實驗室用水相關常識
問題：實驗室中常見規格的水及使用注意點?
1、自來水（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, ?uoride), and is generally unsuitable for use in important experiments.Tap water is ?ne for washing glassware but should always be followed by a rinse with a higher-grade water (distilled, deionized, etc.).
2、蒸餾水（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 signi?cant stir occurred within a large university’s biochemistry department when the ?rst 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、去離子水（Deionized Water ）
Deionized water can vary greatly in quality depending on the type and ef?ciency 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 ?lter disks for particulates. Some may include ?lters 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 ?nal 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 ?ow 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 Te?onTM or similar materials. Never use highly plasticized ?exible 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、水中有哪些有機物質：
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 ?ltered 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 puri?er is responsible for polishing the water to a grade ?t 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 ?lters 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 puri?er 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-?er system. The solution was to install two pre-?lters of decreas-ing porosity in line ahead of the reverse osmosis unit. The ?rst ?lter needed replacing monthly, but the second ?lter 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-?lter which had a clear housing, admitting sunlight. After cleaning and sterilizing the ?lter 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、在水的使用中還有哪些問題？
Leaks
Leaks are sometimes one of the most serious problems that can occur with in-lab water puri?cation systems. Leaks come in three kinds, typically. Leaks of the ?rst 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 back?ush 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 ?ooding in the lab. The likelihood of a leak of the second kind seems to increase exponentially with the cost of instrumentation in laboratories on ?oors directly below the lab with the water puri?er system.
Leaks of the third kind result when a person places a relatively large vessel beneath the water system, begins ?lling, and walks away to tend to a few minor tasks or is otherwise distracted. The vessel over?ows, ?ooding the lab with the extent of the ?ood 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 puri?-cation system immediately above a sink, so that any vessel being ?lled can be placed in the sink, usually prevents this type of cata-strophe. If placement above a sink is not possible, locating the water puri?cation system in a (relatively) high-traf?c 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 ?rst or second type are highly uncommon, but do occur. The best prevention is to have the system periodically
inspected and maintained by quali?ed 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.
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