pH and Conductivity Meters Questions and Answers

Answer: A

pH is defined as the negative logarithm of hydrogen ion activity, not concentration, and the distinction matters in concentrated solutions where activity coefficients depart from unity. Total acid concentration is what a titration measures and can be very different for a weak acid, ionisable group count is a molecular property, and conductivity responds to every ion present rather than to hydrogen ions alone. Because a glass electrode responds to activity, what an instrument reports is an operational quantity fixed by the buffers used to calibrate it.

Answer: B

Taking the negative logarithm of a thousandth gives 3, since ten to the power minus three has a logarithm of minus three. The scale runs conveniently from about 0 to 14 in water at room temperature because the ion product of water is ten to the minus fourteen. A value of 1 would need a tenth molar activity and 11 would be alkaline. Getting comfortable with the logarithmic step is essential, since one pH unit is a tenfold change in activity and instrument resolution of 0.01 units means one part in a hundred of that decade.

Answer: C

Substituting the gas constant, 298 kelvin and the Faraday constant gives:

\[ \tfrac{2.303 \times 8.314 \times 298}{96485} \approx 0.05916 \]

volts, that is 59.16 mV per pH unit. This is the theoretical slope every pH meter is calibrated against, and an electrode returning less than about 95 per cent of it is judged to be ageing. The figure rises with temperature, which is the root of the compensation problem that follows.

Answer: D

Slope is the measured millivolts per pH unit, expressed as a percentage of the theoretical 59.16. A new electrode gives 98 to 100 per cent; below about 92 per cent it should be replaced, because the response has become compressed and readings far from the calibration points will be wrong. Drift with time is a separate symptom, usually of a fouled junction, mounting angle matters only for the electrolyte level, and buffer temperature behaviour is tabulated separately. Slope and offset together are what a two point calibration determines.

Answer: A

A thin glass membrane is a resistance of 50 to 500 megohms, so any appreciable current drawn would drop most of the potential inside the electrode itself. Input impedances of ten to a thousand gigohms are therefore normal, which is why the cable and connector must be kept scrupulously clean and dry: a film of moisture across the connector is a resistance in parallel with the input and will pull the reading towards zero. Corrosion, reference current and temperature stability are real concerns but none of them sets the impedance requirement.

Answer: B

Ion exchange between solution hydrogen ions and alkali metal ions in the swollen gel layer a few tens of nanometres thick on each face of the glass is what generates the potential. This is why a new electrode must be soaked before use and why one allowed to dry out responds sluggishly until rehydrated. The internal silver wire and its filling solution form a fixed reference half cell, and the ceramic junction belongs to the reference electrode and is a source of error rather than of signal.

Answer: C

A potential can only be measured as a difference, so the reference half cell contributes a fixed potential against which the pH dependent glass potential is read. Silver silver chloride and saturated calomel are the two standard choices. Amplification is the meter's job, the reference supplies chloride ions through its junction rather than hydrogen ions, and temperature compensation is done electronically or by a separate probe. Most trouble in pH measurement originates in the reference and its junction rather than in the glass.

Answer: D

A combination electrode packages the measuring and reference half cells concentrically in a single probe, so one connector and one immersion serve for both, and the two elements stay at the same temperature. Calibration against buffers is still required at least daily, most designs still carry a filling solution or a gel equivalent, and non-aqueous work needs special constructions and gives readings that are at best comparative. Convenience is the whole gain, and it is enough to have made the separate pair rare outside teaching laboratories.

Answer: A

Even with the same solution inside and out, a small potential remains because the two faces of the membrane are never identical in strain, composition or degree of hydration. It changes slowly as the electrode ages, and cancelling it is precisely what the offset adjustment of a calibration does. That is the reason a pH meter cannot simply be built to an absolute scale and must be standardised against buffers before each session of work.

Answer: B

The alkaline or sodium error arises because at high pH there are few hydrogen ions and the membrane begins to respond to sodium ions as well, reading the solution as more acidic than it is. Lithium glasses reduce it, and the error is tabulated against sodium concentration for each glass type. The acid error, which does give a reading above the true value, is a separate effect seen below about pH 1 and attributed to water activity in the gel layer. Both are limits of the sensor rather than faults in the instrument.