General Concept of pH and Buffer

pH

Pure water contains, in addition to H₂O molecules, a very small proportion of electrically charged ions formed by electrolytic dissociation, according to the equation

Where H ⁺ is a hydrogen ion, hydrion or proton, produced from a hydrogen atom by the loss of an electron and hence carrying positive charge; and OH ^- is a hydroxyl ion, which is negatively charged.

In pure water the two ions are present in equal numbers, since the water is electrically neutral, and at 25⁰C. their concentration is 10^-7 gm-ions per litre. It can be shown (from the law of mass action) that, in any dilute aqueous solution, the product of concentrations of the hydrogen ions and hydroxyl ions is constant and is equal to 10^-14 gm-ions per litre. i.e.,

[H ⁺] X [OH ^-] = 1.0 X 10^-14

When an acid is added to water it introduces hydrogen ions. Hydrochloric acid, for example, introduces hydrogen ions and chloride ions through electrolytic dissociation

Consequently, hydroxyl ions in the water are reduced so that the product of the hydrogen ion concentration and hydroxyl ion concentration remains equal to 10^-14. For example, if the concentrations of hydrogen ions is increased to 10^-3, the concentration of hydroxyl ions must fall to 10^-11. Similarly, alkalies such as caustic soda introduce hydroxyl ions.

and so cause a reduction in the number of hydrogen ions present in the water. Solutions are acid when the number of hydrogen ions exceeds the number of hydroxyl ions, and alkaline when the hydroxyl ions are in excess of the hydrogen ions. The degree of acidity or alkalinity depends entirely on the relative proportions of the two ions.

In pure water at 25⁰C, the hydrogen ion concentration (H ⁺) = 10^-7gm-ions per litre, or –log₁₀ (H⁺) = 7. The pH value of water is therefore 7, the relation between pH and hydrogen ion concentration being expressed by the equation –

i.e., the pH value is equal to the logarithm of the hydrogen ion concentration with negative sign.

MERITS AND DEMERITS OF NATURAL DYES

ADVANTAGES OF NATURAL DYES: -

1) Health and safety aspects of natural dyes: Though all natural dyes are not 100% safe they are less toxic than their synthetic counterparts. Many of the natural dyes like turmeric, annatto and saffron are permitted as food additives. Many natural dyes have pharmacological effects and possible health benefits.
2) They are obtained from renewable sources.
3) Natural dyes cause no disposal problems, as they are biodegradable.
4) Practically no or mild reactions are involved in their preparation.
5) They are unsophisticated and harmonized with nature.
6) Many natural dyes have the advantage that even though they have poor wash fastness ratings, they do not stain the adjacent fabrics in the washing process because of the non-substantive nature of the dye towards the fabric. An exception to this is turmeric, which shows substantivity for cotton.
7) Natural dyes are cost effective
8) It is possible to obtain a full range of colours using various mordants.

LIMITATIONS OF NATURAL DYES

The limitations of natural dyes that are responsible for their decline are: -

Ø Availability
Ø Colour yield
Ø Complexity of textile dyeing process
Ø Reproducibility of shade
Besides these there are other technical drawbacks of natural dyes: -
These are: -
Ø Limited number of suitable dyes
Ø Great difficulty in blending dyes
Ø Non-standardized
Ø Inadequate degree of fixation
Ø Inadequate fastness properties except few exceptions
Ø Water pollution by heavy metals and large amounts of organic substances.