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Chapter 13: Problem 3

What is the alkaline error and the acid error of a glass membrane \(\mathrm{pH}\) electrode?

Short Answer

Expert verified
The alkaline error occurs in highly alkaline solutions (pH > 12), causing falsely low pH readings. The acid error happens in strongly acidic solutions (pH < 1), resulting in falsely high readings.

Step by step solution

01

Understanding pH Measurement

When a pH electrode is used, it measures the concentration of hydrogen ions in a solution, converting this measurement into a pH value. The glass membrane electrode generally provides accurate readings within the pH range of 1 to 12.
02

Defining Alkaline Error

The alkaline error occurs when a pH electrode is used to measure solutions with a pH above 12. In these highly alkaline solutions, the electrode may mistakenly register alkali metal ions, like sodium or potassium, as hydrogen ions, leading to inaccurately low pH readings.
03

Defining Acid Error

The acid error happens when a pH electrode is used in solutions with a pH below 1. In these strongly acidic solutions, the high concentration of hydrogen ions can saturate the glass membrane, preventing it from accurately reflecting further increases in hydrogen ion concentration, which causes it to give inaccurately high pH readings.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alkaline Error
The alkaline error is a phenomenon that affects the accuracy of pH measurements when using a glass membrane electrode in highly alkaline environments. This occurs when the pH value of a solution exceeds 12.

In such solutions, the electrode may mistake alkali metal ions, like sodium (Na鈦�) or potassium (K鈦�), for hydrogen ions (H鈦�).
This is because at high pH levels, the concentration of hydrogen ions is very low.
Therefore, instead of measuring only H鈦� ions, the electrode may also respond to similar ions present in the solution.

As a result, the pH readings may be inaccurately low, making it seem as if the solution is more acidic than it truly is.
To minimize this error, specialized electrodes or calibration techniques can be used for alkaline solutions.

  • Occurs in pH > 12
  • Involves misinterpretation of Na鈦� or K鈦� as H鈦�
  • Leads to inaccurately low pH readings
Acid Error
The acid error is an issue that arises during pH measurement in highly acidic solutions, particularly when the pH is below 1.

In such environments, the concentration of hydrogen ions is extremely high.
This can saturate the glass membrane of the pH electrode, leading to problems with further increases in hydrogen ion concentration.

The saturation effect means that the electrode cannot accurately discern small changes in the hydrogen ion concentration, causing the electrode to return inaccurately high pH readings.
This makes it seem like the solution is less acidic than it truly is.

To address this, choosing appropriate calibration settings and sometimes using specialized electrodes can help ensure more accurate readings.

  • Occurs in pH < 1
  • Saturation of the glass membrane with H鈦�
  • Results in inaccurately high pH readings
Glass Membrane Electrode
A glass membrane electrode is a tool used for measuring pH levels in a solution. The glass membrane plays a crucial role in pH measurement due to its selective permeability to hydrogen ions.

Here鈥檚 how it works: the electrode is made of a thin, special type of glass that allows only H鈦� ions to pass through.
This selectively permeability is essential for converting the concentration of hydrogen ions into a readable pH value.

Glass membrane electrodes are reliable and accurate under most conditions, typically within a pH range of 1 to 12.
However, extreme conditions like very high or low pH levels can introduce errors, such as alkaline or acid errors.

Despite these limitations, glass membrane electrodes remain popular due to their high sensitivity and ease of use.
By understanding the limitations and proper calibration techniques, more accurate measurements can be obtained.

  • Measures pH by selective permeability of H鈦� ions
  • Accurate in the pH range 1 鈥� 12
  • Could be affected by alkaline or acid errors

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Most popular questions from this chapter

A potassium ion-selective electrode is used to measure the concentration of potassium ion in a solution that contains \(6.0 \times 10^{-3} M\) cesium (activity). From Table \(13.3,\) the electrode responds equally to either ion \(\left(K_{\mathrm{KCs}}=1\right)\). If the potential versus a reference electrode is \(-18.3 \mathrm{mV}\) for a \(5.0 \times 10^{-3} M \mathrm{KCl}\) solution and \(+20.9 \mathrm{mV}\) in the sample solution, what is the activity of \(\mathrm{K}^{+}\) in the sample? Assume Nernstian response.

Describe the different types of ion-selective electrodes. Include in your discussion the construction of the electrodes, differences in membranes, and their usefulness.

The quinhydrone electrode can be used for the potentiometric determination of \(\mathrm{pH}\). The solution to be measured is saturated with quinhydrone, an equimolar mixture of quinone \((\mathrm{Q})\) and hydroquinone \((\mathrm{HQ})\), and the potential of the solution is measured with a platinum electrode. The half-reaction and its standard potential are as follows: What is the \(\mathrm{pH}\) of a solution saturated with quinhydrone if the potential of a platinum electrode in the solution, measured against a saturated calomel electrode, is \(-0.205 \mathrm{~V}\) ? Assume the liquid- junction potential to be zero.

A 50 -mL solution that is \(0.10 M\) in both chloride and iodide ions is titrated with \(0.10 M\) silver nitrate. (a) Calculate the percent iodide remaining unprecipitated when silver chloride begins to precipitate. (b) Calculate the potential of a silver electrode versus the SCE when silver chloride begins to precipitate and compare this with the theoretical potential corresponding to end point for the titration of iodide. (c) Calculate the potential at the end point for chloride. For simplicity, in lieu of activities, use concentrations in calculations.

The perchlorate concentration in a sample containing \(0.015 M\) iodide is determined using a perchlorate ion-selective electrode. All samples and standards are diluted 1: 10 with \(0.2 M \mathrm{KCl}\) to maintain constant ionic strength. A \(0.00100 M \mathrm{KClO}_{4}\) standard gives a reading of \(-27.2 \mathrm{mV}\), and a \(0.0100 M\) KI standard gives a reading of \(+32.8 \mathrm{mV}\). The sample solution gives a reading of \(-15.5 \mathrm{mV}\). Assuming Nernstian response, what is the concentration of perchlorate in the sample?
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