Metal Finishing Guide Book


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Table I. Components of Common Buffers Components Hydrochloric acid Concentration 0.365 g/L Potassium chloride 5.64 g/L Potassium hydrogen phthalate 10.2 g/L Acetic acid 6.0 g/L Sodium acetate 2.08 6.7 g/L Potassium hydrogen tartrate pH 1:1 mixture 8.2 g/L Borax 19.06 g/L 3.57 4.00 1:1 mixture 6.9 9.2 +19.99 pH. In other meters, such as the analog meters, the scale is limited. In this case the millivolt range of the pH meter provides greater range, such as ��1,400 mV, than the normal 0 to 14 pH range, which is ��421 mV at 25��C; therefore, the pH value can be calculated by comparing the millivolt values obtained in a sample. Of course there is reduced readability with the larger millivolt range. Relating pH and millivolt values observed on the pH meter, the pH value can be calculated using the following formula: pHx = pHs - (Ex - Es)/S where pHx is the pH of the sample (x); pHs is the pH of the standard (s); Ex is the millivolt value of the sample as read by the pH meter; Es is the millivolt value of the standard as read by the pH meter; and S is the slope, normally 59.16 mV/pH unit at 25��C. The pH can be determined colorimetrically or electrometrically. Colorimetric Method For a visual indication of the pH a so-called acid-base indicator or mixture of indicators is employed. Such an indicator is a weak organic acid (or base) having the special feature that it changes color within a definite pH range. The standard colorimetric method is carried out by placing a given quantity of solution to be tested in a glass tube. A set amount of indicator is added and compared with standards, in which a solution with known pH has been prepared with the same amount of indicator. This method is cumbersome and rarely used today. The derivative of the standard colorimetric method is the pH test paper. The indicator is impregnated on an absorbent paper strip. A color chart with the appropriate pH values is printed on the strip or a card. After immersion in the solution the strip is compared with the standard color. Reliable pH papers are accurate ��0.3 units from the electrometric value. In the case of a strong oxidizing or reducing solution the pH papers are not reliable. Electrometric Method Certain electrodes, such as hydrogen, quinhydron, antimony and others, when immersed in a solution, develop voltages (called electrical potential), which depend upon the pH of the solution. As a result a pH meter is basically a millivolt meter. The glass bulb electrode is the key to making a pH measurement. The special composition glass used is very selective and sensitive to hydrogen ions. The potential that is developed at the glass membrane can be related to the pH of the solution. To complete the circuit and provide a stable and reproducible referencing potential, another electrode is required. The ���reference��� electrode makes contact with the solution through a junction, which allows slow leakage of the filling solution into 566

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