Acidity, Alkalinity and pH in PRINTING

Acidity, Alkalinity and pH

pH is a measure of the acidity or alkalinity of a solution. It is defined as the logirathm of the acidity of dissolved hydrogen ions (H+ ). Hydrogen ion activity coefficient cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale.

The concept of pH was first introduced by Danish Chemist Soren Peder Lauritz Sorensen at the Carlsberg laboratory in 1909. Sorensen suggested the notation “pH” for convenience. Standing for “power of hydrogen”. Using the co-logarithm of the concentration of hydrogen ion in solution, pH can be measured if an electrode is calibrated with solution of known hydrogen ion concentration.

Pure water is said to be natural. The pH for pure water at 25oC is close to 7.0. Solution with a pH less than 7 said to be acidic and solution with a pH greater than 7 are said to be basic or alkaline. pH measurement are important for medicine, biology, chemistry, food science, environmental science, oceanography and many other applications.

pH = -log(H+ )

Where H+ is the concentration of hydrogen ions, measured in grams of H+ per litre of solution. THE pH SCALE measures how acidic or basic a substance is.

The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic or more than 7 is basic. The pH scale is logarithm and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times acidic than pH 5 and 100 times more acidic than pH 6. The same holds true pH value above 7, each of which is ten times more alkaline than the next lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and 100 times more alkaline than pH 8. Pure water is neutral. But when chemicals are mixed with water, the mixture can become either acidic or basic. Example of acidic substance are vinegar and lemon juice. Ammonia is example of basic substance.

LOGARITHMS: - The logarithm of a number to a given base of the power or exponent to which the base must be raised in order to produce the number. For example, the logarithm of 1000 to the base 10 is 3, because 10 raised to the power of 3 is 1000, the base 2 logarithm of 32 is 5 because 2 to the power 5 is 32. The logarithm of x to the base b is written logb(x) or if the base is implicit as log(x). so, for a number x, a base b and an exponent y. That is the logarithm of the product of two number is the sum of the logarithm of those numbers. The use of logarithm to facilitate complex calculations was a significant motivation in their original development.

 · Ionic concentration and pH value: An aqueous solution of 0.01 molar solution of HCl (acidic)has

· pH = - Log(0.01) = - Log(10 -2 ) = - (-2) Log 10) =2

· An aqueous solution of 0.01 molar solution of NaOH (alkalic)has

· pH = - Log(0.01) = - Log(10 -2 ) = - (-2) Log 10) =2 pOH = 14-2=12

Use of pH equation: - In the case of a strong acid, there is complete dissociation, so the pH is simply equal to minus logarithm of the acid concentration. For example, a 0.01 molar solution of hydrochloric acid has a pH of –log(0.01), i.e., pH= 2.

pH basic (alkaline): - Indicator papers measures alkaline solutions is the range of pH 7.0 – 10. They can be used for making buffers involving baking soda-sodium bicarbonate or hard (mineralise) water. The pH base indicators are mounted on rigid, high contrast white plastic.

(1) VARIOUS METHOD OF MEASURING pH. There are two general methods available for the measurement of a solution. One method is calorimetric and depends on the change of colour of materials called indicators. Which are added to the solution. The other method is electrical and depends on the change of voltage of little electrical cell, as the pH of the solution of electrical cell changes.

(2) DETERMINATION OF pH WITH COLOURED INDICATORS. Indicators useful for determination of pH are complicated organic compound with name like (1) Bromeresol green, (2) Bromphenol blue and (3) Thymol blue. Each of their indicators changes colour over a range of about two pH units. The bromeresol green is yellow at 4.0 and changes through various shades of green until it is blue at a PH of 5.6. such an indicator is useful only in this narrow range of pH. At any pH below 4.0 it remains yellow, and at any pH above 5.6 it remains blue. If we want to measure a solution whose pH is outside this range you must use another indicator. Thus thymol blue is yellow at a pH of 8.0 and changes through various shades of green until it is blue at a pH of 9.6.

(3) DETERMINATION OF pH SHORT RANGE PH PAPERS. Short range pH papers such as hydroid. Such paper strip are impregnated with a solution of one of the indicator mentioned above, and then dried. To use these papers, you merely tear off a one or two such strip of paper and immerse it for two or three seconds in the solution to be tested. The colour developed in the paper is compared with a pH colour chart which comes with the container holding the paper strips. The pH is given under each of the colours on the chart. Short range of pH paper of this kind are useful for approximate pH measurement such as the pH of a press fountain solution. If the colour developed in the test strip is the same as the colour at either and of the colour chart, then you should retest the solution with another short range pH paper containing a different indicator, you must keep doing this until you find a paper strip that will develop a colour that is somewhere between the colours at then two ends of a particular colour chart.

(4) DETERMINATIONOF pH WITH PH METER Principle of Digital pH meter: If two solutions are separated by pH sensitive glass, an electrical potential will be developed across the membrane. If the solution inside the bulb is of H+ ion concentration, the membrane potential will change as the H+ ion concentration of the other solution varies. If the electrical connection are made to these solution a) inside the glass bulb by the pH electrode internal element and b)outside the glass by reference electrode, the membrane potential can be measured.

Electrical pH meter have been available for several years. Several of them operate on 110 – 120 volt, 50 – 60 cycle alternating current. A couple of small portable battery operate pH meter are also available. These electrical pH meters are simple and rapid to operate and very satisfactory to operate in a lithographic plant.

In this method, the solution to be measured forms part of a primary cell or battery. A cell for the measurement of pH is arranged so its voltage will change as the pH of the solution of the cell changes. The cell is calibrated by measuring the voltage when the cell is filled with a solution of known pH. Then by knowing that the e.m.f. of such a cell changes 0.059 volt for a change of 1.00 pH unit, it is possible to calculate the pH of any unknown solution. Suppose that the e.m.f. of a pH cell is 0.235 volt when filled with a solution of known pH 4.1. later the pH 4.1 solution is replaced with a solution of unknown pH and the e.m.f. changes to 0.379 volt. The increase in voltage is 0.144 volt. This correspondence to an increase in pH of 0.144/0.059 or about 2.4 pH units. So the pH of unknown solution is 4.1 plus 2.4 or pH of 6.5. In fact, the instrument makers have made it easier than this since they have calibrated their meters to read directly in pH units, instead of in volts.

GENERAL TESTS

Procedure: - Immerse the glass electrode previously in water. Start the measurement more than 5 minute after switching on. Rinse well the detecting unit with water and blot the water gently with a piece of filter paper. When the pH meter is adjusted at one pH value, rotate the temperature compensation dial to set to the temperature of the pH standard solution, immerse the detecting unit in the pH standard solution which has a pH standard solution which has a pH near that of the sample solution, and after more than 2 minutes, adjust the zero point adjustment dial so that the reading of the pH meter is set to the pH of the pH standard solution indicated in the table for that temperature. When the pH meter adjusted at two different pH values, rotate the temperature compensation dial to set to the temperature of the solutions, usually immerse the detecting unit in the phosphate pH standard solution, adjust the pH by means of the zero point adjustment and proceed in the same manner as described above, with the pH standard solution which has a pH value near that of the sample solution, by means of a sensitivity adjustment dial or a temperature compensation dial irrespective of the temperature of the pH standard solution.

Wash well the detecting unit with water and blot the water gently with a piece of filter paper. Immerse the detecting unit in the sample solution, and the read the pH value.

pH METER A pH meter generally comprises a detecting unit consisting of a glass electrode and a reference electrode and an indicating unit for indicating the pH value corresponding to the electromotive force detected. The indicating unit usually has dial for sensitivity adjustment.

pH INDICATOR An approximate measure of pH may be obtained by using a pH indicator. A pH indicator is a substance that changes colour around a particular pH value. It is a weak acid or weak base and the colour change occurs around 1 pH unit either side. For example, the naturally occurring indicator litmus is red in acidic solution (pH<7) and blue in alkaline (pH>7) solutions.

BUFFER SOLUTION A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the PH of the solution changes very little when a small amount of the acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications.

HYDRION PAPER Hydrion is a trademarked name for a popular line of compound pH indicators, marketed by Micro Essential Laboratory, Inc. exhibiting a series of colour changes (typically producing a recognisably different colour for each pH unit) over a range of pH values. Although solutions are available, the most common forms of Hydrion are a series of papers impregnated with various mixtures of indicator dyes. It is considered a “universal indicator”.

SIGNIFICANCE OF pH CONTROL IN PRINTING: -

1. Dampening solution (should kept between 4-5): -

 Higher pH (too acidic) of fountain/dampening solution causes

(i)              Ink drying problem thus causes set-off problem.

(ii)            Wearing of images in plate due to corrosion, less impression produced.

(iii)           Fading ink colour.

2. pH in paper: -

Uncoated paper – 5.5-7.

Coated paper – 8-10.5.

Higher acidity will cause ink fading.

Higher alkalinity will cause ink fading.

Water based printing inks pH

In a water based ink many of the ink components contribute to the ink pH for example, the common acrylic resins used in water-based inks are acidic. They are not soluble in water but are soluble when alkaline material are added. When these acidic resins dissolve in alkaline water, they become neutralise and form soluble salts. The solubility is depended on the pH. If the pH drops, the resin will tend to become less soluble.

Pigments are insoluble, but they depend on the pH for their fine particles stability. Dispersing resins and agents form salts around the pigment particles, which enhance the pigment stability through like-charge repulsion between particles. Since some pigments are salts themselves, pH compatibility evaluation is essential.

Likewise, resin emulsion can be selected that are more stable in alkaline conditions. Most water-based ink system use ammonium hydroxide and volatile amines to control and maintain alkaline pH in the ink. Since often they are volatile amines to control and maintain alkaline pH in the inks. Other alkaline additives or buffers can be added to maintains ph.

In reality components of the ink must be tested for optimism performances of the working ph range of the inks. Most water based inks run art an alkaline ph of 8-9.5. However, ammonium hydroxide is quite volatile, so ph will drift down unless appropriate formulation steps are taken to buffer the ph range volatile so ph will drift down unless appropriate formation steps are taken to buffer the ph range. If the ph. is allowed to drift down, the inks will increase in viscosity, build up on the printing plate, and produce unacceptable levels of foam. In addition, the high level of ammonium hydroxide can create high level of “ammonia” odour.

Problem associated with a drop of ph are common . Since the viscosities often increase with a drop of ph, foaming and print quality may deteriorate. Dirty printing is often used to describe ink prematurely drying on the flexographic printing plate and causing the printing to show a ragged edge or sports near the print edge.

Today, inks formulators are developing water based inks that can be run at lower ph with an ultimate goal of formation inks, that will run at neutral ph. New resin binders with lower acid number and formation with resin with lower acid number.