Hydrogen Ion Concentration and Water Purification
Application of this Chemical Action to the Destruction of Organisms in Water, in Determining Extent of Corrosion and in Assistance to Coagulation
IT is quite probable that most of you are wondering just what hydrogen ion concentration is and why, whatever it is, it should appear on this program. It probably suggests to you some of the mysteries with which chemists from the time of the alchemists to date have been popularly supposed to occupy themselves.
The purpose of this paper is to attempt to tell you what the hydrogen ion concentration is, and why it is of interest to water works men. You all know also that there are certain colored compounds such as litmus, methyl orange, etc., which have one color in an acid solution and another in an alkaline solution. As will later be shown, different indicators do not give the same result in many acids.
Hydrochloric acid, or as it is commercially called, muriatic acid, is made up of hydrogen and chlorine, HC1, nitric acid HNO3 sulphuric acid H2SO4 acetic acid H Ac, the Ac being a short abbreviation for a complex radical made up of carbon hydrogen and oxygen, boric acid HaBOa often called boracic acid. We see that the only thing common to them all is hydrogen, we must therefore draw the conclusion that the properties we normally think of in an acid must be due to the hydrogen. Likewise, I will give you the chemical formulae for certain of the better known alkalies. NaOH, KOH, Ca(OH)2, etc. We see here that the common constituent is the OH radical made up of oxygen and hydrogen, and it is to this radical that we must ascribe the characteristically basic properties. You will also note that the O and H, if taken together, make up the elements of water.
Different Degrees of Properties in Acid
But we also know that while all acids have certain properties in common they do not always have these properties to the same degree or intensity. I have prepared here solutions of three acids, equal volumes of each solution containing equivalent amounts of acid. The first is hydrochloric or muriatic acid; this acid even in quite dilute solutions, say three per cent., has a sour taste, and will burn the tongue and even the tougher skin of the hand. The second acid solution is acetic, the acid found in vinegar, is about three to four per cent., and is only pleasantly sour, hardly burning the tongue at all. The third solution I have here is boric or boracic acid. This, in from a three to four per cent, solution, is a popular eye wash and does not have a sour taste at all. I can show you that these three solutions in addition to affecting the senses differently also do not affect all indicators in the same manner. I have here a certain indicator (Thymol Blue), which is red in acid solutions and yellow in alkaline solutions. It is not, however, very sensitive. If a small portion of the indicator is added to each solution, we find that the hydrochloric acid turns the indicator red, but that the other two make it yellow. To this indicator, therefore, the acetic and boric acids are alkaline. This second indicator (Brom Phenol Blue) is yellow in acid solutions and blue in alkaline. It is somewhat more sensitive to acids. Adding it to the three acids we find that the hydrochloric and acetic acids react acid, but the boric does not. This third indicator (Brom Thymol Blue), which is very sensitive to all acids, is also yellow in acid solutions and blue in alkaline; adding it to the three acids we find that they all give an acid reaction to this indicator. I will also add a little of this third indicator to a solution of sodium hydroxide to show you that it will turn blue in a strongly enough alkaline solution.
Acids in Water Break Apart Into Ions
This is sufficient to show you that while we may take equivalent amounts of different acids, there is a certain difference in the intensity of the acid properties. It has been found by a large number of experiments carried out in the past thirty to forty years that any acid when put into solution in water tends to break apart into the hydrogen ion and other ions of which it is composed. In the case of hydrochloric acid, hydrogen ions and chloride ions would be produced. Acetic acid gives hydrogen ions and acetate ions a rather complex radical which we abbreviate Ac. Boric acid gives hydrogen ions and borate ions. We call this splitting up into ions “ionization.” Now it has also been proved that not all acids ionize to the same extent. For example, in the hydrochloric acid solution the hydrochloric acid ionizes about ninety per cent, but the acetic acid only ionizes about two per cent, and the boric acid ionizes only very slightly indeed, about .005 per cent. An acid which ionizes greatly, such as hydrochloric, we call a strong acid, and it will give an acid color to all indicators such as we found hydrochloric acid did. Those that do not ionize to such a great extent we call weak acids, and we find that the above mentioned properties vary in intensity according to the amount of ionization.
What Hydrogen Ion Concentration Is
From this reasoning we see, therefore, that the sour taste and ability to change indicators depends not upon the total amount of acid per unit volume but upon the total amount of ionized hydrogen per unit volume. And that is what the hydrogen ion concentration is, the total amount of ionized hydrogen per unit volume of the solution under consideration. Alkalies ionize in a similar manner as do all salts. The properties of the alkalies depend upon the extent of the ionization in a way similar to the acids.
In most natural water we find carbon dioxide or carbonic acid in solution. This like the boric acid is a very weak acid, but it is an acid and gives a definite hydrogen ion concentration. This hydrogen ion concentration has recently attracted much attention. Biologists have found that the small plants and animals growing in water are much affected by changes in it. Some organisms are unable to exist in certain streams because of too high or too low hydrogen ion concentration.
Method of Making Determinations by Indicators
For this reason considerable interest is attached to the determination of the hydrogen ion concentration of surface waters. One way of making this determination is by means of such indicators as you see here. An indicator is selected which changes color at about the hydrogen ion concentration that the water is thought to be and a small portion placed into a solution of known hydrogen ion concentration, an equal amount is placed in a sample of the water being examined and the colors are compared. If the color is the same the hydrogen ion concentration of the water is the same as that of the known solution; if they are different, known solutions with different hydrogen ion concentration are tried until a match is obtained. Making our comparisons in this way and having at hand a selection of indicators and a set of solutions of known compositions, the hydrogen ion concentrations of which vary in a progressive manner, we are able to make these determinations quite accurately. There are other and for some purposes better methods of making these determinations, but there will not be time to go into those methods at this time.
Determining Corrosive Power of Water
The hydrogen ion concentration also has a great deal to do with whether or not water will corrode metals.
I can illustrate that in a rather gross manner with these same acids which we were examining before. If we drop a piece of zinc into the hydrochloric acid which we now know has a high hydrogen ion concentration we see a good deal of gas rising, indicating that the metal is dissolving or corroding rapidly. If I drop zinc in the acetic acid which has a lower hydrogen ion concentration, there is also an evolution of gas, but not nearly so much. The boric acid solution which has a very low hydrogen ion concentration has no apparent effect on the zinc. Now we have always known that free carbon dioxide or carbonic acid in water would cause the water to be corrosive, but we see here that the corrosive effect is to a considerable extent due to the hydrogen ion concentration built up by the carbonic acid. Two waters containing the same amount of free carbonic acid do not necessarily have the same hydrogen ion concentration and do not therefore have the same corrosive action. The hydrogen ion concentration produced by an acid varies inversely with the concentration of the other ion with which the hydrogen is associated in the acid. A water that contains ten parts per million of carbonic acid will have a much higher hydrogen ion concentration and be therefore more corrosive than one containing the same amount of carbonic acid and say two hundred parts per million of carbonates, for the carbonates such as calcium and magnesium carbonates also ionize to give carbonate ions and calcium or magnesium ions. This increase of carbonate ions serves to decrease the hydrogen ion concentration. This effect which is called the common ion effect can be illustrated by means of this acetic acid. I will take this tube which you saw before which contains acetic acid and an indicator which the acetic acid caused to show its acid color and I will add a small amount of sodium acetate, a salt of acetic acid. This addition of sodium acetate serves to greatly increase the concentration of the acetate ions and you will see from the almost immediate change of color of the indicator that this has caused a decided decrease in the hydrogen ion concentration. We see therefore that a study of the hydrogen ion concentration of natural waters may serve to throw some light upon the probable effect of the waters upon metals. We also see that the same information cannot be obtained by simply determining the total amount of carbonic acid which the waters contain.
Suggestion as to Practical Uses of Sedimentation
Then the third point of interest is the precipitation of the alum in the coagulation of water preliminary to sedimentation and filtration. The hydrogen ion concentration is of importance, since if the water is too acid or, as more commonly stated, deficient in alkalinity, the alum will be precipitated incompletely or not at all. making the use of lime necessary. Since this precipitation of aluminum depends upon the hydrogen ion concentration of the water the proper control of the hydtogen ion concentration should be all that is necessary to obtain satisfactory coagulation results. It does not seem too much to hope that very soon automatic machines will be developed which will be controlled by the hydrogen ion concentration of the treated water and which will regulate the dosage of alum and lime that is to be added to the water.
The precipitation of calcium and magnesium in a water softening reaction is very similar to the alum coagulation and it would seem that in this case the controlling of the softening plants by means of the hydrogen ion concentration might not be too much of a chemist’s dream.
The State Water Survey Division is at present engaged in studying the last three reactions with a view to determining the charges in hydrogen ion concentration which take place throughout the progress of the reactions, with the hope that accurate information on this point will be of value in working out better methods of control.
To summarize the hydrogen ion concentration of a water, or for that matter of any solution, is a measure of the extent to which the various acids and acidic compounds in the water are ionized or broken up into hydrogen ions and acid radicals. It is what we might term the intensity factor of acidity as opposed to the quantity factor which we obtain when we measure the total acid or alkali found in the solution. The hydrogen ion concentration of water is of interest to water works men first, because it is one of the factors controlling the lite of animal and vegetable organisms in the water. Secondly, because it is one of the factors in corrosion, and thirdly because the study of this hydrogen ion concentration offers a possible improved means of controlling the coagulation and water softening reactions used in water purification.
Note—Excerpts from a paper read before the joint meeting of the Iowa-Illinois Sections of the American Water Works Association.