What is pH? Why is p in pH lowercase and H uppercase?

pH value, also known as the hydrogen ion concentration index, pH value, is a scale of the activity of hydrogen ions in a solution, which is a measure of the degree of acid and base in a solution in the usual sense.

Ionization of water


The concept was first proposed in 1909 by the Danish biochemist Søren Peter Lauritz Sørensen. p stands for German Potenz, meaning power or concentration, and H (hydrogen ion) stands for hydrogen ion (H+).


Under normal circumstances (25℃, about 298K), the more pH value tends to 0, the more acidic the solution, the more alkaline the solution tends to 14, when the pH<7, the solution is acidic, when the ph>7, the solution is alkaline, when the pH=7, the solution is neutral.




In calculations, [H+] refers to the molar concentration of a substance with hydrogen ions in a solution (sometimes also written as H3O+), the molar concentration of a substance with hydrogen ions hydrated).


pH represents the concentration of hydrogen ions in the solution c (H+), and uses it to judge the pH of the solution, but in fact the concentration of hydrogen ions in the solution can not be measured, so the International Union of Pure and Applied Chemistry (IUPAC) defines pH≈-lg[c (H+)/(mol/L)] from the operation, and lists the reference pH standard value. Since pH is operationally defined, it is generally thought that pH has no unit.


The mathematical expression of pH is -lg[H+], and in mathematics, the negative value of the natural logarithm is represented by p, due to the particularity of pH and the provisions and expressions, etc., the general pH writing is only p lowercase, H capitalized in accordance with the specification, like ph, PH, Ph and so on are non-standard writing!


So do you know why the p in pH is lowercase and the H is uppercase?



There is no doubt that the H in pH represents hydrogenion (H+), but the source of p is debated.


The first one is called "p", which stands for German Potenz, meaning strength and intensity, and "P", which stands for French puissance, also meaning "strength and intensity".


The second is pH, which stands for pondus hydrogenii, Latin for quantity of hydrogen, One is the Latin potentia hydrogenii (capacity of hydrogen), corresponding to the English potential hydrogen.


The third is that p is simply a random symbol chosen by Sorensen, who also used q.


In chemistry today, we add p in front of a dimensionless quantity to represent the negative log of that quantity, so p is lowercase. H means hydrogen ion, because the element symbol for hydrogen is H, so capitalize!

Related knowledge arrangement

1. What is pH Standard buffer solution? What are its characteristics?


A pH buffer is a solution that keeps the pH level stable. If a small amount of acid or base is added to this solution, or a chemical reaction in the solution produces a small amount of acid or base, and the solution is properly diluted, the pH value of this solution is basically stable and unchanged, which can resist a small amount of acid or base or large or small dilution, and the pH value does not change the solution is called a buffer solution.


pH standard buffers have the following characteristics:

1.1 The pH of the standard solution is known and achieved to the specified accuracy.

1.2 The pH value of the standard solution has good repeatability and stability, with a large buffer capacity, a small dilution value and a small temperature coefficient.

1.3 The preparation method of the solution is simple.

2. How to prepare pH standard buffer solution?


For general pH measurements, a complete set of pH buffer reagents can be used (250mL can be prepared), and when preparing the solution, deionized water should be used and pre-boiled for 15-30 minutes to remove dissolved carbon dioxide. Cut the plastic bag and pour the reagent into the beaker, dissolve it with appropriate amount of deionized water, rinse the bag, and then pour it into a 250mL volumetric bottle, dilute it to the scale, and shake well.

3. How to store and use pH buffers correctly?


After the buffer solution is prepared, it should be packed in glass bottles or polyethylene bottles (alkaline pH buffers such as pH 9.18, pH 10.01, pH 12.46, etc., should be packed in polyethylene bottles) the bottle cap should be tightly capped and stored in the refrigerator at a low temperature (5-10℃), generally for about six months, if it is found to be turbidity, mildew or precipitation. Can not continue to use. When using, several 50mL polyethylene bottles should be prepared, and the group punch solution in the large bottle should be poured into the small bottle, and placed at the ambient temperature for 1-2 hours, and then used after the temperature balance. After use, the buffer solution in the bottle can be used for 2-3 days under environmental conditions of > 10℃, and the general pH 7.00, pH 6.86, pH 14.00 three solutions can be used for a longer time. pH 9.18 and pH 10.01 solutions are more susceptible to changes in pH because they absorb carbon dioxide from the air.

4. What are the uses of pH buffers?


4.1 Calibrate the pH meter before pH measurement.

4.2 is used to verify the accuracy of the pH meter. For example, after calibrating the pH meter with pH 6.86 and pH 14.00, insert the PH electrode into the pH 9.18 solution to check whether the displayed value of the instrument is consistent with the pH value of the standard solution.

4.3 Check whether the pH meter needs to be re-calibrated during the general accuracy measurement. The pH meter may drift or change after calibration and use, so the electrode is inserted into the standard buffer that is close to the tested solution before the test, and the need to re-calibrate is determined according to the size of the error.

4.4 Testing pH electrode performance.

5. Why should the pH electrode be soaked? How to properly soak the pH composite electrode?


The pH electrode must be soaked before use, because the pH bulb is a special glass film, and there is a very thin gel layer on the surface of the glass film, which can only have a good effect on the hydrogen ions in the solution under fully humid conditions. At the same time, after soaking the glass electrode, the asymmetric potential can be greatly reduced and tends to be stable. The pH glass electrode can generally be soaked in distilled water or a pH 4.00 buffer solution. It is usually better to soak with a pH 4.00 buffer solution, and the soaking time is 24 hours or longer, depending on the thickness of the bulb glass film and the degree of electrode aging. At the same time, the liquid junction of the reference electrode also needs to be soaked. Because if the liquid junction dries up, the liquid junction potential will increase or be unstable, the soaking solution of the reference electrode must be consistent with the external reference solution of the reference electrode, that is, 3.3mol/L KCL solution or saturated KCL solution, and the soaking time is generally a few hours.


Therefore, for the pH composite electrode, it is necessary to soak in the pH 4.00 buffer containing KCL, so that the glass bulb and the liquid junction can work at the same time. Special attention should be paid here, because in the past, people used to soak a single PH glass electrode with deionized water or pH4 buffer, and later used such soaking methods when using pH composite electrodes, even in some incorrect pH composite electrode instructions will also carry out this wrong guidance. The direct consequence of this wrong soaking method is that a pH composite electrode with good performance becomes a slow response and poor precision electrode, and the longer the soaking time, the worse the performance, because after a long time of soaking, the KCL concentration inside the liquid junction (such as inside the sand core) has been greatly reduced, so that the liquid junction potential increases and is unstable. Of course, as long as you resoak in the correct soaking solution for a few hours, the electrode will recover.


In addition, the pH electrode can not be soaked in neutral or alkaline buffer solutions, long-term immersion in such solutions will make the pH glass film insensitive.


Correct preparation of pH electrode soaking solution: take a package of pH 4.00 buffer (250mL), dissolve in 250mL pure water, then add 56 grams of analytical pure KCL, heat properly, stir until completely dissolved.

6. What is the difference between refillable and non-refillable pH composite electrodes?


The pH composite electrode housing is differentiated between plastic and glass. The refillable pH composite electrode is provided with a liquid filling hole on the electrode shell. When the external reference solution of the electrode is lost, the liquid filling hole can be opened to refill the KCL solution. Instead, the refillable pH composite electrode is filled with gel-like KCL, which is not easy to lose and has no liquid hole.


The characteristics of the refillable pH composite electrode are that the reference solution has a higher penetration rate, the liquid junction potential is stable and reproducible, and the measurement accuracy is high. And when the reference electrode is reduced or contaminated, the KCL solution can be supplemented or replaced, but the disadvantage is that the use is more troublesome. When the refilled pH composite electrode is used, the filling hole should be opened to increase the liquid pressure and accelerate the electrode response. When the electrolyte level is lower than 2 cm from the filling hole, a new electrolyte should be added in time.


Non-rechargeable pH composite electrode is characterized by easy maintenance and easy use, so it is also widely used. However, when used as a laboratory PH electrode, under long-term and continuous use conditions, the KCL concentration at the liquid junction will be reduced, affecting the test accuracy. Therefore, when the non-refilled pH composite electrode is not used, it should be immersed in the electrode soaking solution, so that the electrode performance will be very good during the next test, and some laboratory pH electrodes are not long-term and continuous tests, so the impact of this structure on the accuracy is relatively small. The industrial PH composite electrode has low requirements for test accuracy, so easy to use has become the main choice.

7. How to use pH composite electrode correctly?


7.1 There should be no bubbles at the front of the bubble. If there are bubbles, they should be thrown away by force.

7.2 After the electrode is removed from the soaking bottle, it should be shaken in deionized water and shaken dry, do not wipe the ball with paper towels, otherwise due to the transfer of electrostatic induced charge to the glass film, it will extend the potential stability time, the better way is to use the tested solution to wash the electrode.

7.3 After the pH composite electrode is inserted into the solution under test, it should be stirred and shaken several times before being placed stationary, which will speed up the response of the electrode. Especially when the plastic-case pH composite electrode is used, the stirring shake is more severe, because there will be a small cavity between the bubble and the plastic-case, and the gas in the space and time cavity after the electrode is immersed in the solution is too late to eliminate bubbles, so that the bubble or liquid junction and the solution are not well connected, so it must be stirred vigorously to eliminate bubbles.

7.4 After testing in a viscous specimen, the electrode must be repeatedly rinsed with deionized water several times to remove the specimen adhering to the glass film. Sometimes it is necessary to wash the sample with other reagents first, then wash the solvent with water, and immerse in the soaking solution to activate.

7.5 Avoid contact with strong acid and alkali or corrosive solutions. If testing such solutions, the immersion time should be minimized and carefully cleaned after use.

7.6 Avoid using in dehydrating media such as anhydrous ethanol and concentrated sulfuric acid, which will damage the hydrated gel layer on the surface of the bulb.

7.7 The shell material of the plastic shell pH composite electrode is polycarbonate plastic (PC) PC plastic will dissolve in some solvents, such as carbon tetrachloride, trichloroethylene, tetrahydrofuran, etc. If the test contains the above solvent, the electrode shell will be used, at this time, the pH composite electrode of the glass shell should be used.

8. How to clean the pH electrode?


After the bulb and liquid junction are polluted, clean with the following solvent, then use deion water to remove the solvent, and immerse the electrode in the soaking solution to activate.

(1) Pollutants: ① inorganic metal oxides, ② organic grease substances, ③ resin polymer substances, ④ protein blood cell precipitates, ⑤ pigment substances.

(2) Cleaning agent: ① less than 1 mol/L dilute acid, ② dilute detergent (weak acid), ③ dilute alcohol, ④ acetone, ⑤ ether, ⑥ acidic enzyme solution (food mother tablets), ⑦ dilute bleaching solution, ⑧ hydrogen peroxide.

9. How to repair pH electrodes?


The "damage" of the pH composite electrode, which is the phenomenon of reduced sensitivity gradient, slow response, and poor reading repeatability, may be caused by the following three factors, and the general customer can use the appropriate method to repair it.


9.1 The electrode bulb and liquid junction are contaminated, and the dirt can be carefully removed with a fine brush, cotton or toothpick. Some of the protective cover of the head of the plastic shell electrode can be turned off, and it is convenient to clean, if the pollution is serious, it can be cleaned with detergent according to the method of article 8.

9.2 The external reference solution is contaminated, and the structure of some electrodes can be added to the solution. At this time, the external reference solution of the electrode can be extracted using the syringe, and a new 3.3mol/L or saturated KCL solution can be prepared, and then added. Pay attention to the first and second time to be extracted in order to clean the inner cavity of the electrode.

9.3 Aging of glass sensitive film: Soak the electrode bulb with 0.1mol/L diluted hydrochloric acid (9mL hydrochloric acid diluted with pure water to 100mL) for 24 hours, wash with pure water, and then soak with electrode soaking solution for 24 hours. If the passivation is more serious, the lower end of the electrode can also be soaked in 45 hydrofluoric acid solution for 3-5 seconds (solution preparation :4 mL hydrofluoric acid diluted with pure water to 100mL), washed with pure water, and then soaked in the electrode soaking solution for 24 hours to restore its performance.

10. What is a point of pH meter calibration?


Any kind of pH meter must be calibrated by the pH standard solution to measure the pH value of the sample, for the measurement accuracy of the sample below 0.1pH, you can pick a calibration method to adjust the instrument, generally choose pH 6.86 or pH 7.00 standard buffer solution. Some instruments themselves are only 0.2pH or 0.1pH, so the instrument only has a positioning adjustment twist, the specific steps are as follows:


10.1 Measure the temperature of the standard buffer, check the table to determine the pH value at this temperature, and adjust the temperature supplement knob to this temperature.

10.2 Rinse electrode with pure water and shake dry.

10.3 Immerse the electrode in the buffer solution and place it motionless after shaking. After the reading is stable, adjust the positioning knob to make the instrument display the pH value of the standard solution.

10.4 Remove electrodes, rinse and shake dry.

10.5 Measure the sample temperature and adjust the pH meter temperature compensation knob to this temperature value.

10.6 Dip the electrode into the sample solution, place it motionless after shaking, and display stable reading.

11. What is the two point calibration of pH meter?


For precision pH meters, in addition to "positioning" and "temperature compensation" adjustments, there is also an electrode "slope" adjustment, which requires calibration with two standard buffers. Generally, the "positioning" calibration is performed at pH 6.86 or pH 7.00, and then the "slope correction" is performed with a pH 4.00 (acidic) or pH 9.18 or pH 10.01(alkaline) buffer solution, depending on the pH base of the test solution. The specific steps are as follows:


11.1 Wash and dry the electrode, dip it into pH 6.86 or pH 7.00 standard solution, and place the temperature compensation knob at the solution temperature. After the indicated value is stable, adjust the positioning knob to make the instrument indicate the pH value of the standard solution.

11.2 Remove the electrode, wash it, shake it dry, and immerse it in the second standard solution. After the indicated value is stable, adjust the instrument slope knob so that the indicated value of the instrument is the pH value of the second standard solution.

11.3 Wash the electrode and shake it dry, and then immerse it in pH 6.86 or pH 7.00 standard solution. If the error exceeds 0.02pH, repeat steps (1) and (2). Until the correct pH value is displayed in both standard solutions without the need for adjusting knobs.

11.4 Take out the electrode, wash it and shake it dry, adjust the pH temperature compensation knob to the sample temperature, dip the electrode into the sample solution, shake it and place it at rest, display the reading after stability.

12. How much influence does temperature have on pH accuracy measurement?


For pH electrodes, the temperature affects each pH with a pH of 0.003pH/ ° C, such as a 0.2 grade pH meter, calibrated in a 30 ° C pH buffer, and then tested in a 60 ° C solution (assuming the pH range of the solution is between pH6-8 and pH 7.00 differs by one pH unit). The maximum error caused by temperature is 30×0.003=0.09pH. If it is 3 pH units (in the range of pH4-10), the maximum error is 0.27pH, which can be seen that the influence of temperature on pH is very large. Of course, we can also draw the conclusion that in order to reduce the error of temperature to pH measurement, we should pay attention to the following three points:


12.1 Calibrate the pH meter with a buffer solution as close to the pH value of the solution under test as possible.

12.2 Try to keep the temperature of the calibrated solution consistent with or close to the temperature of the measured solution.

12.3 A pH meter with temperature compensation should be selected.


pH meters with accuracy higher than 0.1pH have temperature compensation adjustment, while pH meters with 0.2 pH do not have temperature compensation. Some 0.2 pH meters are also known to have 0.1 accuracy, in fact, this is impossible, some people are the resolution of 0.1pH and 0.1pH accuracy of the two concepts are confused. Even in a pH unit, the pH error of 60 ° C is 0.003×60=0.18pH, so the highest accuracy of a pH meter without temperature compensation is only 0.2pH.

13. Can temperature compensation eliminate all errors caused by temperature?


It must be noted in particular that the temperature compensation set on the pH meter only compensates the slope term of the electrode (2.303RT/F). The temperature also affects the standard potential of the glass electrode, the liquid junction potential, etc., which is not strictly linear with the temperature. At the same time, the pH electrode also needs a certain time to reach the equilibrium at the new temperature. Therefore, whether manual temperature compensation or automatic temperature compensation is not very sufficient. According to the operational definition of pH measurement, in order to obtain precise measurement results, the sample solution and the standard solution should be measured at the same and constant temperature, which is the principle of isothermal measurement. For pH measurements requiring general accuracy, temperature compensation can be used when the temperature of the sample solution is different from that of the standard solution.

14. How can you tell if your pH meter is accurate?


There are many users in the use of pH timing have doubts, this pH meter in the end is accurate? Some people judge by work experience, some people judge by pH test paper, and some people judge by the pH meter used in the past, these are not reliable. In fact, the only reliable and easiest method is to test the pH standard buffer solution. That's the only test. Take three standard buffer solutions: pH 6.86, pH 4.00, pH 9.18(preferably freshly prepared and at the same temperature), perform positioning calibration at pH 6.86, slope calibration at pH 4.00, and then test pH 9.18 to see whether the pH meter is accurate and qualified.

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