去离子水和双蒸水区别

cao1976

去离子水和双蒸水区别
在无机和分析化学实验中，根据任务及要求的不同，对水的纯度要求也不同，纯水分为“纯水”和“超纯水”。我们一般在购买纯水机的过程中，常常会混淆这两个概念，造成用户选型困难，无故增加物资供应成本。要分清纯水的类别，必须要弄清纯水是怎样产生的-即纯水的制备过程。

       纯水的制备常用以下三种方法

1、蒸馏法： 目前使用的蒸馏器有玻璃、石英和铜等材料制成，蒸馏法只能除去水中非挥发性的物质，并不能除去溶解在水中的气体，而且，根据制备材料不同，所含杂质的种类和数量也不同，比如，用铜蒸馏器，水中会含有少量的铜离子，用玻璃蒸馏器制备，水中会含有少量的钠离子和硅酸根离子。从经济角度讲，蒸馏制水存在着耗水量大、用电成本高等弊病，如果是偏远地区，运输也会是一个麻烦，尤其是玻璃和石英蒸馏水器。

2、离子交换法 用离子交换法制备的纯水称为去离子水，是目前用的比较多的一种方法，一般采用阴、阳离子交换树脂的混合床装置，这种方法的优点是：成本低、树脂可再生后反复使用，制备水量大，去离子能力强，但每种方法都有缺点，反渗透法也不例外，其缺点就是设备与操作比较复杂，而且不能除去有机物等非电解质杂质，并有微量树脂溶在水中。

3、渗析法 渗析也叫渗透法，渗透是在外电场的作用下，利用阴阳离子交换膜对溶液中离子的选择性透过而使杂质离子从水中分离出来，现在用的比较多的是一种反渗透技术，反渗透能除掉90-99%的绝大多数污染物，但除去杂质的效率比较低，单独使用的话，只适用于一些要求不是很高的实验。通常作为一种预处理手段。

根据以上三种制备方法生产的三种水，我们叫做蒸馏水、去离子水和反渗透水，理解了这三种水的概念，那么再去理解“超纯水”就很简单了。超纯水所使用的纯化技术和过程简单如下，第一步和第二步，就是渗析和去离子的一个过程，然后是活性炭过滤（用化学吸附去除氯，有机吸附除去可溶性有机物）、微孔过滤（或称亚微米过滤，用一个0.2微米孔径的膜或者中空纤维滤膜，滤除大于0.2微米的污染物。微孔过滤掉来自碳柱的碳微粒、离子树脂碎片和任何可能进入纯化水系统的细菌）、超滤（用来除去纯化水中所有直径大于0.01微米的微粒、热源和微生物）。还有一些特别手段，如紫外氧化或光氧化（采用254nm的紫外光除去系统中的细菌）等等。

cao1976

补充一些内容：

水是实验室内一个常常被忽视但至关重要的试剂。实验室用水有那些种类？能达到什么级别？不同实验对水的要求有那些？这些问题以前对我来说具有一些模糊的概念，前几天参加学校的纯水装置的招标，阅读有关的一些资料，初步了解了相关的知识，现在拿来和大家分享，绝大多数都是本人从外文资料翻译过来的，不当之处还望各位批评。这些资料也包括freecell战友在该版块的精华贴，在此也表示感谢！

实验室常见的水的种类：

1、蒸馏水（Distilled Water ）：
实验室最常用的一种纯水，虽设备便宜，但极其耗能和费水且速度慢，应用会逐渐减少。蒸馏水能去除自来水内大部分的污染物，但挥发性的杂质无法去除，如二氧化碳、氨、二氧化硅以及一些有机物。新鲜的蒸馏水是无菌的，但储存后细菌易繁殖；此外，储存的容器也很讲究，若是非惰性的物质，离子和容器的塑形物质会析出造成二次污染。

2、去离子水（Deionized Water ）：
应用离子交换树脂去除水中的阴离子和阳离子，但水中仍然存在可溶性的有机物，可以污染离子交换柱从而降低其功效，去离子水存放后也容易引起细菌的繁殖。

3、反渗水（Reverse osmosis Water）：
其生成的原理是水分子在压力的作用下，通过反渗透膜成为纯水，水中的杂质被反渗透膜截留排出。反渗水克服了蒸馏水和去离子水的许多缺点，利用反渗透技术可以有效的去除水中的溶解盐、胶体，细菌、病毒、细菌内毒素和大部分有机物等杂质，但不同厂家生产的反渗透膜对反渗水的质量影响很大。

4、超纯水（Ultra-pure grade water）：
其标准是水电阻率为18.2MΩ-cm。但超纯水在TOC、细菌、内毒素等指标方面并不相同，要根据实验的要求来确定，如细胞培养则对细菌和内毒素有要求，而HPLC则要求TOC低。

评价水质的常用指标：

1、电阻率（electrical resistivity）：
衡量实验室用水导电性能的指标，单位为MΩ-cm，随着水内无机离子的减少电阻加大则数值逐渐变大，实验室超纯水的标准:电阻率为18.2MΩ-cm。

2、总有机碳（Total Organic Carbon ，TOC）：
水中碳的的浓度，反映水中氧化的有机化合物的含量，单位为ppm 或 ppb。

3、内毒素（Endotoxin）：
革兰氏阴性细菌的脂多糖细胞壁碎片，又称之为“热原”，单位cuf/ml。

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实验室中常见规格的水及使用注意点?

1、自来水（Tap water）
Tap water is usually of uncontrolled quality, may have seasonal variations such as level of suspended sediment depending on the source (municipal reservoir, river, well), may contain other chem-icals purposely added to drinking water (chlorine, fluoride), and is generally unsuitable for use in important experiments.Tap water is fine for washing glassware but should always be followed by a rinse with a higher-grade water (distilled, deionized, etc.).

2、蒸馏水（Distilled Water ）
Distillation generally eliminates much of the inorganic con-tamination and particularly sediments present in tap water feedstock. It will also help reduce the level of some organic con-taminants in the water. Double distilling simply gives a slightly higher grade distilled water, but cannot eliminate either inorganic or organic contaminants.
Distilled water is often produced in large stills that serve an entire department, or building. The quality of the water is dependent on how well the equipment is maintained. A significant stir occurred within a large university’s biochemistry department when the first mention of a problem with the house distilled water was a memo that came out from the maintenance department that stated: “We would like to inform you that the repairs have been made to the still serving the department. There is no longer any radium in the water.” The next day, a follow-up memo was issued that stated:“Correction—there is no longer any sodium in the dis-tilled water.”

3、去离子水（Deionized Water ）
Deionized water can vary greatly in quality depending on the type and efficiency of the deionizing cartridges used. Ion exchange beds used in home systems, for instance, are used primarily to reduce the “hardness” of the water usually due to high levels of divalent cations such as magnesium and calcium. The resin bed consists of a cation exchanger, usually in the sodium form, which releases sodium into the water in exchange for removing the diva-lent ions. (Remember that when you attempt to reduce your sodium intake!) These beds therefore do not reduce the ionic content of the water but rather exchange one type of ion for another.
Laboratory deionizing cartridges are usually mixed-bed cartridges designed to eliminate both anions and cations from the water. This is accomplished by preparing the anion-exchange bed in the hydroxide (OH-) form and the cation-exchange resin in the acid (H+) form. Anions or cations in the water (including monovalent) are exchanged for OH-or H+, respectively, which combine to form neutral water. Any imbalance in the removal of the ions can result in a pH change of the water.Typically water from deion-izing beds is slightly acidic, often between pH 5.5 to 6.5.
The deionizing resins can themselves increase the organic con-taminant level in the water by leaching of resin contaminants, monomer, and so on, and should always be followed by a bed of activated carbon to eliminate the organics so introduced.

4、18MΩ 水 (Reverse Osmosis/MilliQTM)
The highest grade of water available is generally referred to as 18MW water. This is because when the inorganic ions are completely removed, the ability of the water to conduct electric current decreases dramatically, giving a resistance of 18 MW.Com-mercial systems that produce this grade of water usually apply a multiple-step cleanup process including reverse osmosis, mixed-bed ion exchangers, carbon beds, and filter disks for particulates. Some may include filters that exclude microorganisms, resulting in a sterile water stream. High-grade 18 MW water tends to be fairly acidic—near pH 5. Necessary pH adjustments of dilute buffer solutions prepared using 18 MW water could cause discrep-ancies in the final ionic concentration of the buffer salts relative to buffers prepared using other water sources.

5、When Is 18MΩ Water Not 18MΩWater?
Suppose that your research requires 18 MW water, and you pur-chased the system that produces 500ml/min instead of the 2L/min version. If your research doesn’t require a constant flow of water, you can connect a 20L carboy to your system to store your pris-tine water. Bad Move.
18MW is not the most inert solvent; in practice, it is very aggres-sive. Water prefers the presence of some ions so as your 18 mW water enters the plastic carboy, it starts leaching anything it can out of the plastic,contaminating the quality of the water.The same thing happens if you try to store the water in glass. 18mW water loves to attack glass, leaching silicates and other ions from the con-tainer. If you need the highest purity water, it’s best not to store large quantities, but rather prepare it fresh.
For the same reason, the tubing used to transfer your high-grade water should always be the most inert available, typically TeflonTM or similar materials. Never use highly plasticized flexible plastic tubing. Absolutely avoid metals such as copper or stainless steel, as these almost always guarantee some level of contaminants in your water.

6、水的初始pH值是多少？
As mentioned above, the initial pH of typical laboratory-grade distilled and deionized water is often between 5.5 and 6.5. Check your water supply from time to time, particularly when deionizing beds are changed to ensure that no major change in pH has occurred because of seasonal variation or improperly conditioned resin beds.
Although the initial pH of laboratory water may be slightly acidic, the good news is deionized water should have little or no buffer capacity, so your normal pH adjustment procedures should not be affected much. Pay particular attention if your buffer concentrations are very low (<10mM) resulting in low buffer capacity.

7、水中有哪些有机物质：
The answer to this important question depends on the upstream processing of the water and the initial water source. Municipal water drawn from lakes or streams can have a whole host of organics in them to start with, ranging from petroleum products to pesticides to humic substances from decaying plant material to chlorinated species like chloroform resulting from the chlorina-tion process. Well water may have lower levels of these contami-nants (since the water has been filtered through lots of soil and rock, but even groundwater may contain pesticides and chlori-nated species like trichloroethylene depending on land use near the aquifer.
Municipal processing will remove many organic contaminants from the tap water, but your in-lab water purifier is responsible for polishing the water to a grade fit for experimental use. Most commercial systems do a good job of that, but as mentioned pre-viously, care must be taken to not introduce contaminants after the water has been polished. Plasticizers from tubing or plastic storage tanks, monomer or resin components from deionizer beds, and surfactants or lubricants on filters or other system compo-nents are the most common type of organic to be found in a newly installed system.
Another common, yet often overlooked source, is microbial contamination. In one case, a high-grade water purifier mounted on a wall near a window suddenly started showing evidence of organic background. Changing the carbon cartridge did not help the situation. Close inspection of the system showed the translu-cent plastic tubing connecting the reverse osmosis holding tank to the deionizer beds, and ultimately the lines that delivered the polished water to the spigot, had been contaminated by microbial growth. It was surmised that the intense sunlight during part of the day was providing a more hospitable environment for microorganisms to gain a foothold in the system. The clear tubing was replaced with opaque tubing and the problem disappeared.
In a second instance, a facility changed its water source from wells to a river draw-off. This drastically changed the stability of the incoming water quality. During periods of heavy rain, silt levels in the incoming water increased dramatically, quickly destroying expensive reverse osmosis cartridges in the water puri-fier system. The solution was to install two pre-filters of decreas-ing porosity in line ahead of the reverse osmosis unit. The first filter needed replacing monthly, but the second filter was good for three to six months. The system functioned properly for a while, but then problems reappeared in the reverse osmosis unit. Inspec-tion showed heavy microbial contamination in the second pre-filter which had a clear housing, admitting sunlight. After cleaning and sterilizing the filter unit, the outside of the housing was covered with black electrical tape, and the microbial contamina-tion problem never returned.
As discussed in Chapter 12, dispensing hoses from water reservoirs resting in sinks can also lead to microbial contamination.

8、在水的使用中还有哪些问题？

Leaks
Leaks are sometimes one of the most serious problems that can occur with in-lab water purification systems. Leaks come in three kinds, typically. Leaks of the first kind start as slow drips, and can be spotted and corrected before developing into big unfriendly leaks.
Leaks of the second kind are generally caused by a catastrophic failure of a system component (tubing, valve, automatic shutoff switch, or backflush drain). Although highly uncommon, they usually occur around midnight on Fridays so as to maximize the amount of water that can escape from the system, therefore max-imizing the resulting flooding in the lab. The likelihood of a leak of the second kind seems to increase exponentially with the cost of instrumentation in laboratories on floors directly below the lab with the water purifier system.
Leaks of the third kind result when a person places a relatively large vessel beneath the water system, begins filling, and walks away to tend to a few minor tasks or is otherwise distracted. The vessel overflows, flooding the lab with the extent of the flood depending on the duration of the distraction.
Leaks of the third kind are by far the most common type of leak, and are also the most preventable. Locating the water purifi-cation system immediately above a sink, so that any vessel being filled can be placed in the sink, usually prevents this type of cata-strophe. If placement above a sink is not possible, locating the water purification system in a (relatively) high-traffic or well-used location in the lab can also minimize or eliminate the possibility of major spills, since someone is likely to notice a spill or leak.
Leaks of the first or second type are highly uncommon, but do occur. The best prevention is to have the system periodically
inspected and maintained by qualified personnel, and never have major servicing done on a Friday. Problems seem to be most likely after the system has been poked and prodded, so best to do that early in the week. Then the system can be closly watched for a few days afterward before leaving it unattended.