Med-Hycos - les Données
Base de connaissance
STORY OF CONDUCTIVITY ?
(Text and Image from HORIBA Company)
This chapter gives you an easy-to-understand explanation of conductivity. Therefore, it is free of strictly scientific discussion or professional content, to help you become familiar with the basics that will be helpful in using a conductivity meter. Anyway, please read on with ease of mind.
When Alessandro Volta, born in Italy in 1745, was a child, his parents thought he was mentally retarded. As an adult, he became known in 1800 as the inventor of the first electric battery. Unlike the friction batteries known up to that time, the Volta battery provided continuous electric current, and was one of the great inventions of the century. This achievement by Volta paved the way for the likes of Georg Ohm, the German physicist who measured the conductivity of metals, and in 1827 discovered the now-famous Ohm's law.
Michael Faraday was born in 1791, the son of an English blacksmith. At age 13, he became a bookbinder's apprentice, which gave him access to many books. In 1833, he became an assistant to Professor Davies of the Royal Research Laboratory. He did prominent work in the fields of chemistry and physics, and in 1833, he conceived the law of electrolysis, and he envisioned ion as made of corpuscles that conveyed electricity in solution.
The conductivity of electrolytes was energetically measured by Friedrich Kohlrausch of Germany between 1869 and 1880. It is said that he started measuring conductivity as a means of obtaining ionic product. The Kohlrausch bridge, which he invented at that time for the purpose of measuring conductivity, is still well known today
Do you remember Ohm's law, which you learned about in school as one of the basic principles of electricity? In our daily lives, however, we don't need to use Ohm's law. In fact, most of us have our own electrical resistance--we simply walk away when the conversation turns to technobabble about electricity. Since a detailed discussion of electricity would merely cloud the issue, we will try to stick to the basics and make a long story short--and a lot easier to understand.
The above formula describes Ohm's law.
The electricity we use at home in Japan has electromotive force of 100 volts, flashlight batteries 1.5 volts. This is called voltage.
When you connect a little bulb to a battery, the bulb glows. It glows because electric current flows through the bulb. But the flow of current is hindered by what is called resistance, and the larger the resistance, the harder it is for the current to flow.
Try to apply Ohm's law to a river. The difference in height between the head of the river and its mouth is the voltage, and the volume of water that flows is the current. The length and width of the river and the obstacles in the river are the resistance.
It means that the greater the difference in the heights of the water flowing in the river, the shorter and wider the river and the fewer obstacles in the river, the greater the volume of water that can flow.
If you have not heard of the term conductivity or of its unit of measurement (mS/cm or S/cm), we would like to first tell you in simple terms what conductivity is all about.
The electric power generated at a power station is transmitted through an electric wire and reaches your home. When we say we use electric power at home, it means that electric current flows from a distant power station to reach your home. If it were the force of flowing water, the power station would be the high-ground source of a river, and your home would be downstream near the sea. Electric resistance hinders this flow from upstream to downstream, as we have mentioned already. Let's think a little further about electric resistance.
Let's think for a moment of electricity as a man and the substance through which electricity flows (usually electric wire) as a road. There are paved roads, gravel roads and muddy roads. Furthermore, a narrow road hinders the man's passage, causing him to expend much more energy on a long trip.
The difficulty of passage on a bad road may be represented by the following formula:
Think about the resistance of an electric wire. The greater the length and the smaller the cross-sectional area, the greater the resistance. You may also understand that the greater the resistivity with the same length and the same area, the greater the resistance value. Each substance has its own resistivity value. For example, aluminum wire has a resistivity of about 1.6 times the resistivity of copper wire. This means that when an aluminum wire and a copper wire of the same size are compared, the aluminum wire has more resistance against the flow of electric current than the copper wire.
Thus, resistivity becomes an index of difficulty of flow of electric current. And the reciprocal of resistivity (1/resistivity) is conductivity. So, conductivity becomes an index of ease of flow of electric current. Its unit is written S/cm, meaning siemens per centimeter. Also, 1/1000 S/cm is a called millisiemens per centimeter, and 1/1000 of a millisiemens per centimeter is a microsiemens per centimeter.
We have so far dealt with Ohm's law and conductivity in general, and hope you understand the concept. You may wonder, however, what it has to do with the measurement of the conductivity of water--the real question from the beginning. So, we are now going into the main subject.
So far, we have discussed the flow of electricity through an electric wire. A metal, such as in an electric wire, contains a great number of free electrons. These electrons pass electric current from one to the next, just like a line of people forming a bucket brigade. Such a metal is called a conductor.
The next subject involves what is called an ion conductor, basically ions in an electrolyte solution, which also affect the conducting of electricity.
We will now discuss some of the new terms that have come up. When a certain substance is dissolved in liquid--water in the case of Twin--and if the liquid thus obtained can conduct electricity, such a liquid is called an electrolyte solution, and the dissolved substance is called an electrolyte. And each corpuscle that carries electricity is called an ion (a Greek word meaning wanderer).
Common table salt (NaCl) is an electrolyte, and when this is dissolved in water to form salt water, it becomes sodium ions (Na+) and chlorine ions (Cl-), each of which is a corpuscle that conducts electricity.
Let's go back to conductivity. Conductivity is an index of how easy it is for electricity to flow. In water, it is the ions that pass electricity from one to the next. This means that the more Na+ and Cl- contained in water the more electricity is carried, and the higher the conductivity.
To sum up, if we know the conductivity of a sample of
salt water, we can calculate just how salty the water is. (This is what
happens in the salinity conversion to arrive at the value displayed by
the Twin conductivity meter.)
Salinity (density of salt in salt water) and conductivity
You now understand that we can determine the salinity of salt water by knowing its conductivity. Some of you may wonder whether sugar water can also be measured. Unfortunately, a conductivity meter cannot tell you the density of sugar in water. Although sugar is soluble in water, it does not form ions, which means that it is not an electrolyte. Only when ions are produced in water can the density of the dissolved substance be calculated from conductivity measured using a conductivity meter.
Like a human, an electrolyte has a variety of properties.
Electrolytes can be broadly divided into strong electrolytes and weak electrolytes.
Let's spend some time on this subject.
In a very low density zone, conductivity has a linear relationship with density, as is seen with organic acids. Acetic acid solution is a good example. However, as density increases, the rate of ionization decreases. In the high-density zone, only part of the electrolyte is ionized, and the overcrowding causes most of the potential ions to remain dissolved in water as molecules.
About mol/l (moles per liter): The mol (symbol for the SI unit mole) is one of the chemical units we use for expressing the measured quantity of a substance. The number of atoms or molecules in one mol of a substance is equal to the Avogadro constant, which has a value of 6.022 X 10. Therefore, the unit of density mol/l (moles per liter) indicates how much of a substance (in mol) is dissolved in 1 liter of a solution.
Many people become more active in summer than in winter. The ions in an electrolyte solution likewise become more active when the temperature rises, and this makes it easier for electricity to flow. (Of course you know that heat is the movement of molecules and ions that make up a substance.)
In other words, as the temperature of a conductive solution rises, its conductivity increases. Also, just as each human has a different personality, ions have different characteristics. Even with the same temperature variation, ions become active differently, depending on the kinds of ions and the density of the solution. This indicates that, when making various judgments on the basis of the value for conductivity (when comparing density of salt water, for example), the temperature of the solution to be measured should be kept constant.
But, that can't be done so easily. Therefore, the temperature of a solution is measured, and the value for conductivity obtained at that temperature is converted to the value for conductivity if the temperature were 25 °C. Since, however, the variation in conductivity in relation to temperature differs according to the kind of solution and its density, the conversion must adjust for these factors as well. For simplicity, conversion is made with an assumption of a 2% variation in conductivity per °C. (This is the JIS standard.)
A little more detail may be in order regarding the conversion of temperature, as it is not easily understandable.
For example, the conductivity of a solution of 100 S/cm at a temperature of 25 °C will be 98 S/cm at 24 °C, and 102 S/cm at 26 °C--a change of 2% against a temperature variation of per °C. The following formula represents this:
Accordingly, the conductivity of a solution of 80 S/cm at 15 °C will be 100 S/cm when converted to 25 °C.
An electrolyte solution contains positive ions, each of which has a positive electrical charge, and negative ions, each of which have has a negative electrical charge. As illustrated in Fig. (A), we will now place a pair of metal plates at opposites sides in an electrolyte solution, and connect a battery. With such a setup, the positive ions move toward the plate connected to the negative terminal of the battery, and the negative ions move toward the plate connected to the positive terminal of the battery, and thus electric current flows through the solution
When the above distance is widened to 10 cm, the cell constant will be = 10/1 = 10 (). When the same liquid, at 1 (S/cm) for example, isused for these cells, the resistance value of K = 1 () cell is 1 (),andthe resistance value of K = 10 () cell is 10 ().
When we discussed Fig. (B), we suggested that ions moved in straight lines. Actually, however, all ions do not move in straight lines (remember, the name ion originally meant wanderer). Some ions detour, as illustrated in Fig. (C), and the cell constants can't be measured with a ruler.
Hence, the resistance of a standard solution with a known conductivity is measured to determine the cell constant, as follows:
Cell constant (K) = Resistance (R) X Conductivity (k)
Frequently used as a standard solution is potassium chloride solution (KCl dissolved in water). It was even used by Kohlrausch, who laid the foundation for measurement of conductivity.
Familiar tap water or river water contains various substances. So, it is not actually pure water (). Of course, tap water is fine for household use. For scientific research or industrial engineering, however, we sometimes need highly pure water.
As we try to make pure water by gradually removing electrolytes, its conductivity gradually decreases. So, if all electrolytes are removed, will its conductivity become zero? No. Why? Because an infinitesimal part of the molecules of water--only about one in 500 million--is ionized as hydrogen ions () and hydroxide ions (). Theoretically, at this point, the conductivity becomes 0.0548 S/cm at 25 °C.
The conductivity of water used in the production of VLSIs in the semiconductor industry is below 0.06 S/cm-- that's water of very high purity indeed. Such water is referred to as ultrapure water. Without going to such an ultrapure level, conductivity of water can be lowered to below 1 S/cm through a process of deionization. Such water is called ion-exchanged water or deionized water. When we talk about pure water, we refer to water of this level of purity. When water comes in contact with air, carbon dioxide is dissolved in water, causing its conductivity to rise by about 1 S/cm. This does affect the conductivity of pure water.
The term acid rain has already become a household word. But, what exactly does it mean? Although many people know this word, not many fully understand its meaning. Here's how rain becomes acid rain: First, we know that sea water and lake and river water evaporates into the atmosphere, forming clouds, and comes back to the ground in the form of rain. In other words, rain is distilled water and neutral (pH of 7), and its conductivity is close to that of pure water. But, when the atmosphere is polluted with sulfur oxide () and oxides of nitrogen (), the rain goes through oxidation with ozone () or hydrogen peroxide () before falling to the ground, and forms some and . These are contained in the rain that pounds your umbrella. These compounds and are sulfuric acid and nitric acid, respectively. When these acids fall to the ground--and they do--it's a serious problem.
The definition of acid rain is not very clear, but generally rainwater with pH below 5.5 or 5.6 is called acid rain. The pH alone, however, does not tell us the actual quantity of pollutants contained in rain.
Some acid rain with pH of 5 has a conductivity of 50 S/cm, while other acid rain with pH of 5 can have conductivity of 100 S/cm. Naturally, the acid rain of 100 S/cm has a higher concentration of pollutants. Incidentally, in the case of rain with very small content of pollutants and conductivity of around 10 S/cm, reliable measurement of pH is difficult, and in some cases it may show a pH of around 5, caused by aspects other than pollutants.
When measuring acid rain, therefore, we recommend that you first measure conductivity in order to determine the density of pollutants before measuring pH.
In Japan, an acid rain incident occurred in 1973 where people complained of irritation to the skin or eyes, and this occurred with a drizzling rain.
The relationship between large-scale logging and acid rain was brought to the forefront especially in Europe and North America, and now investigations are being undertaken worldwide regarding the chronic effects of acid rain.
Have you ever turned on a water faucet in some remote place and been surprised with strange smell coming from the water? Perhaps a musty or chlorine odor? In supplying tap water, the water is taken from a lake or river, run through a treatment process and then a small amount of chlorine is added to kill germs before it is sent through pipes to faucets in the area. Therefore, the taste and smell can vary, depending upon the origin of water. The cleaner the original water and the more upstream its source, the more delicious the water. Does this mean that pure water, which contains no impurities at all, is delicious? No. If there are no minerals in the water, it has no taste. The water which we think is tasty is natural spring water. That's why you see bottled mineral water in the stores. Those minerals are substances such as calcium, magnesium, sodium, potassium and iron, which are dissolved in water.
When conductivity of tap water and natural water is measured, the results of measurement are sometimes just the reverse of what we might expect. Tap water of high conductivity contains a lot of such components as chlorine that take away from the taste of water. Mineral water contains minerals that make water delicious. (Incidentally, tap water is 100 to 200 S/cm, and good-tasting water is often between 400 to 700 S/cm.)
To make tap water delicious, there are various water purifiers
on the market. Some remove chlorine with activated charcoal, and others
use calcium carbonate to help dissolve calcium. As long as you understand
the system of purifiers, you can determine whether a water purifier is
working properly by measuring the conductivity before and after passing
water through the purifier
Water with high content of calcium ions () and magnesium ions () is called hard water, and water with low content of these ions is called soft water. River water in Japan is soft water in most cases, but underground water is often hard water.
The degree of this is called hardness, but each country has its own criteria for determining hardness. In Japan, when a 100 cc of water contains 1 mg of calculated as calcium oxide, the hardness is calculated as 1 degree, and in the case of , it is calculated as magnesium oxide. When water contains 1 mg, the hardness is calculated as 1.4 degrees. Therefore, water with hardness of over 20 degrees is called hard water, and water below 10 degrees is called soft water.
This hardness, unfortunately, can't be measured with a conductivity meter. The reason is that a conductivity meter cannot selectively measure or . Generally, measurement is made in many cases using a calorimetric method, and in some cases measurement is also made with ion chromatography or ion electrodes.
Making good sake requires good water. Since ancient times, good Japanese sake has depended on the water, rice and climate. "Miyamizu" (shrine water) of Nada, Hyogo Prefecture in Japan is famous as the perfect water for fermentation of sake, and it is said that the technological factors behind Nada's success in the sake industry have been its highly polished rice and use of Miyamizu. When conductivity is calculated from the quality table of Miyamizu, it becomes 600 S/cm, which means that is delicious water. It is further said that Miyamizu was discovered by Yamamura Tarozaemon, an ancestor of Sakura Masamune, in 1840. Tarozaemon carefully examined the difference of taste from one place of fermentation to another, and through trial and error by changing chief brewers and comparing methods of fermentation, he finally found the water to be a key factor in the taste of sake. Today, it is clear through analysis of Miyamizu which components in the water affect fermentation, and more and more brewers have came to use water that is appropriate for sake fermentation. Still today in Nada, Miyamizu is used, and the region takes great pride in the taste of its sake. Naturally, when making whiskey, wine or beer, there are differences in taste and color, depending upon where they are produced and what kind of water is used. The famous Munich black beer uses hard water, and Pilsner light-colored beer uses soft water.
When ice produced in a home freezer melts, does its conductivity return to that of tap water?
When water freezes, it freezes from its pure portion and
from its surface. Because of this, the conductivity of the outside part
of an ice cube goes down because impurities migrate away from the surface
during freezing. Conversely, the center portion of the cube contains a
concentration of the ions and pollutants that were contained in the original
tap water, and therefore exhibits higher conductivity. Accordingly, when
you drink juice or whiskey with home-made ice, try to drink it before the
ice melts completely, and the original taste will not be adversely affected
(although it will be slightly diluted).
When looking at the color of river water, many people may think that if the water is transparent, it must be clean. But having read this far, you may wish to reserve judgment until you have measured the conductivity of the river water.
Let's now compare the conductivity of water in a river with its dirtiness. By measuring the conductivity of river water, we can obtain a rough guide as to the concentration of ions in the water. However, the ions contained in river water are originally and predominantly the components of the soil with which the water is in contact, and because of differences in the soil, the conductivity of river water differs from one river to another. Therefore, the degree of conductivity of river water alone does not tell how dirty the river is.
There is a saying "The water must be clearer three feet downstream," meaning that the river itself has the capability of getting rid of dirt and is somehow helpful in the dilution, deposition, absorption or penetration of pollutants, as well as purification with microbes. These actions differ from one river to another, and although the conductivity does increase generally as we move from upstream to downstream, conductivity downstream is sometimes lower, thanks to purification.
Recently, however, residential sewage and industrial waste flow into rivers in amounts that far exceed any river's capacity for self-purification. Rerouting of a river can also sometimes compromise its purification capabilities. As a result, rivers in general have become polluted, and in many cases, conductivity is higher downstream.
Rain is another thing to consider when talking about rivers.
Compared with river water, rainwater has lower conductivity, and depending upon the season, river water may be diluted or enriched. In Japan, conductivity is low in many places during the rainy season of June and July. With this taken into consideration, when judging the dirtiness of a river by its conductivity, it is necessary to fully understand the characteristics of that river. We must make a global judgment by not only knowing its characteristics upstream and downstream, but also how the river changes on a long-term basis rather than making a one-time judgment.