- an invitation to troubles in
pH of the dampening solution is the
main cause of lot many printing
problems. Attention to pH is
very much decisive in the quality of
printing product. Described here are
printing problems that may
be caused by wrong pH – value in
the dampening solution. The focus is
on alkaline printing papers
which are increasingly offered in
the market and they could
cause unwelcome chemical reactions
with the acidic dampening solutions.
What is pH?
Water (H2O) not only consists of
neutral hydrogen-oxyn molecules. The
electric charge of hydrogen-oxyn
particles determines whether a
liquid is acid or alkaline.
letters this is
H = one hydrogen atom (neutral)
H+ = one hydrogen ion (positive)
O = one oxygen ion (negative)
the chemical formula is
H2O = H+ + OH-
The arrows of this equation mean
that one normal water molecule H2O
may split into 2 oppositely
charged ions plus reserve.
The quantity of the (dissociated)
water molecules determines whether a
hydrous solution is acidic or
alkaline. If a hydrous solution
contains more than 10-7 mol (the
quantity unit for small particles)
of H+ ions per litre, it is acidic.
If, However, it contains less than
10-7 mol of H+ ions per litre, it is
alkaline. The pH value is calculated
on the basis of these figures.
In order to simplify the way by
which the acidity or alkalinity of
hydrous solutions is determined in
figures, the clumsy expression 10-7
was dropped. Now only the exponent
is used. Thus the pH value is a
It will do to determine the H+ ions
since the ratio of hydroxide ions is
x [OH] = constant
12 13 14
to measure pH?
In most cases, so-called indicators
are used for measuring pH. They are,
the indicators are integrated in
paper strips, which only have to be
put into a test solution in order to
determine the pH fairly accurately.
They are called indicator strips.
This method can only be used for
solutions without colour or that are
faintly coloured. The electrical
resistance is measured by means of
the conductivity of ions. Ions
conduct current depending on how
they are charged, in neutral
solutions (e.g. pure distilled
water), however, no current will be
conducted due to a lack of ions. In
print shops electrical measurements
are seldom made.
pH in offset printing
For the printing process, a
pH-value of approx. 5.5 has proved
Dampening solution that is too acidic
has the following effects:
printing layer of the plate is
fretted resulting in sharp
pointed halftone dots. The
useful life of the plate is
Ink drying is delayed. In
extreme cases, ink does not
dry at all.
effects of alkaline solutions are:
Tendency towards scumming and
Inks with metallic pigments
will oxydate resulting in
blunt quality in printing
adjustment of pH-value with
phosphoric or citric acid requires
constant supervision. Meanwhile,
dampening solution additives with
buffer effect are being offered
which help maintain a pH of 5.5.
These buffers stabilize the
The pH-value has nothing to do
with alcoholic dampening. Alcoholic
additives facilitate the guidance of
water. They do not change the
are papers alkaline?
Alkaline papers contain calcium
carbonate. Calcium carbonate
prevents a decrease of acidity and
in turn the “ageing” of paper. In
paper manufacturing, calcium
carbonate is an inexpensive raw
material with good flowing
properties, and it gives the paper a
good opacity. Due to these positive
properties, more and more
papers are being made alkaline.
do alkaline papers and dampening
solutions behave and which reaction
While dampening solutions have a
pH of 4.8 to 5.5, alkaline papers have
a pH of 8 - 11. A reaction between these
two cannot be avoided. The calcium
carbonate in the paper is dissolved
by the acidic dampening solution.
The calcium salts can enrich in the
circuit of the dampening solutions,
and this enrichment can result in a
series of troubles in printing for e.g.
of the pH-value
on the blanket
The dissolved components of
the coating mostly accumulate in
the last printing unit.
rollers running blank
Salts that do not dissolve
easily will build up in the
pores of the inking rollers and
render them hydrophilic. The
water infiltrates the ink film,
the ink will be displaced and
the inking rollers will partly
run without ink. The
regeneration of blind inking
rollers can only be achieved
with specific washing agents.
plates running blind
for the prevention of problems
has a decisive effect on the
dissolving speed of calcium
carbonate. The higher the pH value,
the slower the reaction of calcium
carbonate and dampening solution. It
is recommended therefore to
stabilize the pH at the upper level
of the tolerance range at approx.
higher pH values may result in
printing plates not running clean
and well (scumming) and they may
result in an increased water
absorption by the printing ink.
alcohol concentration must also be
reduced to the lowest possible
present dampening additives are
being offered that keep the pH value
constant (buffers). A harmonization
of the pH of the dampening solution
with the pH of the paper should be
achieved. On principle, the use of
suitable dampening additives is recommended.
It would be a good idea to contact
once ink supplier for further