[Dry goods] What are the precautions for laboratory water?

With the development of science, all kinds of laboratory instruments are becoming more and more sophisticated, and the requirements for reagents and water used in the testing process are also getting higher and higher. Especially laboratory water, which is used everywhere in the experiment process, is even more important. So, how much do you know about the common sense of laboratory water?

Classification of laboratory water

According to the provisions of GB/T 6682-2008 “Analytical Laboratory Water Specifications and Test Methods”, the raw water for analytical laboratory water should be drinking water or water of appropriate purity. And laboratory water is divided into three levels: primary water, secondary water and tertiary water.

Primary water is used for analytical experiments with strict requirements, including experiments with requirements on particles, such as water for high performance liquid chromatography. The first-grade water can be produced by treating the second-grade water through the distilled water of quartz equipment or the ion exchange mixing window, and then filtering it through a 0.2 nanometer microporous membrane.

Secondary water is used for experiments such as inorganic trace analysis, such as atomic absorption spectroscopy. Secondary water can be produced by multiple distillation or ion exchange.

Grade 3 water is used for general chemical analysis tests. Tertiary water can be produced by distillation or ion exchange.

Types of water most commonly used in laboratories

1. Distilled Water

Distilled water is the most commonly used pure water in the laboratory. Distilled water can remove most of the pollutants in tap water, but it cannot remove volatile impurities such as silica, ammonia, carbon dioxide, and some organic compounds. Fresh distilled water is sterile, but bacteria are prone to multiply after storage, so it is recommended not to store it for a long time. In addition, the container for storing distilled water is also very particular. It is necessary to choose a container made of non-inert material as much as possible. Otherwise, the ions and plastic substances in the container will be precipitated and cause secondary pollution of the distilled water, making it unusable. In early laboratories, distilled water was widely used because of its convenient preparation. However, although distilled water equipment is cheap, it is extremely energy-intensive, water-intensive and slow in water production. In recent years, laboratory applications have gradually decreased.

2. Deionized Water

Deionized water is obtained by removing anions and cations from water using ion exchange resins. There are still soluble organics in the deionized water, which can contaminate the ion exchange column and reduce its efficiency, affecting the water quality.

Like distilled water, deionized water is easy to cause bacteria to multiply after storage, so it should be used immediately to reduce storage time.

3. Reverse osmosis Water

Reverse osmosis water is a kind of laboratory water that is more and more widely used in laboratories. The principle of reverse osmosis water generation is that under the action of pressure, water molecules pass through the reverse osmosis membrane to produce pure water, and the impurities in the water are intercepted and discharged by the reverse osmosis membrane.

The production process of reverse osmosis water has low energy consumption, fast water production, safety, pure physical process, and no chemical reaction. Therefore, this economical, practical, safe and efficient water production method is now being used by more and more laboratories. Reverse osmosis water can effectively remove impurities such as dissolved salts, viruses, bacteria, colloids, bacterial endotoxins and most organic matter in water, and overcome many shortcomings of distilled water and deionized water. The desalination rate of a good reverse osmosis membrane can reach about 99%, but the reverse osmosis membrane produced by different manufacturers has a great influence on the quality of reverse osmosis water.

Because of its high purity, reverse osmosis water is prone to secondary air pollution. Because of its high storage requirements, try not to expose it to the air for a long time.

4. Ultra-pure grade water

The standard of ultrapure water is that the resistivity of water is 18.2MΩ.cm (25°C) (Please note that we are talking about water conditions in an ideal environment of 25 degrees). In addition to water, ultra-pure water has almost no ion content. It is a strong oxidant and is very easy to be polluted by the air again. Even if it is stored, the resistivity will drop quickly. Because, in the laboratory, in order to ensure the accuracy of the experiment, we recommend that ultrapure water be used immediately. Ultrapure water is different in terms of TOC, bacteria, endotoxin and other indicators due to the requirements of various experiments. Therefore, it is necessary to determine the relevant standards of ultrapure water according to the requirements of the experiment.

The following introduces several commonly used indicators for evaluating ultrapure water quality:

A. Electrical resistivity, the unit of resistivity of ultrapure water is MΩ/cm (25°C), which is an index to measure the conductivity of laboratory water.

The resistivity of ultrapure water increases with the reduction of inorganic ions in the water, and the resistivity and conductivity have an inverse relationship.

Laboratory ultrapure water standard: the resistivity of water is 18.25MΩ/cm (25°C).

B. Total Organic Carbon, TOC is the concentration of carbon in ultrapure water, in ppm or ppb, which can reflect the content of oxidized organic compounds in water, and has a great influence on some organic matter experiments.

C. Endotoxin, also known as “pyrogen”, is mainly lipopolysaccharide cell wall fragments of Gram-negative bacteria, and its unit is cfu/eu. Ultrapure water for removing endotoxin is mainly used in life sciences.

Precautions for use of laboratory ultrapure water machine

When using the ultrapure water machine in the laboratory, you need to know these things:
Raw water inflow matters
Raw water inlet requirements, raw water quality TDS < 200PPM (municipal tap water can meet), water temperature 5-45 ℃. When the influent water quality TDS>400ppm, the consumables in the filtration system are easy to wear out, and the frequency of replacement will become very high.

Drainage system needs to be checked before use
The drainage system will affect the water quality and the service life of consumables. When the drainage is not normal, the machine cannot be rinsed and cleaned, bacteria are easy to breed, and the reverse osmosis system is easy to block.

Maintenance of laboratory ultrapure water machine

The maintenance of laboratory ultrapure water machines are relatively simple, mainly including regular replacement of consumables, keeping the machine clean, and checking and cleaning without water leakage.

Consumable replacement period: pre-filtration system is generally 3-6 months, reverse osmosis filtration system is generally 12-15 months, the consumables of the deion exchange system are determined according to the water quality of the ultrapure water. The resistivity of the water quality of the first-grade laboratory water is above 12MΩ.cm (25°C), and it needs to be replaced when it is lower than this value. The higher requirement for laboratory water is that the resistivity is above 15MΩ.cm (25°C). Therefore, it depends on the conditions of laboratory water use. Pay attention to clean the internal water stains after replacing the consumables.

When the ultrapure water machine is not used in the laboratory for a long time, the water inlet valve and the power supply should be closed. If there is no use for more than 30 days, it is necessary to check whether the water circuit and circuit are normal before use. You need to put ultrapure water for about 15 minutes before using water. If the water quality is abnormal, you need to replace the consumables as soon as possible before using.