pH and Conductivity Meters Questions and Answers

Answer: A

Both hold, and the second is why the first is necessary. Every electrode has its own offset and its own slope, both of which drift as it ages, so a calibration belongs to an electrode and not to a meter. Good practice recalibrates daily in routine work and immediately after any change of electrode, filling solution or measuring temperature. Recording the slope at each calibration also gives an early warning of an electrode approaching the end of its life.

Answer: B

Rubbing the bulb with a tissue charges the glass electrostatically, and the charge takes minutes to leak away through the very high resistance of the membrane, during which readings are noisy and drift. Rinsing with distilled water and blotting gently avoids it. The gel layer is not removed by a wipe, though it can be damaged by abrasion, speed is not the argument, and rinsing does not usefully equalise temperature. It is a small point of technique that accounts for a surprising amount of irreproducibility.

Answer: C

In an ISFET the metal gate of a field effect transistor is replaced by an ion sensitive insulator in contact with the solution, and the surface potential modulates the drain current directly. It is rugged, needs no glass bulb, can be made very small and does not have to be kept wet, which suits food, skin and field measurements. It still needs a reference electrode and still requires calibration, and its drift is generally worse than good glass, so it complements rather than replaces the classical electrode.

Answer: D

Conductivity measures the total ability of a solution to conduct electricity, summed over every ion present and weighted by its charge and mobility. It is therefore a bulk, non-selective measurement: it tells you how much ionic material is present but not what it is. That is its strength in water purity monitoring and its limitation in analysis of mixtures. pH responds to one ion, diffusion rates are a transport property, and identifying an ion needs a selective electrode or a separation.

Answer: A

Conductivity is a property of the material rather than of the sample cell, so its unit carries a length: siemens per metre, or in practice microsiemens per centimetre for water and millisiemens per centimetre for stronger solutions. The plain siemens is conductance, which depends on cell geometry, ohm metres is resistivity, and siemens square metres per mole is molar conductivity, the conductivity divided by concentration. Keeping the three apart avoids most of the confusion in this subject.

Answer: B

The cell constant is the ratio of the distance between the electrodes to their effective area, in units of reciprocal metres or reciprocal centimetres, and multiplying the measured conductance by it yields the conductivity. Cells of constant 0.1 suit pure water, 1.0 general work and 10 concentrated solutions, because each keeps the measured resistance in a comfortable range. It is determined in practice by measuring a certified potassium chloride standard rather than from the geometry, since fringing fields make the effective area uncertain.

Answer: C

Conductivity is conductance multiplied by the cell constant:

\[ \kappa = 1.41 \times 1.0 = 1.41 \]

giving 1.41 mS per cm, which is close to the value of the 0.01 molar potassium chloride standard. Had the cell constant been 10 the same conductance would have meant 14.1 mS per cm. This is why the cell constant must be entered into the meter or measured against a standard: the raw conductance on its own says nothing about the solution.

Answer: D

A steady current would drive electrode reactions, building up a layer of products that opposes the applied voltage and makes the measured resistance rise steadily, as well as slowly changing the sample. A few hundred hertz to a few kilohertz reverses the field before significant products accumulate. Direct current passes perfectly well, which is the trouble, the signal is not intrinsically larger, and heating is negligible at the voltages used. Polarisation is the single dominant difficulty in conductance measurement.

Answer: A

Polarisation is the interfacial impedance formed as ions accumulate at each electrode, which appears in series with the solution resistance and makes the reading too high. It grows worse at high conductivity and low frequency. The remedies are platinising the electrodes to increase their real area, raising the excitation frequency with conductivity, and above all using a four electrode or toroidal design. Dipole alignment and wall effects are not the mechanism, and cell constant drift is a mechanical matter.

Answer: B

A finely divided platinum deposit multiplies the true surface area many times, so the interfacial impedance falls and polarisation error with it. The trade is that the black is fragile, traps sample and is unsuitable for very pure water, where a bright platinum or a four electrode cell is preferred instead. Catalysing electrolysis would be exactly the wrong aim, corrosion is not the concern for platinum, and the cell constant still varies slightly with temperature through thermal expansion.