TM 10-6630-222-12&P
0004 00
OPERATOR AND UNIT MAINTENANCE MANUAL
(INCLUDING REPAIR PARTS AND SPECIAL TOOLS LIST)
FOR WATER QUALITY ANALYSIS SET: PURIFICATION (WQAS-P)
DESCRIPTION AND USE OF CONTROLS AND INDICATORS FOR ULTRAMETER 6P
Samples containing chlorine, sulfur, or ammonia can "poison" any pH
pH Glass
ORP
electrode. If it is necessary to measure the pH of any such sample,
Electrode
Electrode
thoroughly rinse the sensor well with clean water immediately after
taking the measurement. Any sample element which will reduce (add an
electron to) silver, such as cyanide, will attack the reference electrode.
Replacement sensors are available only from the Myron L. Company or
Teflon*
our authorized distributors.
Reference
*DuPont Company
Junction**
Sensor
Under Glass
Body
pH Bulb
pH AND ORP MEASURING
pH/ORP Sensor
A.
pH
Top View
1.
pH as an Indicator
pH is the measurement of Acidity or Alkalinity of an aqueous solution. It
is also stated as the Hydrogen Ion activity of a solution. pH measures the effective, not the total, acidity of a solution.
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.
2.
pH Units
The acidity or alkalinity of a solution is a measurement of the relative availabilities of hydrogen (H+) and hydroxide (OH-)
ions. An increase in (H+) ions will increase acidity, while an increase in (OH-) ions will increase alkalinity. The total
concentration of ions is fixed as a characteristic of water, and balance would be 10 7 mol/liter (H+) and (OH-) ions in a
neutral solution (where pH sensors give 0 voltage and a pH of 7x).
pH is defined as a negative logarithm of hydrogen ion concentration. Where (h+) concentration falls below 10 7 , solutions
are less acidic than neutral and so are alkaline, so a concentration of 10 9 mol/liter of (H+) would have 100 times less (H+)
ions than (OH-) ions and be called an alkaline solution of pH 9.
3.
The pH Sensor
The active part of the pH sensor is a thin glass surface which 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 trick 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 coated with silver
chloride. This is as inert a connection as can be made from metal to an electrolyte. It still can produce an offset voltage,
but using the same materials to connect to the solution on the other side of the membrane allows the 2 equal offsets to
cancel.
The trouble is, the other side of the membrane is some test solution, not potassium chloride. The outside electrode, also
called the Reference Junction, is the same construction with a porous plug in place of a glass barrier to allow the junction
fluid to contact the test solution without significant migration of liquids through the plug material. The pH/ORP Sensor Top
View figure shows a typical component pair. Migration does occur, and this limits the lifetime of a pH junction, from
depletion of solution inside the reference junction or from contamination. The junction is damaged by drying out because
insoluble crystals may form in a layer, obstructing contact with test solutions.
4.
The Integral pH Sensor
The sensor in the Ultrameter is a single construction in an easily replaceable package. The sensor body holds an oversize
solution supply for long life. The reference junction "wick" is porous teflon to provide a very stable, low permeability
interface. It is formed in a ring around the pH glass stem. The construction combines all the best features of any pH sensor
known.
5.
Sources of Error
The basics are presented in pH/ORP (see Section D 2, Page 10)
