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HYDROGEN ION CONCENTRATION AND ITS APPLI-
CATION TO WATER PURIFICATION!
By R. E. Gbeenfield'
It is quite probable that most of those present are wondering just
what hydrogen ion concentration is and why ,whatever it is, it should
appear on this program. It probably suggests some of the mysteries
with which chemists from the time of alchemists to date have been
popularly supposed to occupy themselves.
The purpose of this paper is to attempt to tell what the hydrogen
ion concentration is and why it is of interest to waterworks men.
Most of them have some idea what is meant when one speaks of an
alkaline solution and an acid solution. All know that the things
called acids usually have a sour taste, and what are called alka-
lies have a soapy taste. All know, also, that there are certain colored
compounds, such as litmus and methyl orange, 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. Some of those present may have heard the excellent paper on
this subject given by Dr. Washburn before this society in 1910.
Hydrochloric acid, or as it is commercially called, muriatic acid,
is made up of hydrogen and chlorine, HCl; nitric acid is HNO3, sul-
phuric acid, H2SO4; acetic acid, HAc, the Ac being a short abbrevia-
tion for a complex radical made up of carbon, hydrogen and oxygen;
and boric acid, HjBOj often called boracic acid. The only thing com-
mon to them all is hydrogen, and it is natural to conclude, therefore,
that the properties persons normally think of in an acid must be due
to the hydrogen. Now consider the chemical formulas for certain of
the better known alkalies, NaOH, KOH, Ca(0H)2, etc. The
common constituent is the OH radical made up of oxygen and hydro-
gen, and it is to this radical that the characteristic basic properties
must be attributed. The OH and H, if taken together, make up the
elements of water.
•Read before the Illinois Section, March 23, 1921. Discussions are invited
and should be sent to the Editor.
•State Water Survey, Urbana, 111.
397
398 B. E. GREENFIELD
While all acids have certain properties in common they do not al-
ways have these properties to the same degree or intensity. Take
solutions of three acids, equal volumes of each solution cohtaiiiing
equivalent amounts of acid. The first is hydrochloric or muriatic
acid; most of those present know that this acid, even in quite dilute
solutions, say 3 per cent, has an extremely sour taste, in fact it will
quite severely burn the tongue and even the tougher skin of the hand.
The second acid solution is acetic, the acid found in vinegar; ordinary
vinegar is about 3 to 4 per cent acetic acid and everyone knows that
it is only pleasantly sour, hardly burning the tongue at all. If one
were so rash to as place some in his eye he would suffer some incon-
venience. Those here may never have tried this, but some may have
had sour orange juice spattered in his eyes and that is about the same
as vinegar. The third solution is boric or boracic acid. This, in
from a 3 to 4 per cent solution, is a popular eye wash, causing no dis-
comfort when dropped in the eye and does not have a sour taste at
all. These three solutions, in addition to affecting the senses differ-
ently, also do not affect all indicators in the same manner. Just as
the sense of taste and feeling vary with the intensity of the acid reac-
tion, indicators are also of different sensitivity. For instance, thymol
blue, which is red in acid solutions and yellow in alkaline solutions,
is not very sensitive, being only affected in the strongest acids. If
a small portion of the indicator is added to each of the three acid solu-
tions already mentioned, the hydrochloric acid turns the indicator
red but that the other two make it yellow. To this indicator, there-
fore, the acetic and boric acids are alkaline. Another indicator,
bromphenol blue, is yellow in acid solution and blue in alkaline. It
is somewhat more sensitive to acids. Adding it to the three acid
solutions, the hydrochloric and acetic acids react acid, but the boric
does not. A third indicator, bromthymol blue, which is very sensi-
tive to all acids, is also yellow in acid solutions and blue in alkaline.
Adding it to the three acid solutions, each gives an acid reaction to
this indicator. A little of this third indicator added to a solution
of sodiimi hydroxide, which is strongly alkaline, will turn it blue.
This is sufficient to show that while one 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 nuihber of experiments that any acid,
when put into solution in water, tends to break apart into the hydro-
gen ion and other ions of which it is composed. In the case
HYDROGEN ION CONCENTRATION 399
of hydrochloric acid, hydrogen ions and chloride ions would be
produced. Acetic acid gives hydrogen ions and acetate ions, the lat-
ter being rather complex radicals which chemists abbreviate into Ac.
Boric acid gives hydrogen ions and borate ions. This splitting up.
into ions is called "ionization." Now it has also been proved that
not all acids ionize to the same extent. For example, in the hydro-
chloric acid solution the hydrochloric acid ionizes about 90 per cent,
but the acetic acid only ionizes about 2 per cent, and the boric acid
ionizes only very slightly indeed, about 0.005 per cent. An acid
which ionizes greatly is called a strong acid, and it will have the very
sour taste, ability to smart and burn the skin and give an acid color
to all indicators, like hydrochloric acid. Those that do not ionize
to such a great extent are called weak acids and the above mentioned
properties vary in intensity according to the amount of ionization.
From this reasoning, it will be seen, therefore, that the sour taste
and ability to change indicators depend not upon the total amount
of acid per unit volume but upon the total amount of ionized hydro-
gen per unit volume. And that is what the hydrogen ion concentra-
tion 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 carbon dioxide or carbonic acid is present
in solution. This is a very weak acid like the boric 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 con-
centration. 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 indicators. An
indicator is selected which changes color at about the hydrogen ion
concentration that the water is thought to be. A small portion is
placed in a solution of known hydrogen ion concentration, an equal
amoimt is placed in a sample of the water being examined and the
colors are compared. If the color is the same, the hydrogen ion con-
centration of the water is the same as that of the known solution;
if the colors are different, known solutions with different hydrogen ion
concentration are tried until a match is obtained. Making compari-
400 R. E. GREENFIELD
sons 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, it is practicable to make these
determinations quite accurately. There are other and, for some pur-
poses, better methods of making these determinations but there will
not be time to go into them here.
The hydrogen ion concentration also has a great deal to do with
whether or not water will corrode metals. It is possible to illustrate
this in a rather gross manner with the same acids which were pre-
viously selected as types. If a piece of zinc is dropped into the hydro-
chloric acid, which has a high hydrogen ion concentration, a good
deal of gas rises, indicating that the metal is dissolving or corroding
rapidly. If zinc is dropped 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 hydro-
gen ion concentration, has no apparent effect on the zinc. It has
long been known that free carbon dioxide or carbonic acid in water
would cause the water to be corrosive; the experiments just men-
tioned show 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 nec-
essarily have the same hydrogen ion concentration and do not, there-
fore, have the same corrosive action. The hydrogen ion concentra-
tion 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 10 parts per million of carbonic acid will have a
much higher hydrogen ion concentration and be, therefore, more cor-
rosive than one containing the same amount of carbonic acid and
say 200 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 acetic acid. Into a tube containing acetic acid and an indicator
which the acetic acid caused to show its acid color, add a small amount
of sodium acetate, a salt of acetic acid and there results a great in-
crease in the concentration of the acetate ions. There is also an
almost immediate change of color of the indicator, showing that
there has been a decided decrease in the hydrogen ion concentration.
Therefore a study of the hydrogen ion concentration of natural waters
HYDROGEN ION CONCENTRATION 401
may serve to throw some light upon the probable effect of the waters
upon metals. The same information cannot be obtained by simply
determining the total amount of carbonic acid which the waters
contain.
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 hydrogen ion concentration would 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 alimi and lime that is to
be added to the water.
The precipitation of calciiun and magnes'um 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 changes in
hydrogen ion concentration which take place throughout the pro-
gress of the reactions, with the hope that accurate information on this
point will be of value in working out better methods of control.
Summary. To siunmarize, 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 might be termed the intensity factor of acidity as opposed to
the quantity factor which is obtained by measuring 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 life of animal and vegetable organisms in
the water; second, because it is one of the factors in corrosion; and,
third, because the study of this hydrogen ion concentration offers
a possible improved means of controlling the coagulation and water-
softening reactions used in water purifications.