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pH Paper

pH papers contain compounds which will undergo a specific color change at a particular pH value.  pH sticks have different indicator papers sealed along the length of the stick.  pH papers and sticks can be used to obtain quick pH measurements.  The pH paper is simply dipped in the sample and the color compared to the reference chart supplied with the product to indicate the pH.  This method will not give highly accurate pH readings as the comparison of the paper color to the reference chart is subjective.  However, an accuracy of between 0.3 and 1 pH units are obtainable.

Product pH Range Color Change
Litmus red paper 5-8 Red to blue
Litmus blue paper 5-8 Blue to red
Phenolphthalein paper 8.5-10 Colorless to red
Indicator paper 2-12 in 2 unit steps -
Indicator paper 0.5-5 in 0.5 unit steps -
Indicator strip 0-14 in  1 unit steps -
Indicator strip 1.7-3.8 in 0.3 unit steps -

Table 1: Examples of available pH papers and strips.

 

Figure 1: Examples of pH paper and strips.


pH Meter

For more accurate pH readings a pH meter measuring system should be used.  A pH measurement system consists of three components, a pH measuring electrode, a reference electrode, and a high impedance pH meter.  In most modern pH measuring systems the pH measuring electrode and the reference electrode are mounted into the same device, known as a combination electrode.

The pH sensing electrode contains a solution with a constant pH value.  When the electrode is placed in a solution a potential is developed which is caused by the activity of the H+ ions in the sample solution.  The reference electrode has a defined, stable potential which is not affected by the H+ activity of the sample solution.  The pH meter will measure and convert the electrode voltages into a pH reading.

The response (or slope) of the pH electrode is defined by the Nernst equation (Equation 1):

Where:
E0 = cell potential under standard conditions
R = gas constant (8.314 JK-1mol-1)
F = Faraday constant (9.6485x104 Cmol-1)
T = temperature in K
n = ionic charge

The theoretical slope for an electrode is 59.16 mV where the H+ ionic charge (n) = 1 at 25 °C (298 K).  Therefore, a one unit change in pH will result in the system sensing a change of 59.16 mV.  Measuring the electrode slope gives an indication to the working condition of the electrode.

From equation x it can be seen that temperature can have an effect on the electrode slope, therefore, this may need to be taken into account when measuring pH.  Automated Temperature Compensation (ATC) probes are available which can be used to correct for temperature.

 
 
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