH in the smaller syringe. I suggest filling it exactly to an even mark. When we are done, we will need to exactly how much NaOH we have added to the sample.

Begin slowly adding NaOH to the sample. Constantly swirl the solution to keep it mixed. As you do this you will begin to notice purple or pinkish color swirls. When this happens, slow down. You are nearing the end point. When the solution turns purple and stays that way, stop. It is sometimes helpful to have a white piece of paper as a background.

Figure out how much NaOH you used to turn the sample purple. Suppose you needed 6 ml NaOH. Then we would say the titratable acidity is 0.6 % as tartaric. Sometimes this is also expressed as 6.0 grams per milliliter. Notice the difference in the placement of the decimal point. These two expressions are actually the same. There are 1000 grams of water in a liter, so 6/1000=0.006. Dividing by 100 to express as a percent gives 0.6%

We use tartaric acid as a standard in wine making. When we say, "0.6% as tartaric", We are saying that our sample, which has many different acids in it, has the same measure of titratable acid as a 0.6% tartaric would.

The solutions in the test kit are rigged to give the right numbers if the directions are followed. Follow the directions for your kit, just in case the concentrations are rigged differently, but I think the test is pretty standard.

Now that we know how to do the test, let's take a look at what is happening chemically. When organic acids dissociate, only a few hydrogen ions are formed. An example would be like acetic acid. For every 100 molecules of acetic acid only one dissociates. It is only that one out of the 100 that gets measured with pH.

The NaOH is a base. When we add the NaOH, it dissociates into Na+ and OH-. We now have a solution with the following.

Acetic Acid	Water	Sodium Hydroxide
CH3COOH, H+, CH3COO-	H2O, H+, OH-	Na+, OH-
(acid adds H+)	(Neutral)	(base adds OH-)

The water was neutral, ie equal amounts H+ and OH-.

We added acetic acid which gave us extra H+.

Now we add NaOH which adds extra OH-.

The H+ and OH- will now combine and make water.

The CH3COO- and the Na+ are the leftover acid and base "globs"

The CH3COOH is an entire acid molecule. It has not dissociated yet. It is like an unopened box of acid in solution.

I like to use the analogy of kids playing in a swimming pool. All the kids have balloons to play with, but many of them keep them in their pockets. (It's more fun to bat them back and forth than to just hang on to your own.) The balloons are H+, the kids without balloons in their pockets are the dissociated acid "globs", and the kids with balloons still in their pockets are the undissociated, complete acid molecules. The pH measures how many balloons they're playing with at any given time. This is probably the most useful information, but we would also like to know how many balloons there really are. Adding a molecule of NaOH is like throwing a dart at a balloon. NaOH is a strong base. Every molecule associates. Every OH- dart hits a balloon. We throw a few darts and the kids want more balloons to play with, so they take them out of their pockets. -A new equilibrium is reached. We keep throwing darts until they run out of balloons. This is the titration end point. The color indicator turns purple. We then count up how many darts we threw.

We apparently taste both free and bound H+ ions. I think I have heard that we taste the free ones "more loudly" than bound ones, but there are so many more bound ones that titration gives a more accurate measure of taste. The pH gives a better measure of whether or not the wine will go bad, whether the sulfite will work, or whether the mash will produce the right amount of sugar.

To expand on our knowledge of chemistry and exactly how we are counting those NaOH darts, we must know more about the concentration of NaOH. The list above said 0.2 N NaOH. N means normal. We must therefore understand what normality is in relationship to molarity. First lets look at acids. We will look at two examples.

Nitric acid dissociates as follows:

HNO3 -> H+ + NO3-

Note that for each mole of HNO3 we get one mole of the H+ and one mole of "glob", NO3-. We would make a 1 molar solution by adding one mole of HNO3 to enough water to make 1 liter. Remember that one mole is it's

molecular weight in grams. Therefore, one mole of HNO3 is 63 grams.

Our second acid will be sulfuric acid, H2SO4. It has two dissociation reactions:

H2SO4 -> H+ + HSO4-

HSO4- <-> H+ + SO4--

If we ignore that the second reaction will probably not be complete, we can say that one mole produces 2 moles of H+. In a sense it is twice as strong as HNO3 because it produces twice as many H+'s We can define yet another term. the EQUIVALENCE, as the number of moles of H+ produced by one mole of acid. Therefore. the equivalence of HNO3 is 1 and that of H2SO4 is 2. From there we can define normality.

NORMALITY = EQUIVALENCE X MOLARITY

rearranging:

MOLARITY = NORMALITY / EQUIVALENCE

So a 0.1 normal solution of HNO3 has a molarity of 0.1 / 1 = 0.1 but, a 0.1 normal solution of H2SO4 has a molarity of 0.1 / 2 = .05

The important part of all this is solutions with the same normality, have the same amount of H+. The same theory goes for bases except we are now talking about OH-.

Now back to our 0.2 N NaOH

Again it dissociates : NaOH -> Na+ + OH-

One mole NaOH produces one mole H+ so its equivalency is 1. It's molarity is then 0.2 / 1 = 0.2

Tartaric acid has the chemical formula HO2.CHOH.CHOH.CO2H. It can release 2 H+. Its molecular weight is then 150. Therefore a one molar solution would be made up of 150 grams to one liter, and it would be 2.0 normal. More importantly, our 0.2 normal solution NaOH has 0.2 moles H+ per liter, or 0.0002 moles per ml. It took 6 ml. of this solution to counteract all the H+'s in our 15ml sample. That means there were 6 X 0.0002 = 0.0012 moles H+ in our 15 ml sample. We divide by 15 and multiply by 1000 to get how many H+ there would be per liter. This would be 0.08. Since tartaric acid has two H+ per molecule (2.0 equivalency) there are 0.04 moles per liter. By multiplying the molarity by the atomic weight we get the number of grams per liter. This is equivalent to 150 X 0.04 = 6 grams per liter. And 6 grams / 1000 grams = .006 or 0.6% tartaric. This number should be familiar as It is the same number as I started with in my original explanation of what the units of the results of titration were. For wines these numbers will range from about 1 to 9 grams per liter tartaric or .1% to .9% as tartaric. I have seen numbers outside this range but not in anything really drinkable.

Titrations of the sort I just explained are relatively simple in clear solutions and white wines. In red wines, the color of the wine is already so close to that of the end point, that it becomes difficult to know when to stop. So what do you do if you have a dark red or purple wine and you can't see the color change. The answer is to use a pH meter to determine the end point.

A number of years ago I was told to titrate to a pH of 8.2. This is the process I have been using, but it would seem more logical to go to pH = 7.0. I have found the 8.2 number in essays of standard processes outlined in wine making texts, so it is correct but I can't explain why.

I am experimenting and researching ML bacteria for acid reduction. It may prove an interesting article as well.

Good luck and good brewing.

Master Gerald Goodwine

Check out "Principles of Brewing Science" by George Fix, ISBN 0-937381-17-9 for more brewing chemistry.