Why is the pH value measured by a pH meter more than 1 point higher than the pH value measured by a

The pH test paper only roughly measures the pH value, and the accuracy of the instrument is much higher than that of the test paper, and the error of the pH test paper is relatively large.

There is definitely an error between the two. It is recommended to use the pH test result as the pH test paper is too intuitive to judge the result, and the error is very large.

Let's figure out the principle first.

pH test strips. Taking the discoloration of phenolphthalein under different acid-base conditions as an example to illustrate the discoloration principle of pH test paper

Phenolphthalein is dropped into concentrated H2SO4, which appears orange and its color does not change regardless of how long it oscillates. If the orange solution is mixed with a large amount of water, the colorless solution is obtained. Phenolphthalein drops into dilute NaOH solution is purple-red, phenolphthalein drops into concentrated NaOH solution (concentration greater than 2mol L), at the beginning of the concentrated NaOH solution surface phenolphthalein drops will appear some purple-red, slightly oscillating purple-red disappears immediately.

In fact, this is phenolphthalein under different acid and alkali conditions, due to the change of its structure, showing four corresponding discolorations.

There are three indicators on the pH test paper: methyl red, bromocresol green, and thymol blue. Methyl red, bromocresol green, thymol blue and phenolphthalein, like phenolphthalein, will change color according to a certain law in solutions with different pH values. The discoloration range of methyl red is red) yellow), the discoloration range of bromocresol green is yellow) green), and the discoloration range of thymol blue is yellow) blue).

It is not difficult to understand that the pH test strip prepared by drying the pH test strip is impregnated with a mixed indicator of quantitative methyl red plus quantitative bromocresol green and quantitative thymol blue.

pH meter. The electrodes used in potentiometric analysis are called galvanic cells. A galvanic cell is a system that converts the energy of a chemical reaction into electricity.

The voltage of this battery is known as electromotive force (EMF). This electromotive force (EMF) consists of two half-cells, one of which is called the measuring electrode, and its potential is related to a specific ion activity, such as H+; The other half-cell is the reference half-cell, usually called the reference electrode, which is usually connected to the measuring solution and to the measuring instrument.

For example, an electrode is made of a silver wire inserted into a salt solution containing silver ions, and at the interface between the wire and the solution, due to the different activities of silver ions in the two phases of metal and salt solution, the charging process of ions is formed, and a certain potential difference is formed. Silver ions that lose electrons enter the solution. When no external current is applied for countercharging, i.e. no current, the process eventually reaches an equilibrium.

The voltage present in this equilibrium state is known as the half-cell potential or electrode potential. This electrode, which (as mentioned above) consists of a metal and a solution containing this metal ion, is known as a type 1 electrode.

This potential is measured relative to a reference electrode whose potential is independent of the composition of the salt solution. This reference electrode with independent potential is also known as a second electrode. For these electrodes, the metal wires are covered with a layer of slightly soluble salts of the metal (e.g.,

ag agcl) and insert into an electrolyte solution containing the salt-limiting ions of this metal. The magnitude of the half-cell potential or electrode potential depends on the activity of this anion.

Broad pH test strips can be accurate to 1 and pH meters can be accurate to two decimal places. There is also a precision pH test strip that can be accurate to one decimal place.

Usually the pH is a number between 0 and 14, at a temperature of 25, when the pH is <7, the solution is acidic, when the pH is >7, the solution is alkaline, and when the pH = 7, the solution is neutral.

pH value is one of the most important physical and chemical parameters of aqueous solution, and all natural phenomena involving aqueous solutions are involved. Chemical changes as well as the production process are related to the pH value, which is why pH is measured in industry, agriculture, medicine, environmental protection and scientific research.

The pH value of the solution measured by the extensive pH test paper is in the range of 1--14, compared with the colorimetric card, only the pH integer value can be measured. The pH meter is accurate to two decimal places.

How to use test strips correctly.

After soaking the filter paper with some solution, dry it to prepare test strips, there are many types of test strips, and they are commonly used in middle school chemistry experiments.

Red litmus test paper, blue litmus test paper, pH test paper, starch potassium iodide test paper, and lead acetate test paper.

1) When testing the properties of the gas with the test paper, the test paper is generally moistened with distilled water, glued to one end of the glass rod, and glass.

Put the test paper near the mouth of the container containing the gas to be measured (do not touch the solution), observe whether the test paper changes color, and judge the gas.

Quality. (2) When testing the properties of the solution with test strips, a small piece of test strip is generally placed on a surface dish or glass plate, and it is used to wait.

The glass rod of the test solution is dotted in the middle of the test strip to observe whether the color changes and judge the properties of the solution, especially when using pH test strip.

The glass rod should not only be clean, but also free of distilled water.

3) After taking out the test strip, the container containing the test strip should be tightly covered to avoid being contaminated by some gases in the laboratory.

4) Uses of various test strips:

Red litmus paper turns blue when it encounters an alkaline solution, and blue litmus paper turns red when it encounters an acidic solution, and they can be qualitative.

to determine the acidity and alkalinity of a gas or solution.

The iodine ions in the starch potassium iodide test strip are oxidized to iodine when they encounter oxidizing substances, and iodine shows blue when it encounters starch, which can qualitatively test the presence of oxidizing substances, such as chlorine, bromine vapor (and their solutions), N02, etc.

When the lead acetate test strip encounters hydrogen sulfide and sulfur ions, it generates black lead sulfide, which can qualitatively test hydrogen sulfide and content.

A solution of sulfur ions.

When the pH test paper encounters solutions with different acidity and alkalinity, it shows different colors, which can be compared with the standard color card to determine the solution.

The pH value of the solution, it can roughly test the strength of the acidity and alkalinity of the solution The red litmus paper turns blue when it encounters an alkaline solution, and the blue litmus paper turns red when it encounters an acidic solution

The deepest, which is the true pH of the solution, causes the distribution from shallow to dark due to the infiltration of the solution into the test strip.

The test strip is put into the water for half a second and immediately reads the lowest position. The long time of the water flow above and the long time of entering the water will cause large errors.

You take the reading too slowly, and you won't have this problem if you take out the reading immediately after dipping the pH test strip into the solution.

There are many ways to determine the pH of a solution:

1.Use a pH indicator. Add a pH indicator to the solution to be measured, different indicators will change color according to different pH values, and the pH range can be determined according to the study of the indicator. During titration, a precise pH standard can be made.

2.Use pH test strips. pH test paper has a wide range of test strips and precision test strips, dip a glass rod into the test strip with a little solution to be measured, and then according to the color change of the test strip against the standard color card can get the solution of the test strip can not show the pH value of the oil, because the pH test paper uses hydrogen ions to measure the pH value of the solution to be measured, but the oil does not contain hydrogen ions, so the pH test paper can not show the pH value of the oil.

3.Use a pH meter. A pH meter is an instrument that determines the pH value of a solution, and it can be accurate to two decimal places by a pH selector electrode (such as a glass electrode).

In addition, there are many other more advanced and accurate pH measurement methods and methods.

A: There are 3 methods:

pH test strip method: the commercially available "universal pH test strip", which can determine the pH range: 1--14

Indicator method: by adding different indicators, the pH range can be determined. For example:

1) Methyl orange: yellow is displayed, and the pH is greater than ; Shows red, pH less than 2) Methyl red: shows yellow, pH greater than ; Shows red, pH less than 3) neutral red:

Orange-yellow, pH greater than ; Shows red, pH less than 4) phenolphthalein: shows red, pH greater than ; Colorless, pH less than acidity meter method: using acidity meter, the pH value can be accurately determined in the range.

How to measure soil pH, today I will hand over to you a simple little method!

Measured with pH test strip, the results are compared with the color table