The Method of Reducing the Measurement Error of PH Meter
Familiarity with and understanding of the working and design principles of the equipment will help observers to use and maintain the equipment correctly, accurately diagnose and analyze measurement errors, master troubleshooting skills, and improve the level of acid rain observation and business quality.
1 Working principle of pHS-3B-S pH meter
The working principle of the pHS-3B-S pH meter. The pre-amplifier converts the composite electrode input signal into a low-impedance signal, which is then input to the pH-t hybrid circuit for calculation. The temperature measurement circuit has two functions, one is to automatically display the temperature of the solution to be tested, and the other is to manually measure the solution temperature by switching the switch on the back and adjusting the temperature knob on the panel to complete the manual temperature compensation. The obtained signal is calculated with the temperature signal obtained by the temperature sensor, which can be used as automatic temperature compensation or manual temperature compensation. During manual temperature compensation, the adjusted temperature is displayed digitally, thereby avoiding subjective measurement errors. The AD converter converts the analog signal into a digital signal and then displays the measured signal digitally.
2 Correct use of pHS-3B-S pH meter
2.1 Normal operating conditions of the pHS-3B-S pH meter
(1) The pHS-3B-S pH meter is a precision instrument with a 3.5-digit LED (digital tube) digital display with high measurement accuracy. According to the requirements of the "Acid Rain Observation Standards", the pH meter is placed in the acid rain observation laboratory The indoor temperature should be controlled at 10~30°C throughout the year, and the relative humidity should not exceed 85%. In addition to keeping it clean and tidy all the year round, no debris should be piled up in the laboratory. It is strictly forbidden for observers to smoke indoors or use food, beverages, etc. that may cause acid rain to measure the environment. The affected items are brought into the laboratory, and the working platform where the pH meter is placed should be kept dry and clean all year round to avoid vibration and electromagnetic field interference.
(2) The power supply used by the pH meter should be stable, and the power supply should meet and meet AC220±22V50±0.5Hz, and use a dedicated line for power supply, and be equipped with a regulated power supply of not less than 500W to ensure stable acid rain observation readings and reduce data errors.
2.2 Correct usage of pHS-3B-S pH meter
(1) When using the pHS-3B-S pH meter for the first time, the composite electrode must be activated before use, that is, the electrode must be soaked in potassium chloride solution with a concentration of 3mol/L for more than 24 hours. Preheat the pH meter for 30 minutes before each acid rain measurement. The temperature of the precipitation sample must be balanced with the temperature of the standard buffer solution to ensure that the temperature difference is less than ±2°C. In order to shorten the temperature difference equilibrium time, the precipitation sample can be installed Put the beaker and the reagent bottles of two different standard buffer solutions into the water close to the room temperature at the same time. After the temperature balance, it should be measured immediately, and the temperature balance time generally does not exceed 2 hours. Immediately after the measurement, a protective cover filled with 3mol/L potassium chloride solution should be installed to keep the electrode ball wet, and the power should be turned off.
(2) Correct calibration of the pH meter. Before each measurement, a standard buffer solution with a known pH value must be used for positioning calibration, and the closer the value is to the measured value, the better. The two standard buffer solutions used for calibration in this station are neutral solution with pH=6.86 and acidic solution with pH=4.00. Generally speaking, when the instrument is in continuous use, it should be calibrated once a day by two-point calibration method. The correct way to calibrate is:
a. Set the "selection" switch to the ℃ position, and the digital display value is the solution temperature value;
b. Turn the slope adjustment knob clockwise to the end (that is, to the 100% position);
c. Insert the cleaned electrode into the neutral standard buffer solution of pH=6.86;
d. Adjust the positioning knob so that the reading on the instrument is consistent with the pH value of the buffer solution at the current temperature;
e. Clean the electrode with distilled water, then insert it into an acidic standard buffer solution with pH = 4.00, adjust the slope knob to make the reading displayed by the instrument consistent with the pH value of the buffer solution at the current temperature;
f. The instrument is calibrated. After calibration, the positioning adjustment knob and slope adjustment knob must not be changed in any way. Pay special attention to gently kneading and slow movements when adjusting the positioning and slope knobs.
(3) Preparing an accurate standard buffer solution can also reduce the measurement error of pH value. Therefore, you need to pay attention to the following points:
a. Chemical reagents used to prepare standard solutions should be stored in a dry place to prevent deliquescence;
b. The acid rain observation station must regularly measure the K value of the pure water used in the station every month, and it shall not exceed 10μS/cm;
c. When preparing, use a smaller beaker to dilute to reduce the pH standard solution sticking to the beaker wall. In addition to emptying out the plastic bags used to store chemicals, they should also be rinsed with distilled water several times, and then poured into the prepared pH standard solution together;
d. The storage period of the prepared standard solution shall not exceed 3 months, and if turbidity, mold or precipitation are found during the storage period, it cannot be used any longer.
3 Use the temperature compensation function to reduce measurement errors
Since the change of electrode potential is different at different temperatures, the pHS-3B-S pH meter is equipped with automatic and manual temperature compensation devices in order to adapt to the measurement of pH values under various temperature conditions. Check the position of the temperature compensation switch on the rear panel of the pH meter. If the temperature automatic compensation function is used, it must be set to the "automatic gear" position. If the manual temperature compensation method is used, it must be set to the "manual gear" position. In order to achieve a sufficient response to the pH electrode, the measured acid rain pH data is true and reliable.
The temperature compensation on the pHS-3B-S pH meter is only the slope item of the compensation electrode. Therefore, in order to obtain accurate measurement results, the measured precipitation sample and the standard buffer solution should be measured at the same and constant temperature as much as possible.
In order to reduce the pH value measurement error caused by temperature, the temperature measuring probe should be calibrated once a month. Calibration should also be performed after the instrument is installed and repaired, and the difference between the temperature measuring probe temperature value and the standard temperature should not exceed 0.5°C.
In order to reduce the measurement error of the pH value of the precipitation sample, the observer should pay special attention to the temperature balance process of the measured precipitation sample from the collection to the laboratory and the standard solution during normal observation. Put a clean watch glass on it to prevent dust from falling in and contaminating the measured precipitation sample, which will cause errors in the measurement results.
4 Proper maintenance and maintenance of the instrument
The pHS-3B-S pH meter has a high input impedance, and the use environment is often exposed to chemicals. In order to ensure the normal use of the instrument, it is necessary for the observer to maintain and maintain it correctly and reasonably, so that the equipment is always in good condition. state of the art, thereby reducing measurement errors.
4.1 Maintenance of pHS-3B-S pH meter host
(1) The input terminal (measuring electrode socket) of the instrument must be kept dry and clean. When the instrument is not in use, it can be covered with a clean and soft cloth to prevent dust and water vapor from entering. When the ambient temperature is high, the electrode plug should be wiped with clean gauze. Dry.
(2) The leading wire of the electrode should be kept still during measurement, otherwise it will cause unstable measurement.
(3) The instrument uses a MOS integrated circuit, so the electric soldering iron should be well grounded during maintenance.
(4) When calibrating the instrument with a buffer solution, the reliability of the buffer solution must be ensured, and the wrong buffer solution cannot be mixed, otherwise it will cause errors in the measurement results.
(5) The temperature sensor adopts Pt100 linear thermistor, which has a long service life, but do not knock or drop it. If the temperature sensor is damaged, you can use the manual temperature compensation function for measurement.
4.2 Maintenance of composite electrodes
(1) When the electrode cover is removed, the sensitive glass bubble of the electrode should not be in contact with hard objects, because any damage or rubbing will make the electrode invalid.
(2) After the measurement, put the electrode protective sleeve on in time, and put a small amount of external reference supplement solution in the electrode sleeve to keep the electrode bulb moist, and do not soak in distilled water.
(3) The external reference replenishment solution of the composite electrode is 3mol/L potassium chloride solution, and the solution volume should be kept at least 1/2 of the inner cavity capacity, and less than 1/2, and add it from the small hole on the upper end of the electrode with a dropper . When the composite electrode is not in use, the rubber sleeve should be pulled on to prevent the supplementary solution from drying up.
(4) The lead-out end of the electrode must be kept clean and dry to prevent short-circuit at both ends of the output. The electrode should not be soaked in distilled water or other protein solutions and acidic fluoride solutions for a long time. The electrode should also avoid contact with silicone oil.
(5) The service life of the composite electrode is generally 1 year, and the station should be replaced with a new composite electrode regularly in April of each year. During the service period, if you find that the slope decreases slightly, you can soak the lower end of the electrode in 4% HF (hydrofluoric acid) for 3~5s, wash it with distilled water, and then soak it in 0.1mol/L hydrochloric acid solution to make it recover.
