By C.F. ‘Chubb’ Michaud, CWS-VI
Purity, like beauty, is oft in the eye of the beholder. Simply said, purity is defined as the absence of impurity (or contaminants). Bottom line… pure water is 100-percent water with zero impurities. Pure gold, often defined as 99.9 percent, still contains elements other than gold and is, therefore, not pure. Does ‘pure water’ thus exist? No, it does not…at least not as an entity on its own. All water on the planet exists as a dilute salt solution and therefore, contains a measurable amount of impurities. Even if the impurities are measured in parts per trillion, that still excludes it from being 100-percent pure. Strangely enough, water containing double-digit, part-per-billion levels of impurities (and even water produced by RO) is sometimes referred to as ‘ultra’ pure water, even though it isn’t even pure (100 percent) water. This is a little like ‘soft’ water containing up to 17 ppm of hardness. It’s just a term we use and misuse.
If you were to do an Internet search for pure water, you would find hundreds of companies willing to sell you pure water or equipment to make pure water and hundreds more with the word pure in their names, presumably to imply the quality of their product. There is nothing wrong with that, providing the reader also understands that pure water does not exist, other than as a chemical formula H2O, and in fleeting reference to the relative level of potability (i.e., pure drinking). In this case, pure is confused with purified (unless you mean that the jug contains only drinking water and nothing else). It is 100-percent pure drinking water (but not pure water). Seawater is, therefore, 100-percent pure seawater and well water is 100-percent pure well water, even though it may be contaminated with chrome. Pure mountain spring water contains only water from mountain springs (even though those springs may be contaminated). You get the point. In these cases, the word pure only defines the source and not the quality. One hundred ppb of arsenic in distilled water leaves us with 99.99999-percent pure water. It is, however, toxic!
So how is pure water defined and how do we use it? In water treatment terms, pure water may simply be interpreted as meaning pure enough for its intended use. The word means different things to different industries. Do not confuse the word pure with its cousin purified. Purified water is not pure water. It has simply been treated to make it safe to drink.
Measuring water purity
Not many years ago, wet chemistry was the only method of measuring the salt content of processed water. This was a long and laborious procedure not easily done in the field. There was a developing need for a quick and reliable method of determining the quantitative and relative residual salt content of water and, hence, its purity. It was noted that relatively pure (distilled) water was a fairly poor electrical conductor. Contrast this to seawater, which is a moderately good conductor. It was theorized and later shown that the electrical conductivity of water increased with increasing ionic (salt) content and could, therefore, be used to approximate the relative salt content of water. The conductivity of a substance is defined as its ability to conduct or transmit (be it electricity, heat, sound or some other form of energy). The SI (International System of Units) unit of measure for conductivity is Siemens per meter (S/m). Test meters used to measure minute electrical activity were extremely fine tuned at an early date and very sensitive to the relative small trickle of current in the rapidly developing semi-conductor field. A new testing technology emerged that could now differentiate between two waters with salt contents differing by only a few parts per billion.
Silver metal has the highest electrical conductivity (EC) of all of the metals at 63×106 S/m. Copper is next at 59.6×106. . Among the worst conductors (best insulators) are glass (10-15 S/m), paraffin (10-18 S/m) and Teflon (10-25 S/m). EC is highly dependent on the number of electrons available for the transfer (conduction) of energy. Thus, the ionic (salt) content is proportional to the EC. The EC of high quality, deionized (DI) water measures 5.5×10-6 S/m (0.0000055 S/m). To put this in a more familiar form, the metric system allows us to move zeros around and rename the unit. So we can multiply by 106 to eliminate the zeros. The unit then becomes the microSiemen and is written 5.5μS/m. Then we convert the meter to centimeter (move the decimal two more places to the left) and come up with the conductivity of DI water of 0.055 μS/cm. Since electrical resistivity (ER) is the inverse of conductivity and the unit of resistivity is called the ohm, this translates to (1/0.055 or) 18.2 and the unit is called ‘megohms’ (million ohms). The inverse of the ohm is also called the ‘mho’ (ohm cleverly spelled backwards), so the unit μS/cm can also be written simply as the micromho. DI water has a resistivity of 18.2 megohms and a conductivity of 0.055μS/cm (or 0.055 micromhos). Drinking water might run 50-500 μS/cm and seawater logs in at 50,000 μS/cm. How do these numbers translate the salt content of water?
Conveniently, hand-held and in-line meters can accurately determine the EC of a water solution in real time and display that value as EC or ER. The reading is basically a determination of the number of ions in the solution. Since one ppm of sodium (molecular weight = 23) will have a different number of ions than one ppm of calcium (MW = 40), how can this translate to ppm as TDS? Fortunately, the EC of a solution and its TDS compare quite well in dilute solution by the relation of TDS = EC/2 (as CaCO3). Therefore, 10 μS water is about 5 ppm TDS. The conversion factor (2) decreases as the concentration of salt increases. By the time one measures seawater at 50,000 μS, we have 35,000 ppm TDS (not 25,000 as would be calculated).
NOTE: Since EC is really a measure of ionic ‘activity’ (as opposed to actual ionic content), factors such as temperature and pH will impact the reading. Hot water is more ‘active’ and will give higher conductivity readings. Low or high pH water is more ‘ionic’ and also gives higher readings.
Pure electrical conductivity is, at best, a poor estimator of the actual TDS of water because it converts an EC measurement using a fixed ratio of conductivity to salt. There are many factors that can change the conductivity of water without changing the TDS. Hot water is more conductive than cold water but contains the same TDS. Soft water is more conductive than the hard water from which it was produced but contains the same TDS (measured as CaCO3). De-cationized water (the effluent from the cation portion of a two-bed deionizer), will read three to four times higher in TDS than the feed that produced it, even though half the salt molecule has been removed. (Explanation: the acid produced by the cation exchanger is more conductive than the salt from which it started.)
Since each salt ionizes to different degrees, there is a different conversion factor for each salt in solution. The EC/2 is an approximation and can be routinely used for relative comparisons of salt content before and after treatment. Unless the specific meter is calibrated for the particular salt mix being tested, however, TDS meters can not be used to determine the absolute value of the solution. A TDS meter measuring the permeate from an RO system measures largely the conductivity of the carbon dioxide content of the water.
The TDS reading derived from a two-bed DI utilizing strong acid cation and strong base anion measures largely the conductivity of the sodium hydroxide leakage and, with a weak base anion, you will measure sodium bicarbonate leakage. The following table shows the conversion numbers for various solutions. Results are expressed as ppm as CaCO3 over the range of 1-10 ppm:
Technology today comes awfully close to making pure water. Salt contaminants are measured in parts per trillion and ER is well beyond the 18 meg mark. One quirky observation is that 0.055 μS DI water would have a calculated salt content of approximately 25 to 30 ppb (0.055/2 = 0.0275 ppm or 27.5 ppb). Specific ion measurements for sodium, silica and chlorides, however, may only add up to 5-10 ppb. How can this be? What is causing the higher reading?
Autodissociation of pure water
High-purity water will undergo a chemical reaction whereby a proton (a hydrogen ion) will transfer from one water molecule to another to create two ions: a hydronium (H3O+) and a hydroxide (OH–) as shown in Reaction 1. This is known as the autodissociation or self-ionization of water and it adds to the ionic activity of the solution—even in the absence of salt. So even if pure water did exist, we would not have the means of measuring it.
Reaction 1 2 H2O ←→ H3O+ + OH–
As the need for a higher and higher purity environment evolved in the industrial sector, so did the specifications for the water. Technical standards were established by a number of professional organizations including the American Chemical Society (ACS), ASTM International, the US National Committee for Clinical Laboratory Standards (NCCLS, now CLSI) and the US Pharmacopeia (USP). US EPA is more concerned about specific contaminants such as arsenic, hex chrome, lead and chlorine than the overall totals. Listed in Table 2 are guidelines for various grades of purified water:
Reducing the above contaminants to below maximum contaminant levels (MCLs) would produce a water that would be a good candidate for municipal supply. But salts aren’t the only contaminants with which we are concerned. US EPA lists seven primary microbial limits, four different DPBs, three disinfectants, 16 inorganics (above), 53 organics plus four radionuclides. In addition, they list 15 secondary standards which deal primarily with the aesthetic effects of water (these include color, taste odor, pH foaming agents and others). Note: US EPA considers tooth and skin discoloration caused by excess fluoride, silver or arsenic within the non-enforceable guidelines of the secondary regulations. Jeese!! And just when I was beginning to trust the government. Visit http://water.epa.gov/drink/contaminants/index.cfm for a complete listing of the US EPA drinking water standards (it is extensive) . ‘Purified’ water must meet all of these standards.
Treatment choice will vary
Since different methods of treatment are employed for the removal of different contaminants, water purification is rarely a single step process. Distillation is about as close as we come to single step but this usually involves post treating with activated carbon for the removal of VOCs as well (VOCs will carry over with the distillate). US EPA also wants us to regulate TDS and has chosen the number 1,000 ppm (as would be indicated with an EC of <2,000).
Water purification is usually a well-orchestrated combination of treatments that systematically address the removal of certain contaminants while protecting the device imediately downstream, whose job is to remove someting else. Disinfectants (oxidizers) might be added to a raw water stream to precipitate certain elements, such as heavy metals, while inactivating microbes. The floc that forms from the precipitation is taken out by slow-flow media filters (which also address cysts and turbidity). This would prepare the water for actvated carbon filtration to remove organics and color (and chlorine), which would protect ion exchange or RO to reduce TDS. Then there is final polishing, which could involve pH adjustment. No single device can do all of this in a single step.
The MCL set by US EPA is supposed to be a protective level concentration below which we are safe. The MCL, however, simply means the highest level of a contaminant that is allowed (in drinking water). It does not mean that that level is absolutely safe. Beyond the MCL is the maximum contaminant level goal (MCLG) that more accurately states the level of a contaminant in drinking water, below which there is no known or expected risk to health. There is a gap between many MCLs and MCLGs. For many contaminants, the MCLG is zero. Municipalities are supposed to stay within the MCL (most of the time) but there are exceptions where the output from the treatment plant may be considered unsafe. Since drinking water MCLs are a time-weighted average based exposure, there may be times when the output is actually above the standard. Not ALL municipally supplied water is below ALL of the MCLs ALL of the time. In addition, natural disasters (floods, earthquakes, tsunamis, fires) wreak havoc on local water supplies. Boiling water might make it microbiologically safe but does nothing else. It is everybody’s responsibility to protect themselves and their families with proper filtration and storage of a clean (purified) source of water.
When the word pure is used in conjunction with drinking water, it usually means that the water meets the US EPA MCL standards and is pure enough (to drink). It can be better understood as purified water, in that it has undergone treatment (purification) to bring it into compliance with safe water standards. Purified can also mean highly processed to remove all impurities (such as particulates, microbes, dissolved salts and gases, suspended solids, organics, etc.) and the product actually approaches the true definition of pure. Since pure means 100-percent free of impurities, it should suffice that pure is simply pure enough to do the job.