Laboratory water knowledge
2017-11-15Li Ji Bio
1. The basic nature of water
One water molecule (H2O) is formed by bending and bonding one oxygen atom and two hydrogen atoms. Since the centers of positive and negative charges are inconsistent, they are polar molecules. When two water molecules are present at the same time, the two will be hydrogen-bonded by electrostatic interaction, attracting each other and maintaining a certain distance. And one water molecule can combine with four water molecules at the same time to form a crystal-like neat structure.
In the water molecule polymer, since the hydrogen-bonded network structure is partially broken, and a state of successive movement changes is formed, the water appears to be in a liquid state as a whole, and the structural change can reach 1012 times per second.
In general, if water contains a proper amount of minerals such as sodium, potassium and silicate, it will feel good. If it contains a large amount of residual salts, such as magnesium, calcium and other non-acid-base neutral salts, it will Feel hard to drink. That is to say, the so-called water contains many other ingredients in addition to H2O, and the type and content of these ingredients determine the taste of water.
Water is extremely soluble in salts, and even if the anions and cations are strongly combined by electrostatic interaction, they are easily electrolyzed in water. This is because water molecules can combine with ions to produce "hydrated ions." The radius of the ions is small, and the ions with large charges interact with the water molecules strongly, and the water molecules are closely arranged around the ions. At this time, the cation interacts with the oxygen atom with the negative moment, while the anion forms the opposite structure.
2. Impurities present in water
Source water impurities (5 types in total)
A. Electrolyte
B. Organic matter
C. Particulate matter
D. Microorganisms
E. Dissolved gas
A. Electrolytes: Electrolytes in water include soluble inorganics, organics, and charged colloidal particles. Affect the conductivity of water. Usually, the more impurities in the water, the higher the conductivity of the water. We know that water with a lot of impurities can have a very negative impact on our chemical analysis. Electrolytes are mainly composed of anions and cations. It can generally be determined by ion chromatography or atomic absorption spectroscopy. It can also be analyzed by mass spectrometry to identify the unknown components.
B. Organic matter: The organic matter contained in water generally refers to organic acids and organometallic compounds, and the measurement method is usually to determine the chemical oxygen demand.
C. Particulate matter: sediment, dust, organic matter, microorganisms, and the like. Can be measured with a special particle analyzer
D. Microorganisms: bacteria, plankton, algae, etc.
E. Dissolved gases: N2, O2, CL2, CO, CO2, etc., which can be determined by gas chromatography.
3. Purity required for experimental water
The so-called experiment refers to the action of verifying the hypothesis assumed by the phenomenon. It is important to assume that it can be proved to be truth, and whether the hypothesis can be reproducible. In addition to good skill, the reproducibility of the experiment is also affected by the purity of the chemical used and the precision of the analytical instrument. The chemical reagents used to configure the solution in the experiment, as well as the purity of the water used, are also very important. Assuming that the contaminants in the water have an impact on the experimental test, these substances must be removed. In addition, in order to obtain good reproducibility results, it is necessary to use pure water capable of maintaining stable water quality.
As the sensitivity of the analytical system used in the experiment increases, there is a higher demand for water purity.
1ppm = 1mg/L
1ppb = 1μg/L
1μg/L=1ng/ml=1ppt
4. Representation of the purity of water
In the water, two electrodes with a surface area of ​​1 cm2 and a distance of 1 cm are energized to monitor the conductivity between the two poles. The resistance between the two poles can be known by the applied voltage and the measured current. This value is usually used in water quality analysis. It is called resistivity or specific resistance, and its unit is expressed by MΩ.cm (mega ohm-centimeter).
The reciprocal of resistivity is called conductivity or conductivity and is expressed in μs/cm (micro Siemens per centimeter).
These two parameters are the most commonly used parameters for the purity of water.
Removing ions from tap water will increase the resistivity value (conductivity decreases), but not an unlimited increase, because some water molecules will ionize into hydrogen ions and hydroxide ions, and their resistivity values ​​limit. 18.248 MΩ.cm (25 ° C). In addition, the resistivity value changes with the ionization constant of water and is therefore affected by the water temperature. For example, ultrapure water at 25 ° C has a resistance value of 18.2 MΩ·cm, but is 84.2 MΩ·cm at 0° C. and 1.3 MΩ·cm at 100 ° C. At around 25 °C, when the temperature rises by 1 °C, its resistance will drop by 0.84 MΩ.cm. Therefore, the resistivity value compensated to 25 °C is often used as a measure.
In addition, total organic carbon content (TOC), endotoxin content, bacterial content, particulate content, microbial content, total dissolved solids (TDS), etc. are often used as an important parameter to supplement water quality. Therefore, the purity standard of water is usually comprehensively described and classified by one or more of the above parameters.
5, grading standards for pure water
Laboratory pure water can be divided into 4 conventional grades: pure water, deionized water, laboratory grade II pure water and ultrapure water
Pure water: The purification level is the lowest, usually the conductivity is between 1-50μs/cm. It can be made via a single weakly basic anion exchange resin, reverse osmosis or single distillation. Typical applications include glassware cleaning, autoclaves, constant temperature and humidity chambers, and washer water.
Deionized water: The conductivity is usually between 1.0 and 0.1 μs/cm. Made by mixed bed ion exchange with strong anion exchange resin, but it has a relatively high level of organic and bacterial contamination, which can meet a variety of needs, such as cleaning, preparation of analytical standards, preparation of reagents and dilution of samples.
Laboratory Grade II pure water: Conductivity <1.0 μs/cm, total organic carbon (TOC) content less than 50 ppb and bacterial content below 1 CFU/ml. Its water quality can be applied to a variety of needs, from reagent preparation and solution dilution to nutrient solution and microbial research for cell culture. This pure water can be double steamed, or integrated with RO and ion exchange / EDI technology, can also be combined with adsorption media and UV lamps.
Ultrapure water: This grade of pure water is close to the theoretical purity limit in terms of electrical resistivity, organic matter content, particle and bacterial content, pre-purified by ion exchange, RO membrane or distillation, and then purified by nuclear ion exchange purification. Ultra-pure water. Generally, the ultrapure water has a resistivity of 18.2 MΩ-cm, a TOC of <10 ppb, and a particle size of 0.1 μm or less, and a bacterial content of less than 1 CFU/ml. Ultrapure water is suitable for a variety of precision analytical experiments such as high performance liquid chromatography (HPLC), ion chromatography (IC) and ion trapping-mass spectrometry (ICP-MS). Low-purity ultrapure water is suitable for biological applications such as eukaryotic cell culture. Ultrafiltration is commonly used to remove macromolecular bioactive substances such as heat sources (results <0.005 IU/ml) and undetectable nucleases and proteases.
At present, the most common pure water standards in the world are as follows:
International Organization for Standardization (ISO), American Society of Clinical Pathology (CAP) reagent-level water standards, American Society for Testing and Materials Testing (ASTM), International Committee for Clinical Trial Standards (NCCLS), American Pharmaceutical Association (USP), etc. At the same time, China also has corresponding pure water standards: China's national electronic grade ultra-pure water specification GB/T11446-1997 and China's national laboratory water specification GB6682-92. Therefore, most of the pure water systems on the market, whether imported or domestic, are designed according to these standards.
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