ph in plain language

pH IN PLAIN LANGUAGE

To understand and learn about pH measurements is not difficult as long as we stick to fundamentals. That is, let us be concerned with the question, "What is the pH of my solution?", and then we can think of the instruments and techniques as tools to help us get the answer.

In plain language we'll discuss what pH means, why it is important and the results that you may expect to get from reading this paper.

If you can study this paper and at the same time have an operating pH meter in front of you, the results will be even more dramatic. Here you'll read about it, see it, use your hands to make it work, and finally get the results, all in one tremendous learning effort.

Our format is simple - each subject heading asks a question, and the paragraphs that follow provide the answer.

To get off to a good start and give you some incentive, we'll begin with a look at why pH is important.

WHY IS pH IMPORTANT?

If the pH of the blood in your body were lowered one unit, you would die. Living things grow and survive in a particular pH environment and when the pH is not correct their growth and survival are threatened. For example, wheat, corn and other foodstuffs grow best in soil of a particular pH. To get the greatest yield, the farmer must condition his soil to achieve the proper pH. This explains, in part, why the yield per acre has increased in recent years since soil science has shown the farmer how to provide optimum conditions for best growth.

Here's a list of pH important areas.

1. Drinking water purification depends on correct pH for its operation.

2. In sugar manufacture, improper pH can result in formation of unwanted acids and very little sugar.

3. In sewage treatment, pH must be adjusted for proper operation of disposal plants. This is, of course, another reason why polluting sewers with high acid or alkaline material spills is undesirable.

4. Milk turns sour at a pH of 6.00. Thus good milk requires good pH control.

5. In old-fashioned jelly making the pH must be below 4.0. Otherwise the jelly will not jell.

6. The brightness of chrome coating on our auto bumpers is directly related to the pH of the plating solution.

To sum up, almost every manufacturing process that involves even the simplest chemical reaction is sensitive to pH and will usually produce best results at some optimum pH value. One of the key factors in keeping consistent, uniform products is the ability to measure and maintain pH at the proper level.

WHAT IS pH?

pH is a number that exactly describes the degree of acidity or basicity of a solution. We can make a good analogy with the measurement of temperature. Here we have the terms cold and hot and we immediately realize that these are very general terms which cannot be used with any degree of accuracy. Accordingly, the temperature scale was developed and now a temperature reading of, say 50°C, means the same to everyone and is scientifically accurate.

In the same way, the pH scale was developed. Centuries ago, man discovered that certain materials possessed properties which he called acid, while others possessed other properties which he called bases. Between these two was a neutral area in which the material showed neither acid or base properties and he termed this a neutral.

Now just as in hot and cold, the words acid and base do not give us a scientific value that we can use. We needed a scale such that we all would be in exact agreement when we discussed the degree of acidity or basicity.

Some time ago a fellow named Sorensen developed such a scale and at the same time came up with the symbol pH.

Before we study the scale, let's take a moment to find out what makes one material an acid while another is a base.

1. An acid must have ionized (or free) hydrogen ions, H⁺.

2. A base must have ionized (or free) hydroxyl ions OH^-.

3. The pH is directly related to the ratio of H⁺ to OH^-. If the H⁺ is greater than OH^-the material is acid. If the OH^- is greater than the H⁺, the material is a base. If equal amounts are present, the material is a neutral salt.

Back to the development of the scale. A molar solution of hydrochloric acid is about a 3.6% solution of the acid. Let's assign this solution a pH of 0.

A molar solution of sodium hydroxide (lye) is about a 4.0% solution of the base. Let's assign this solution a pH of 14.

If we now dilute the acid by adding one milliliter of acid to nine milliliters of pure water we will have a 1/10 molar solution. Let's assign this solution a pH of 1. In the same way, let's dilute the molar solution of sodium hydroxide by adding 1 milliliter to 9 milliliters of pure water. We1ll assign this resultant solution a pH of 13.

Notice that with the acid, our 1/10th dilution increased the pH from 0 to 1, while the same dilution of the base reduced the pH from 14 to 13. Now here's the significant point - in both cases we were going toward 7. As we'll see shortly, a pH of 7 is the exact middle of the scale at which we have neither acid nor base, but neutral.

Continue to dilute both the acid and base by 1/10th and each time increase the number in the case of the acid and decrease the number in the case of the base.

The results look like this

Acid pH (molar)		Base pH (molar)
1.0	0	14	1.0
0.1	1	13	0.1
0.01	2	12	0.01
0.001	3	11	0.001
0.0001	4	10	0.0001
0.00001	5	9	0.00001
0.000001	6	8	0.000001
0.0000001	7	7	0.0000001

Notice an interesting fact on the acid column - the number of decimal places exactly equals the pH!. Therefore a change of 1/10th or 10 times in acid concentration will make a change of 1 pH unit either up or down. The same holds true for bases. A change of either 1/10th or 10 times will change the pH by 1 either up or down.

The following is really not plain language and can be skipped without too great a loss.

pH is really a number that is the negative logarithm of the molar concentration of H⁺ (Hydrogen ions).

Another interesting fact from the chart: if we mix equal volumes of pH 3 (0.001 HCl) and pH 11 (0.001 NaOH), the resultant solution will be a pH of 7. The same applies to 0 and 14, 1 and 13, and so on. Equal amounts of H⁺and OH^- neutralize and result in a pH of 7.

A logical question at this point is "Do all acids and bases act the same?" Unfortunately they do not. The major difference comes in the amount of free H⁺ or OH^- that is present in solution. For example, when you dissolve hydrochloric acid in water, all of the hydrogen ions H⁺ are free in solution. The same is true of the sodium hydroxide and the hydroxyl ions OH ^-. However if you dissolve acetic acid in water, all of the hydrogen ions H⁺ are not free (only about. 1.3% of the total H⁺) and the pH is considerably higher than what you would expect for a molar solution of the acid. Thus, acetic acid (vinegar) is called a weak acid. A good example of a weak base is ammonium hydroxide (household ammonia).

The subject of ionization and dissociation will not be covered here. Just bear in mind that not all acids or bases are completely ionized.

Refer to the scale below to picture just how pH relates to acid and base. Adding acid on the left tips the scale pointer to the left and the number gets smaller (more acidic). Adding base on the right tips the scale pointer to the right and the number gets larger (more basic).

HOW DO WE MEASURE pH?

Although there are paper and dye indicators that change at different pH values, we shall be concerned only with pH meters since we are interested in a precise number that represents the true pH of our solution.

First, there is a pH probe (which we'll discuss shortly) which produces a voltage that can be directly related to the pH of the solution in which we place the probe. Secondly, there is an electronic circuit within the pH meter cabinet that receives the voltage from the probe and then presents it to the meter scale. This voltage developed at the probe will cause the meter pointer to move. The value of the number at which the pointer stops is the pH of the solution.

Let's.say we had three separate solutions. No. 1 contains 0.001 molar hydrochloric acid. No. 2 contains pure water and No. 3 contains 0.0001 molar sodium hydroxide.

No. 1	No. 2	No. 3

Fill in the blanks with the number that you would expect the pH meter to read as you move the pointer from solution to solution. (If you wrote in No. 1 pH 3, No. 2 pH 7 and No. 3 pH 10, you get 100%.)

There are ten small divisions between 7 and 6. Each small division is equal to 0.1 pH units. Thus if the meter pointer came to rest at the fourth small division to the right of the number 6, the pH would be 6.4. If the pointer came to rest between the small divisions, one can estimate and write the value as 6.45.

The shiny strip just below the scale is called mirror-backing. Its purpose is to allow you to look at the pointer and if you see a reflection of the pointer in the mirror you move your head slightly until you cannot see the reflection. This means that you are looking directly at the pointer and therefore the value you read is correct. You can readily see that if two people read the same meter but from different angles, they would report different values. The mirror backed scale helps us all to look at the same angle.

Of course this is now pretty well outdated by the advent of digital pH meters. (Ed.)

The probe can be thought of as a battery whose voltage changes as the pH of the solution in which it is inserted changes. It consists of two parts (in fact many pH measurements are made with two separate probes), first the hydrogen sensitive glass bulb and second the reference electrode. The special glass of the bulb has the ability to pass H⁺. This ability then allows the H⁺ inside the bulb to compare themselves with those outside the bulb and to develop a voltage that is related to this difference. This bulb then is a half-cell and will need a companion reference to function.

On the drawing notice that just above the bulb is a reference electrode frit. This is actually a small opening in the glass through which the inside filler solution can very slowly leak out. Now the relationship between the reference electrode and the solution also produces a voltage and this is the other half cell. Together the pH sensitive bulb and the reference electrode constitute the complete probe.

The value of voltage produced by the probe is fortunately a linear function of the pH. For example, at pH 7.00, the probe produces zero volts while at pH 6.00 it produces +0.06V or +60 millivolts. Notice the plus polarity mark; if the voltage were of minus polarity the meter pointer would go to the right to a pH reading of 8.00.

Generally, a probe will produce about 60 millivolts for each change of 1 pH unit. Thus, a probe voltage of +300 millivolts would cause the meter to read pH 2.00 (+300 / 60 = 5 units, 7 - 2 = 2).

Since the pH meter and probe are both electronic-type devices, you might like to have some standard pH solution that you can measure and be sure that everything is calibrated correctly. These solutions are available and they are called standard buffers. You'll soon find that a buffer is a vital part of all pH measurements.

A buffer is a solution of a particular pH that has the ability to resist change in pH. (By the way, buffers in our blood system are what keeps us alive and healthy.)

Here's an experiment to demonstrate the ability of buffers to resist change. Take two glasses and in one place distilled water. In the second, place a buffer near pH 7.00. Next measure and record the pH at the start. They should both be near 7.00. Now, add one drop of concentrated hydrochloric acid to each glass. Stir, and again record the pH. Do this for several drops with a measurement between each drop. You should find the distilled water drop in pH with each drop. The buffer, on the other hand, should resist change until you exceed its buffering capability.

We won't go into why buffers act this way. Suffice to say it has to do with a phenomenom called common-ion effect. Look it up in your chemistry text.

Temperature is also a very important factor in pH measurements and here's why. We said earlier that the voltage output of a probe was about 60 millivolts per pH unit. However we didn't confuse you by telling you that this is true only at 25°C (room temperature). If you reduce the temperature to 0°C, the probe will provide only about 54 millivolts. And if you increase the temperature to 100°C the probe will provide about 70 millivolts per pH unit.

You can easily see that you must have some way of correcting for this, or make all of your measurements at 25°C.

Let's re-state this situation as a problem and see if we can get a clear understanding of what's happening.

problem: How to make a pH meter read the correct value of a buffer or sample solution at all temperatures.

Probe output for each pH unit

0°C	54 millivolts
25°C	60 millivolts
50°C	64 millivolts
75°C	68 millivolts
100°C	70 millivolts

In looking at this table we realize that our pH meter must be able to move one pH unit with 54 millivolts input or 70 millivolts or any value in between. Most meters can do this.

Solution: Let us put a sensitive attenuator in the meter and adjust it first to work with 54 millivolts to move 1 pH unit. We'll have a knob on this attenuator and mark the point at which 54 millivolts equalled 1 pH unit. Next, let's move the attenuator to a position where it requires 70 millivolts to move the pointer 1 pH unit. (By the way, we can apply these voltages from a voltage source, and do not require a probe.) In the same way we can set 60 millivolts and so forth. When we are finished we'll have a knob control in which we can set the proper sensitivity of the meter to correspond to the temperature of the sample solution.

Finally, since we are really concerned only with pH (and not millivolts), let's erase the 54 millivolts and write instead 0° C. For the 60 millivolts write 25°C and so forth around the circle.

At this point let's review a little and then we'll make a bona fide pH measurement.

First we discussed the meter and its mirror-backed scale. Next we found that probes are really two-piece affairs (reference and pH sensitive bulb) that produce millivolt outputs and vary with pH. Then the important buffers were described and finally we found that temperature played a vital role in accurate measurements.

To pass the course, we must successfully use this information and produce an accurate pH measurement. Assume for the moment that someone has just handed us a bottle containing a watery solution and asked "What is the pH of this?" Here's how to proceed.