Myron-l TH1 User Manual download pdf (Page 34)

A 4% solution of acetic acid (pH 4, vinegar) can be quite palatable, but

a 4% solution of sulfuric acid (pH 0) is a violent poison. pH provides the

needed quantitative information by expressing the degree of activity of

an acid or base.

In a solution of one known component, pH will indicate concentration

indirectly. However, very dilute solutions may be very slow reading, just

because the very few ions take time to accumulate.

B. pH Units (TPH1 & TH1)

The acidity or alkalinity of a solution is a measurement of the relative

availabilities of hydrogen (H

) and hydroxide (OH

) ions. An increase in

) ions increases acidity, while an increase in (OH

) ions increases

alkalinity. The total concentration of ions is xed as a characteristic of

water, and balance would be 10

mol/liter (H

) and (OH

) ions in a neutral

solution (where pH sensors give 0 voltage).

pH is dened as the negative logarithm of hydrogen ion concentration.

Where (H

) concentration falls below 10

-7

, solutions are less acidic than

neutral, and therefore are alkaline. A concentration of 10

-9

mol/liter of

) would have 100 times less (H

) ions than (OH

) ions and be called

an alkaline solution of pH 9.

C. pH Sensor (TPH1 & TH1)

The active part of the pH sensor is a thin glass surface that is selectively

receptive to hydrogen ions. Available hydrogen ions in a solution will

accumulate on this surface and a charge will build up across the glass

interface. The voltage can be measured with a very high impedance

voltmeter circuit; the dilemma is to connect the voltmeter to solution on

each side.

The glass surface encloses a captured solution of potassium chloride

holding an electrode of silver wire coated with silver chloride. This is the

most inert connection possible from a metal to an electrolyte. It can still

produce an offset voltage, but using the same materials to connect to the

solution on the other side of the membrane causes the 2 equal offsets

to cancel.

The problem is, on the other side of the membrane is an unknown test

solution, not potassium chloride. The outside electrode, also called the

Reference Junction, is of the same construction with a porous plug in

place of a glass barrier to allow the junction uid to contact the test solution

without signicant migration of liquids through the plug material.

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 43 44

Comments to this Manuals

No comments

Myron-l TH1 User Manual Page 34

Comments to this Manuals

Related products and manuals for Tools Myron-l TH1