pH
Return to the... Group I pH Prelab.

Background

pH is a measurement of the hydrogen ion concentration and is defined as:

pH = -log[H+]

For aqueous solutions, the hydrogen ion, H+, is the Ahrrenius definition of acid and the hydroxide ion OH- is the definition for base. A pH of 7.00 at 25C is defined as neutral. At this pH, the hydrogen ion concentration equals the hydroxide ion concentration. When the hydrogen ion concentration increases, the pH drops.

Note the logarithmic nature of pH. This is convenient since in practice it is easy to change the hydrogen ion concentration over 16 orders of magnitude. So instead of saying the solution has a concentration of [H+] = 1.0 x 10-13 we can say it has a pH of 13.00 .It becomes a little less clear when the concentration increases to 5.0 x 10-13 since now the pH is 12.30 . You should check this with your calculator.

Finally a word about Sig Figs.
The pH, 12.30, reflects an accuracy of ...........
two sig figs!
The 12 (this is called the logarithm's characteristic) reflects the order of magnitude and is derived from the 10-13. Therefore it is not part of the sig figs. It is only the logarithm's mantissa, .30, which can count as sig figs.

MEASURING pH

USING the pH METER

The pH meter must be calibrated before use. You will use a pH =7.00 buffer.

A buffer is a solution which maintains its pH despite dilution and minor contamination.

  • To CALIBRATE:
    • Rinse the electrode with distilled water and pat dry with a towel. Immerse in the pH=7.00 buffer.
    • Switch the pH/standby Switch to pH.
    • Adjust the meter using the pH 7 Calibration knob so the METER reads exactly 7.00
    • Return pH/standby Switch to standby.
  • To READ
    • Rinse and dry the electrode after calibration
    • Put electrode into SOLUTION.
    • Switch pH/standby Switch to pH
    • Swirl and read Meter.
    • CAUTIONS
      • The pH electrode is fragile. Do not let the glass bulb at the bottom of the electrode hit the container. There is a plastic guard to protect the electrode but it isn't idiot proof !!
      • Always rinse and calibrate the electrode before using.

PARALLAX ERROR in reading the METER

The following photos are reading the pH of the same solution.

The center photo is an overall view of the meter. pH is the top scale and goes from 0 to 14.

On the left is a closeup of the needle reading. Note you can see the red needle but not its reflection in the mirror strip.The camera is "looking" perpendecular to the needle. This reading is giving the correct pH...... 3.34

In the picture on the right, the same meter reading is made but with the camera's point-of-view on a oblique angle from the right. Here you can not only see the needle, but also its reflection in the mirror strip.
This
parallax error results in the meter being read as pH = 3.18 rather than the true 3.34!

SOME COMMON LIQUIDS

Are they Acids or Bases?

Do you know the pHs of these common solutions?

Not to panic.....

You will measure them in this experiment!

However you might want to remember
Acids tend to be sour, bases bitter.

RWK