Ion Selective Electrode (ISE): It has a selective response to a specific ion. It can convert the specific ion content in solution into the corresponding potential, thus realizing the conversion of chemical quantity → electrical quantity, while the ion concentration in solution is measured.
Indicator electrode: the electrode potential and the activity (or concentration) of the ion to be measured in solution is Nernst response electrode is called the indicator electrode. In fluoride determination of ion-selective electrode method in fluoride electrode is the indicator electrode.
Reference electrode: refers to the conditions of a certain temperature, the electrode potential is known, and does not change with the composition of the solution to be measured. In the fluoride determination of ion-selective electrode method in the glymercury electrode as a reference electrode
Techniques related to the determination of fluoride
Fluoride electrode membrane potential is with the change of fluoride ions in the test solution changes in the activity of the response in a certain activity interval between the potential and the activity in line with the Nernst equation. The equation is:
T= 273.15 + t(temperature of the measured solution), ni =
aF = r - ρF, r is the activity coefficient, when in dilute electrolyte solution r ≈ 1, ρF is the concentration of the measured ion.
So, in dilute solution the activity and concentration are close to each other, from equation (1), the potential E is linearly related to -log aF or -log ρF, so that aF or ρF can be found by determining the value of E .
Characteristic parameters of ion-selective electrodes
Selectivity of electrodesIn fact, all ion electrodes are affected to varying degrees by interfering ions. Only those electrodes that have a selective response to the ions to be measured are of practical use. Therefore, selectivity is one of the most important performance indicators of ion electrodes. The selectivity of the electrode is described by the selectivity coefficient.
When considering the influence of ****existing ion interference, the electrode potential can be expressed by the modified Nernst equation.
Linear range and lower limit of detection
(1) Linear range: various ion electrodes under certain conditions, the electrode potential and the activity of the ion to be measured between the Nernst relationship. The concentration range corresponding to the straight line part of the obtained E -log(ai) curve is called the linear range of ISE.
⑵ Lower limit of detection: indicates that the ion selective electrode can effectively measure the lowest concentration of the ion to be measured. At present, the lower limit of detection of most commercial electrodes is 1×10-7~1×10-5mol/L.
Factors affecting the lower limit of detection
①The main factor is the solubility of the active substance of the electrode membrane in solution, i.e., the lower limit of measurement can not be lower than the solubility of the active substance of the electrode membrane.
②The test method and the composition of the solution.
③Pre-treatment of electrode and stirring speed, etc.
Electrode slope s
In the linear range, when the activity of the ion to be tested changes by one order of magnitude, the change in electrode potential (mV) caused by the change is called the slope of the electrode for a given ion, i.e., the slope of the E-logai curve .
Theoretical value: expressed as s = 2.303RT/(niF). It reflects the ability of the membrane electrode to convert the activity of the measured ion into potential when it changes by 10 times, which is 59.16mV for monovalent ion at 25°C. The slope of the electrode will deviate from the theoretical value due to the change of the electrode's performance in the practical application. If the slope of the electrode is too low, it will increase the error of measurement.
Judgment: Generally speaking, it is considered that the measured s of the electrode reaches more than 90% of the theoretical value, and the electrode is considered unqualified if it is less than 70%
Response Time and Stability
Response Time: It is the time required for the electrode to reach the stabilized potential (±1mv) after being immersed in the test solution. Generally a few seconds to a few minutes. Electrode response time and stability of the factors affecting:
① and the electrode membrane structure, nature, solubility, thickness, finish and so on.
② and the concentration of the liquid to be measured.
③Related to the speed of the measured ions to reach the electrode surface: stirring the solution can accelerate the rate of the measured ions to reach the electrode surface, thus speeding up the time for the electrode to reach equilibrium. Therefore, when the measurement is an unknown solution, it should be carried out at the same stirring speed as the standard.
4 and *** storage ion type and concentration: when *** stored in the measured solution of ions for the non-interfering ions, its presence can shorten the response time, when *** stored ions for the interfering ions, will increase the response time.
Temperature: when the temperature increases, it will shorten the response time of the electrode. Speed up the ion exchange rate, reduce internal resistance, speed up the charge conduction in the membrane.
Stability: refers to how long the E-value can remain constant when the electrode is kept at a constant temperature. Measured by the degree of drift and reproducibility.
Drift: refers to the degree to which the potential of a cell consisting of a membrane electrode and a reference electrode changes slowly and in an orderly manner over time in a solution of constant composition and temperature.
Reproducibility: the reproducibility of an electrode is the degree to which the electrode potential is reproducible between multiple measurements.
Life of an electrode
Life of an electrode: the time for which an electrode maintains its function in accordance with the Nernst equation.
Factors affecting electrode life:
①Mechanical damage.
②Chemical corrosion of the sensitive membrane.
③Continuous use in hot or corrosive solution use, the life may be only a few days or even shorter. Normal use may usually reach 1 to 2 years.
Electrode aging and poisoning
Electrode aging: refers to the phenomenon of electrode use for some time after the increase in internal resistance, sensitivity decline. The performance of the response time is long, the response slope decreases, the linear range becomes narrower, etc., the sensitive membrane loses the active phenomenon.
Reason: ① ions in the sensitive membrane gradually dissolved into the solution, causing the carrier to reduce the exchange current becomes smaller.
② "lattice defects" gradually reduced. The ion exchange between the solution and the sensitive membrane tends to disappear the "defects" in the crystallization.
Electrode poisoning: refers to the electrode surface of the active material and the ions in the test solution chemical reaction, resulting in the electrode on the measured ion activity no longer have the ability to respond to the phenomenon.
For most of the solid film electrode can be used to renew the electrode surface by mechanical polishing. The normal function of the electrode can be restored.
Reference electrode performance and use
Reference electrode (ganmercury electrode) performance
(1) device is simple, the electrode potential reproducibility is good, in the measurement of potential, even if there is a small amount of current through the electrode potential remains constant.
(2) During the use of the calomel electrode, in order to form a good and constant liquid-joint potential, potassium chloride solution is required to leak through the liquid-joint part at a certain rate. The leakage rate of the liquid-junction part of the calomel electrode with porous ceramics is about 1 drop every 6hours. Leakage too fast will cause the potential of the calomel electrode to drift, and too slow will not ensure good ionic contact in the liquid junction, and even increase the internal resistance of the calomel electrode.
When the calomel electrode in contact with the liquid to be measured, if there will be leaching mercury and calomel, or can react with the KCl liquid substances, will affect the potential of the calomel electrode. Therefore, it is necessary to prevent the back diffusion of the components of the liquid to be measured, the back diffusion phenomenon will make the determination of the potential value drift deviation.
Prevention of back diffusion method
A, the addition of a salt bridge, so that back diffusion of harmful ions can only diffuse into the salt bridge solution, and can not enter the inner filling solution of the calomel electrode.
B. The internal reference solution of the calomel electrode should be higher than 2cm above the surface of the liquid to be measured.
Precautions on the use of the electrode:
(1) Before use, the appearance of the reference electrode should be observed, is there any crack, is the wiring good? Is the internal liquid filled to the injection hole? Are there any air bubbles? The tube is saturated KCl solution (GR grade, less impurities, otherwise it will cause drift), and the liquid level of KCl solution is higher than the mercury sphere in the tube, and there is a small amount of KCl crystals in the tube.
(2) Before use, the electrode should be injected into the hole of the small rubber plug removed to maintain a certain flow rate, and to maintain the KCl liquid level and the height difference between the liquid level to be measured.
(3) Clean the liquid connection immediately after use to prevent clogging. When not in use in the liquid filling mouth and liquid joints set on the rubber cap. When not in use for a long time, it should be filled with internal reference liquid. Store in the electrode box or in potassium chloride solution.
Fluoro-electrode method of determining the results of the influence of factors
and its elimination method
1, the influence of factors
1 temperature: because the temperature has an effect on the slope of the electrode,
s = 2.303RT / (niF),
and affects the potential of the glymercuric acid electrode.
So do it at a constant temperature (the temperature of the solution being measured should be the same).
Ionic strength
The ion-selective electrode determines the activity of ions in solution according to the Nernst equation. And the activity of an ion is equal to the product of the activity coefficient and the concentration. Therefore, there is a difference between the calibration curve of the electrode potential versus activity and the calibration curve of the potential versus concentration, and this difference is especially pronounced in the high concentration range.
The activity of a particular ion in solution is determined primarily by the ionic strength of the solution. Obviously, at a certain temperature and a certain ionic strength, the activity coefficient of an ion is certain.
In practice, the standard solution and the unknown solution is used to add an equal amount of high concentration of inert electrolyte, so that the standard solution and the total ionic strength of the test solution is equal to find the concentration of the substance to be measured. For example, the method of adding total ionic strength adjusting buffer (TISAB) is used in the determination of F-. With the addition of TISAB, the response time of the electrode can be shortened at low concentrations.
(total ion strength adjustment buffer , TISAB)
PH value of the solution
For fluoride ion selective electrode, the better reagent acidity condition is pH 5 ~ 6.
When pH<5, the following weak acid coordination reaction occurs in the solution: 2F-+H+ =HF+F-=HF2-, which reduces the F- in the solution and affects the sensitivity of the electrode, making the analysis results low. This is due to the fluorine electrode only F- response to HF or HF2 - no response, and fluorine electrode lanthanum fluoride electrode membrane will increase to be dissolved, affecting the measurement.
When pH>8, OH- response to the electrode, will seriously affect the results of the determination, so that the analytical results are high. It has been shown that the interference of OH- on fluorine electrode is also due to the chemical reaction between OH- and the membrane surface, and the introduction of additional F- of the test solution. The reaction formula is: LaF3 + 3OH - = La (OH) 3 + 3F -
Interference substances
Interference substances have two manifestations:
To be measured in the liquid contained in the composition of the action with the LaF3 single crystal, and La3 + or F - to form a complex or some kind of conjugate, affecting the potential measurement. Such as OH-, as described earlier, so that the measured results are high.
The presence of ions complexed with F- in the liquid to be measured, such as Fe3+, Al3+ Be2+, Th4+, etc., so that the results are low.
Interference elimination method
The method of eliminating these influences is to add the same volume of Total Ionic Strength Adjustment Buffer (TISAB) to the standard solution and the sample solution.
(1) The main components of TISAB and its function:
Complexants (inert electrolytes): such as citrate, CDTA, etc.. These ions are some of the stronger complexing agents than F-, preferentially combining with the above interfering ions, thus freeing the fluoride ions from the complex.
Ion strength regulator: NaCl, etc., a high concentration of electrolytes used to maintain the solution has the same activity coefficient, eliminating the effect of the difference in ion strength between solutions on the potential.
PH regulator: acetic acid, hydrochloric acid, sodium hydroxide, etc., the formation of citrate, acetate pH buffer system.
The use of TISAB should pay attention to the problem:
The reagents used as TISAB should reach the required purity, otherwise it can introduce interfering impurities and increase the background of the blank.
The reagents used to prepare TISAB will be exothermic when they are mixed, and the temperature of the solution will rise, so it is not suitable to use a pH meter to adjust the final pH directly while measuring.
The concentration of citrate in the measurement solution should not be greater than 0.5 mol/L, and the concentration is too high, so citrate may react with the electrode's membrane material.
LaF(solid)+Cit3-(water)=LaCit(water)+3F-(water)
The fluoride ions in the membrane phase are transferred to the solution causing measurement errors.
Example: TISAB solution for urine fluorine determination: weigh 58g sodium chloride, 4g trisodium citrate dissolved in 500mL of water, add 57ml glacial acetic acid, and dilute with water to 1000mL after adjusting pH to 5.0~5.5 with 5 mol/L sodium hydroxide.
Sources of error in the fluorine electrode method
(1) Ion-selective electrode errors: mainly Electrode response characteristics caused by the error, from the membrane potential with time and temperature changes caused by the drift and slope changes, electrode aging and electrode insulation or electrostatic induction on the membrane potential, as well as interfering ions and ionic strength changes affect the value of the membrane potential.
(2) reference electrode error: mainly from the reference electrode potential drift, temperature fluctuations and liquid-joint potential drift caused by the error.
(3) Ion meter error: mainly from the input impedance, input current, the quality of electronic components and these components with the temperature change and electromagnetic interference caused by the drift.
(4) Standard solution error: from the configuration process of reagents, balances, volumetric vessels or reagents placed too long improper storage.
(5) Operational errors: including the washing and pretreatment of the electrode, electrode calibration method or improper use, stirring rate is too fast, the equilibrium potential of the inaccurate readings, data recording graphing and calculation of the improper and sampling and pretreatment, and so on.
The correct use of the test device
(1) Ion meter or acid meter accuracy requirements ± 0.1mV. Before the test, first check the use of the instrument and electrode pairs are in use, the instrument power on the warm-up.
(2) Electrode activation: Fluoride ion selective electrode should be placed in the corresponding standard solution before use to soak the activation for a period of time (urine fluorine, water fluorine determination of the electrode activation can be used to ~ 10 μg / mL fluorine standard solution), 1 ~ 2h or tens of minutes. New unused for a long time longer, often used activation time is short or not activated. (Longer unused fluorine electrodes should be stored dry, do not soak in pure water)
(3) According to the aforementioned reference electrode precautions for the use of checking the ganmercury electrode, the pro-use pre-vertical inserted in pure water, so that the liquid joint potential to stabilize.
(4) Check the fluorine electrode, if you find that there are bubbles attached to the fluorine electrode inner membrane surface in the filling liquid, measures should be taken to eliminate or it will also cause poor contact with the conductor inside the electrode and affect the correct measurement of the potential.
(5) During the measurement process, it should be noted that: the rate of stirring is stable; the depth of the electrode pair into the test solution is basically the same; when measuring the solid film electrode, the stirring speed is generally good for the slow and medium speed, and the value can be read in the stirring.
(6) In the measurement process, how to determine whether the electrode reaches equilibrium potential is extremely important, according to the IUPAC recommended response time definition, the potential change of ≤ 1mV/min can be considered to respond to the equilibrium. It is important that the standard and sample measurements should be performed in exactly the same way. The urinary fluoride measurement method is specified to be read after the potential reading has stabilized (i.e., the electrode potential changes less than 0.1mV in 30s), and the temperature at the time of the measurement is recorded at the same time.
(7) Magnetic stirrer after a long period of operation, may cause the stirrer body temperature rises and into the measurement cup, to the determination of the error, so the measurement of the cup is often added under the adiabatic pad, and in the measurement interval to replace the adiabatic pad.
(8) Pay attention to the shielding and grounding of the instrument and avoid electromagnetic interference. If the instrument potential reading keeps changing after power on (jitter), one of the possible reasons is poor grounding of the instrument.
(9) Because the electrode has a "memory" effect, after measuring samples containing high fluorine, be sure to wash the fluorine electrode to the required blanking potential.
(10) Standard addition method to calculate the electrode slope (s), to be used before and after the measured fluid addition of the measured E1 and E2 corresponding to the fluorine standard liquid concentration range of the electrode measured slope. That is, the measured slope s of the electrode is taken as the measured value of the standard liquid in the concentration range close to the measured liquid, and cannot be calculated as the theoretical slope or the s value of the average slope of the measured standard series (meaning the full interval).
Calculating the regression equation:
Take the measured mV value of the fluoride standard series as x,
Take the logarithm of the fluoride mass concentration of the standard liquid (logCF-) as y,
Establish the equation y = a + bx, or x = a' +b' y equation
Enter the electronic calculator to find the values of a and b;
Fluoride concentration (F-mg/L) = the opposing value of y.