With the rapid development of the economy, the material level has also been greatly improved. Cosmetics have entered all aspects of life. In the dry and cold winter, we use moisturizers and moisturizing lotions to maintain the skin's stratum corneum with moderate moisture, which is used in the hot summer days. Sunscreen to shield or absorb UV rays and reduce skin damage caused by sun exposure. The cosmetics industry is developing rapidly, and the use of cosmetics is becoming more and more widespread. At the same time, its safety incidents are also frequent. The body powder contains asbestos as a carcinogen, and the baby bathing liquid detects formaldehyde and other incidents. The reaction monitoring system collected only 12,790 adverse reactions of special types of cosmetics.
Faced with the frequent discovery of cosmetic safety incidents, the government has increasingly tightened the supervision of cosmetics safety, and the regulatory system has become more and more perfect. It has successively promulgated and implemented the Regulations on Cosmetic Hygiene Supervision, the Administrative Measures for the Administration of Cosmetics License, and the Technical Specifications for Cosmetics. . Among them, the 2015 version of the "Cosmetic Safety Technical Specifications" was approved by the State Food and Drug Administration, and was implemented on December 1, 2016. The specification not only stipulated the list of banned restricted components, but also contained as many as 77 physical and chemical components. Testing method.
Today, Xiao Bian interprets the technical specifications for the determination of pH in the "Technical Specifications for Cosmetics Safety". The determination of the pH value of the lotion will be used as an example to familiarize more users with the compliant pH value by interpreting the experimental steps. Methods and steps.
The scope of application of the pH value determination method of "Cosmetic Safety Technical Specifications"
The method provides for the determination of the pH of the cosmetic by a pH meter.
The method is suitable for the determination of the pH value of the cosmetic.
Summary of pH test method
The glass electrode is used as the indicator electrode, the saturated calomel electrode is the reference electrode, and the test solution is inserted into the battery to form a battery. The potential difference generated by this battery is related to the pH of the solution to be tested, and the relationship between them is consistent.
Nernst equation: E = E0 +0.059 lg [H+] (25°C) E
= E0 -0.059 pH
Where E0 is a constant.
At 25 ° C, the pH per unit corresponds to a potential difference of 59.1 mV. That is, for every 59.1 mV change in potential difference, the pH in the solution changes by 1 unit. The pH can be read directly on the instrument.
Reagents and materials:
The reagents used in this method are excellent grade pure reagents unless otherwise stated. Water used refers to deionized water that does not contain CO2.
3.1 Potassium hydrogen phthalate standard buffer solution: 10.12 g of potassium hydrogen phthalate (KHC8H4O4) dried at 105 ° C for 2 h was dissolved in water, diluted to 1 L, and stored in a plastic bottle. This solution had a pH of 4.00 at 20 °C.
3.2 Phosphate standard buffer solution: Weigh 3.50g of potassium dihydrogen phosphate (KH2PO4) and 3.55g of disodium hydrogen phosphate (Na2HPO4) which are dried at 105 °C for 2h, dissolved in water, diluted to 1L, and stored in plastic bottles. . This solution had a pH of 6.88 at 20 °C.
3.3 Sodium borate standard buffer solution: Weigh 3.81 g of sodium tetraborate (NaB4O7·10H2O), dissolve in water, dilute to 1L, and store in plastic bottle. The solution had a pH of 9.22 at 20 °C.
The pH values ​​of the above three standard buffer solutions vary slightly with temperature, as shown in the following table:
Ohaus supplies three pH buffers in accordance with the Cosmetic Safety Technical Specification, eliminating the need for users to purchase reagent materials and prepare pH buffers.
Bottled pH=4.01 standard buffer solution, 250ml, 25°C
Bottled pH=6.86 standard buffer solution, 250ml, 25°C
Bottled pH=9.18 standard buffer solution, 250ml, 25°C
The equipment used in the technical specifications: Â
4.1 precision acidity meter (accuracy 0.02)
4.2 composite electrode or glass electrode and calomel electrode
4.3 magnetic stirrer (with heating control function)
4.4 beaker, 50mL
4.5 balance
As a supplier of laboratory equipment with a long history of 111 years, Ohaus can provide users with high-precision pH meters, electrodes and stable and reliable products .
Analysis step Â
5.1.1 Dilution method
Weigh 1 part of the sample (accurate to 0.1 g), add 9 parts of deionized water without CO2, heat to 40 ° C, and stir until uniform, and cool to room temperature as the solution to be tested.
If it is a product with high oil content, it can be heated to 70 °C - 80 °C. After cooling, remove the oil block for use; the powder product can be precipitated and filtered for use.
5.1.2 Direct measurement method (not applicable to powder, oil-based and water-in-oil emulsion cosmetics)
Place the sample in the appropriate packaging container into the beaker for use or insert the small package directly into the lid and insert the electrode directly into it.
5.2 Determination
5.2.1 Electrode activation The composite electrode or glass electrode (4.2) should be immersed in water for more than 24 hours before use.
5.2.2 Calibrating the instrument According to the instrument (4.1) factory instructions, two standard buffer solutions close to the sample pH are used to calibrate at the specified temperature or to calibrate under temperature compensation conditions.
5.2.3 Sample Determination Wash the electrode with water, blot it with filter paper, insert the electrode into the sample to be tested, start the stirrer, wait for the acidity meter to stabilize for 1 min, stop the stirrer, and read the pH directly from the instrument. Test twice, the error range is ±0.1, and take the average reading value. After the measurement is completed, the electrode is rinsed with water, and the glass electrode is immersed in water for use.
Precision
A number of laboratories tested 19 commercially available cosmetic samples in 6 to 22 replicates by dilution, with a relative standard deviation of 0.16% to 1.94%.
The Ohaus ST5000 pH meter can store 1000 measurement data and 10 calibration data for 10 electrodes. The measurement data can be saved to the computer through U disk or printed out via RS232 interface to meet the needs of user data statistics and analysis.
Commercial common lotion pH determination steps: Â
In this experiment, a common lotion is used as a sample, a high-precision ST5000 pH meter and an easy-to-use composite glass electrode STMICRO5 (the pH glass electrode and the reference electrode are combined) as a pH measuring instrument.
The Ohaus ST5000 is a 0.001 pH grade, color touch screen laboratory benchtop pH meter with no buttons and intuitive operation.
(ST5000 pH meter)
pH determination:
The first step, sample preparation: The sample measured in this experiment is a lotion, which is suitable for measuring the pH value of the lotion by the direct test method 5.1.2.
The second step, sampling: shaking the container vigorously before sampling, mixing the sample evenly, opening the container, taking out 5 ml of the sample to be analyzed, and sealing the container after sampling.
The third step, the pH meter calibration: three standard calibration of the ST5000 meter using the Ohaus bottle pH=4.01, pH=6.86, pH=9.18.
(STMICRO5 electrode application picture)
ST5000 pH meter calibration steps:
i) Turn on the power and light up the meter screen. Select the language in the boot screen, click the power button in the lower right corner to enter the main interface.
Ii) Click on the buffer set and set the buffer set to the Chinese group (pH 1.68, 4.01, 6.86, 9.18, 12.46 25 °C).
Iii) Wash the electrode with pure water and blot the water droplets to prevent the paper towel from rubbing against the electrode tip. Put in the first buffer, click "Cal" to start calibration, wait for about 30s to stabilize the value, and complete the calibration of the first pH point.
Iv) Remove the electrode from the first calibration solution, rinse with deionized water, dry it in the second calibration solution, click "Next", wait for the value of 30s to stabilize, and complete the calibration of the second pH point.
v) Repeat step (iv) to calibrate the third pH point. After calibration, the meter will display the calibration slope and offset.
In the fourth step, the calibrated STMICRO5 electrode is rinsed with pure water, and the test sample is placed twice in the lotion sample. The average value is taken. After the measurement is completed, the electrode is rinsed clean.
If you need to measure the pH value of multiple parallel samples, repeat the fourth step of pH measurement, no need to calibrate the instrument again, it is convenient and fast, and the test data is printed out through RS232 interface or saved to the computer via U disk, which is convenient for users to carry out data. statistic and analysis.
(Makeup pH measurement experiment)
How to read this article, have a clearer and more intuitive understanding of the technical specifications of the pH specification for Cosmetics Safety Technical Specifications, and familiar with the experimental steps for determining the pH value of lotion using ST5000 pH meter and STMICRO5 microelectrode, with 111 The annual history of American Ohaus not only provides pH detection instruments, but also provides powerful centrifuges, vortex oscillators and Tianping products. Welcome to call 4008-217-188 and leave relevant information, our professional engineers will contact you in the first time!
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