pH and Conductivity Meters Questions and Answers

Answer: A

Glass membrane resistance roughly triples for each ten degree fall, so the time constant of the measuring circuit lengthens and readings take much longer to settle, sometimes minutes near zero degrees. Low temperature electrodes use a lower resistance glass. Buffer values do change but that is a calibration matter, crystallisation blocks the junction rather than slowing the glass, and the Nernst slope falls with temperature but never reverses. Patience, or a warmer sample, is the practical answer.

Answer: B

Protein precipitates in the junction, restricting the flow of filling solution so that response slows and the junction potential shifts unpredictably. It is the standard failure mode in dairy, brewing and clinical work, and it is treated by pepsin in hydrochloric acid or by using a sleeve junction that can be flushed. The glass slope is unaffected, sample ionic strength is unchanged, and recalibration merely papers over a drifting fault that will reappear within the working session.

Answer: C

The cell constant, in reciprocal centimetres, converts a measured conductance into a conductivity characteristic of the solution. It is found by measuring a certified potassium chloride standard rather than from the physical dimensions, because fringing fields make the effective area larger than the geometric one. Impedance describes the electrical behaviour of the cell, polarisation is the interfacial error the design tries to avoid, and compensation refers to the temperature correction applied afterwards.

Answer: D

The hydrogen ion moves by proton hopping rather than by bodily migration, so hydrochloric acid conducts several times better than a fully dissociated salt at the same concentration. Potassium chloride and sodium chloride are similar to each other and well below it, potassium a little above sodium. Acetic acid is far the lowest despite being an acid, because it is only slightly dissociated. The comparison neatly illustrates the two variables at work: how many ions there are, and how fast they move.

Answer: A

Resistivity is the reciprocal of conductivity, and 0.055 microsiemens per cm inverts to 18.2 megohm cm, the figure quoted on every water purification system. The two numbers describe the same limit and either may be displayed. Values of 1 to 15 megohm cm indicate water that is pure but not at the theoretical limit, and anything below about 1 suggests a significant ionic load. Because carbon dioxide alone can halve the resistivity in minutes, the measurement is made in line and never in an open beaker.

Answer: B

Two changes act together: dissolved carbon dioxide adds conductivity and evaporation concentrates the salt, so an open standard drifts upward. Dilute standards are the more vulnerable, which is why 0.001 molar potassium chloride is usually made fresh. Potassium chloride is not light sensitive, good grade containers do not leach appreciably, and the salt does not react with oxygen. The general point is that the lower the level being measured, the more the container and the handling matter.

Answer: C

Calibration comes first, since without it the reading has no scale; the electrode is then rinsed before entering the sample, the reading is allowed to stabilise, and the value is recorded with the temperature. Reporting the temperature is part of the measurement rather than an optional extra, because a pH value belongs to the temperature at which it was measured. The same discipline applies to conductivity, where the compensation reference must be stated.

Answer: D

Equilibria shift with temperature, so the same solution really does have a different hydrogen ion activity when warm: pure water is neutral at pH 7.00 at 25 degrees but at 6.63 at fifty. No compensation can undo that, since it is chemistry rather than instrument error. Electronic drift is small in a modern meter, buffer ageing is a separate quality control matter, and the electrode identity does not change the true value. Quoting the temperature is what makes a reported pH reproducible by someone else.

Answer: A

The ion product of water is ten to the minus fourteen at 25 degrees, making the neutral point exactly 7.00 there. At body temperature neutrality is near 6.8 and at the boiling point near 6.1, which surprises people who take 7 to be a fixed dividing line. The solution is not becoming acidic; the scale itself is moving because water dissociates more as it warms. It is a good illustration of why an operational scale needs its conditions stated.

Answer: B

The first is correct: warming reduces viscosity, ions move faster and conductivity rises about two per cent per degree. The second is the wrong way round, since water dissociates more when warmed, so the hydrogen ion activity rises and the pH of neutral water falls from 7.00 at 25 degrees to about 6.1 at 100. The pair is worth holding together because both effects are temperature driven and both are routinely mishandled, one by omitting compensation and the other by assuming 7 is always neutral.