pH refers to the hydrogen ion concentration (C H +). Mathematically, pH = -log C H +pH is an index of the true acidity or alkalinity of the system. pH is expressed in the form of pH scale which is 1 to 14 pH units. pH of 7.0 is called as neutral pH. pH below 7.0 is acidic while above 7.0 it is basic or alkaline.

pH of normal fresh milk is between 6.6 to 6.8. The slight lower side of pH is due to natural acidity as a function of natural milk constituents contributing towards acidity. During titration of milk with alkali to measure developed acidity inducesshifting of pH through hydrogen ion equilibrium due to the buffering action of milk.

Buffer action simply indicates a state of resistance to a change in hydrogen ion concentration of a solution i.e. milk. The buffer compounds present in milk are the acids, proteins, salts of acids such as phosphate, citrate, carbonates and dissolved CO 2 .

pH of milk can be determined mainly by the following two methods:

i)Indicator Paper Strip Method: With indicator paper strip method pH can be easily determined within few seconds. However, this method gives less accurate results compared to electrometric method. In this method help of pH paper strip is taken. The pH of the strip is so choosen so that it is nearest to the expected pH. On dipping the pH paper strip colour of strip changes. The pH of milk is equal to that of standard pH colour scale to which the dipped paper compares.

ii) Electrometric method: Electrometric method is based on the principle of potentiometer, which measures electromotive force (emf) of the system, the difference of emf is measured with the help of two electrodes. One of the electrode is known as reference electrode which has potential independent of the solution,. The other electrode is pH dependent. It generates emf when dipped in a solution. This emf is generated by H + ion concentration in the solution, which causes emf generation. It is directly related to pH. Due to emf
generation by two electrodes and along with the emf of the reference electrode causes electric current to flow. The magnitude of the electric current flow can be measured accurately and rapidly with modern electromotive devices.

Measuring pH of milk: pH meter is standardized with a standard pH solution e.g pH 4.0, .6.0 or 9.0. After standardization, milk is taken in a beaker and the electrodes are dipped in milk. This results in pH measurement, which can be obtained directly from the pH meter.

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