pH méréshez kapcsolódó fogalmak és leírások


Activity: A thermodynamic term for the apparent or active concentration of a free ion in solution. It is related to con‐ centration by the activity coefficient.

Asymmetry Potential: The potential developed across the glass membrane of the measuring electrode with identical solutions on both sides. Also a term used when comparing measuring electrode potential in pH 7 buffer.

ATC:  Automatic temperature compensation.

BNC: A quick disconnect electrical connector used to inter‐ connect and/or terminate coaxial cables.

Buffer Capacity: A measure of the ability of the solution to resist pH change when a strong acid or base is added.

Buffer: Any substance or combination of substances which, when dissolved in water, produces a solution that resists a change in its hydrogen ion concentration when acid or alkali is added.

Calibration: The process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual value being measured.

Conductance: The measure of the ability of a solution to carry an electrical current.

Dissociation Constant: A value which quantitatively expresses the extent to which substances dissociate in solution. The smaller the value of K is, the less dissociation of the species in solution. This value varies with temperature, ionic strength, and the nature of the solvent.

Drift: A change of a reading or a setpoint value over a period of time due to several factors including change in ambient temperature, time, and the line voltage.

Electrode Potential: The difference in potential established between an electrode and a solution in which it is immersed.

Electrolyte:  Any substance in a solution that will conduct an electric current. Acids, bases, and salts are common electrolytes.

Filling Solution:  A solution of defined composition, also called an electrolyte, that provides a chemical reaction as well as an electrical potential between an internal element and the process being measured. An example is the solution sealed inside a pH glass bulb, typically KCl. A filling solution normally is a buffered chloride, which provides a stable potential and a specific zero potential point. In a reference electrode, the electrolyte, also called the reference filling solution, surrounds the silver/silver chloride wire and periodically requires replenishing.

Hydrogen Ion Activity:  Activity of the hydrogen ion in solution. It is related to hydrogen ion concentration (CH+ ) by the activity coefficient for hydrogen (ƒH+ ).

Impedance:  The total opposition (resistive plus reactive) to electrical flow.

Input Resistance (Impedance): The input resistance of a pH meter is the resistance between the measuring electrode terminal and the reference electrode terminal. A voltage division between the total electrode resistance and the input resistance always affects the potential of a pH measuring electrode circuit.

Internal Reference Electrode Wire: The silver/silver chloride wire used in a reference electrode.

Isopotential Point: A potential which is not affected by temperature changes. It is the pH value at which dE/dt for a given measuring/reference electrode system is zero. Theoretically, for a glass measuring electrode and SHE reference electrode, this potential exists when immersed in pH 7 buffer.

Logarithmic Scale: A method of displaying data (in powers of ten) to yield maximum range while keeping resolution at the low end of the scale.

Membrane: The pH sensitive glass bulb is the membrane across which a potential difference is developed from an ion‐ exchange reaction. The membrane separates the electrolyte in the measuring electrode from the solution being measured.

Millivolt (mV): A unit of electromotive force. It is the difference in  potential  required  to  make  a  current  of 1 millampere flow through a resistance of 1 ohm. One millivolt equals one thousandth of a volt.

Molality: A measure of concentration expressed in moles per kilogram of solvent.

Molarity: A measure of concentration expressed in moles per liter of solution.

Nernst Equation: A mathematical description of electrode behavior in which: E is the total potential, in millivolts, developed between the measuring and reference electrodes; Ex varies with the choice of electrodes, temperature, and pressure; 2.3RT/nF is the Nernst factor (R and F are constants, n is the charge on the ion including sign, and T is the temperature in degrees Kelvin); and ai is the activity of the ion to which the electrode is responding.

Nernst Factor (Slope): The term 2.3RT/nF in the Nernst equation, when T = 25° C, is equal to 59.16 mV when n = 1, and 29.58 mV when n = 2. The term n is the sign of the charge on the ion. The Nernst factor varies with temperature.

pH Junctions: The junction of a reference electrode or combination electrode, typically a liquid junction, is a permeable membrane through which the electrolyte migrates.

Salt Bridge: The salt bridge of a reference electrode is that part of the electrode which is in contact with the electrolyte, establishing the electrolytic connection between the reference  system  and  the   solution   being    measured. An “auxiliary” salt bridge is a glass tube open at one end for electrolyte filling, and connected to the reference electrode cavity at the other end. This type of salt bridge, used for special applications, increases the electrolyte capacity.

Span: The difference between the upper and lower limits of a range expressed in the same units as the range.

Stability: The quality of an instrument or sensor to maintain a consistent output when a constant input is applied.

Standardization: A process, also known as calibration, of equalizing the measuring electrode and reference electrode millivolt output potentials in one standardizing solution (buffer) so that potentials developed in unknown solutions can be interpreted as pH values.

Thermistor: A temperature‐sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors usually have negative temperature coefficients.

Felhasznált és kapcsolódó irodalom


Ion-selective electrodes theory

Ion selective electrodes

Construction of Different Types of Ion-Selective Electrodes.
Int. J. Electrochem. Sci., 7 (2012) 4443 – 4464

Ion selective electrodes
Arthur Covington, Chemistry Cassette