# Presentation: 504253

## pH Meter - Use and Calibration

## What is a pH Meter?

- An instrument that measures the H+ ion concentration (pH) of a solution using an ion sensitive electrode which will ideally respond to one specific ion, in this case H+
- The pH of a solution is the negative logarithm of the H+ ion  concentration
- A typical modern pH meter has a glass and reference electrode in one tube

## How Does a pH Meter Measure H+ Concentration?

- 	The pH meter has two electrodes in one tube, glass and reference electrode.  A saturated KCL and HCL solution is contained in a tube which is inside of an outer tube that will have contact with the solution to be measured.  This outer tube has a  double glass bulb with NA+ which makes an ion specific electrode. When measuring the pH of a solution, a salt bridge forms.  The NA+ ion, not H+, crosses the glass membrane of the pH electrode and allows for a change in free energy which is measured by the pH meter as the concentration of H+.

**Notes:**
The pH meter measure the H+ concentration (the pH ) of a solution using an ion sensitive electrode. An ion selective electrode will ideally respond to only one specific ion.  A typical modern pH probe is a combination electrode, which has the glass and reference electrode in one tube.  The bottom of a pH electrode balloons out into a round glass bulb. The electrode contains a tube within a tube with the inside electrode having the KCL  and HCL solution as well as the cathode terminus of the reference probe. The outer tube wraps itself around the inner tube and  also ends with some type of reference tube like the inner tube. The outer tube only has contact with the solution and ions cross through a porous plug that serves as a salt bridge.  The measuring part of the pH meter, the glass bulb on the bottom is coated both inside and out with a 10nm layer of hydrated gel. These two layers are separated by a 0.1mm layer of dry glass.  The metal cations(NA+) in the hydrated gel diffuse out of the glass and into solution while H+  from solution can diffuse into the hydrated gel. It is the hydrated gel, which makes the pH electrode an ion selective electrode. H+ does not cross the glass membrane of the pH electrode, it is the NA+ which crosses allows  for a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is free energy change and this is what the pH  meter actually measures.  The hydrated gel membrane is connected by NA+ transport  and thus the concentration of H+  on the outside of the membrane is ‘relayed” to the inside of the membrane by Na+.

## Calibrating a pH Meter

- Make sure the meter is in pH mode
- For a 3 point calibration, use high pH (--), pH 7.0 and low (-.0) solution
- Before calibration, rinse probe thoroughly with de-ionized water or a rinse solution
- Immerse the end of the probe completely in the calibration solution
- Stir the probe gently to create                        a homogenous sample

**Notes:**
Some machines will indicate of what type of mode is presently in use with symbols. With the machine is in pH mode rinse the electrodes with de-ionized water before you calibrate. I want to emphasize the importance of rinsing the electrodes off before each calibration and before they are placed in any solution that needs to be pH. The pH bulb should always be stored wet,  preferably in pH  4.00 buffer with 1/100 part of KCL  added. Other pH buffers or tap water are acceptable storage media. This step needs to  done especially at the end of  the day when the machine will no longer be used.  After you have rinsed the electrodes dip the probe in the calibration buffer. It is suggested that you standardize (calibrate) with at least three points before the use of the machine.   Some machines have calibration dials (cal ) dials used do standardization. If you need 2 calibrations to standardize the machine the knobs may say ca1 1,2 or 3.  If your machine has  an “auto read” and it is turned on , it will automatically endpoint when the reading is stable 30 seconds to a minute).
 For a 1 point calibration place the tip of the electrode in the first buffer and press “cal”. Some machine have 5 calibration buffers are shown on the display. Make sure you rinse the electrode between each calibrator.

## Measuring the pH of a Solution

- Always rinse electrodes with de-ionized water prior to placing in a solution for pH measurement
- Allow meter to stabilize for 30 seconds or a minute, then read
- Remove electrodes and rinse with de-ionized solution
- The pH bulb should always be stored wet preferably in pH 4.0 buffer with 1/100 KCl
- Other buffers or tap water can be used for storage

## Maintenance of pH Meter

- Wipe off exterior of pH meter with a damp cloth after use
- The outside of the meter is made of polyester and is not affected by most solvents but is affected by some organic solvents
- pH  the “power of hydrogen”

## Litmus Paper

**A Quick Method for Determining pH**

## Litmus Paper

- Litmus paper is useful to determine the pH of a solution when you do not have a pH meter available
- Proper storage of the litmus paper is essential to maintain quality control

## The pH Scale

- The pH measures the concentration of H+ in a solution
- The lower the number, the more acidic the solution
- The higher the number, the more basic the solution

**Notes:**
Have different beakers of clear liquids and have students dip litmus paper in them to determine acid or base. Examples, bleach, clear soda, vinegar.
Have different beakers of clear liquids and have students dip litmus paper in them to determine acid or base. Examples, bleach, clear soda, vinegar.

## The pH Scale – Another Description

**The pH Scale – Another Description**