Common Acids
1M HCl:  0.0 pH
Sulfuric Acid:  0.3 pH Lemon Juice:  2.0 pH Vinegar:  3.0 pH
Wine:  3.5 pH
Beer:  4.5 pH
Milk:  6.0 pH
Common Bases
Egg Whites:  7.5 pH Seawater:  8.0 pH
Sodium Bicarbonate:  8.4 pH Ammonia:  11.6 pH
Photo Developer:  12.0 pH 0.1M NaOH:  13.0 pH
Lye:  14.0 pH
Principle of Operation
• Standard pH/ORP  electrodes
are also commonly called combination electrodes; a pH/ORP measuring electrode and a reference measuring electrode are combined in a single body.  The pH/ORP sensor measures the amount of hydrogen ions in the liquid.  The pH signal is measured against the steady reference signal. Various chemical elements leaching through the porous reference junction can react with the reference electrolyte, dilute the electrolyte solution, or attack the silver chloride element; in either case, it will disturb the steady reference signal. Stray electrical currents will also affect the steady reference signal. A temperature element is also built into the pH/ORP combination electrode.  Instruments interpret and temperature compensate the pH/ORP and reference signals into pH/ORP readings at 25°C (77°F). Signet offers three different groups of Standard pH/ORP Electrode Models: Models 2714-2717, 2754-2757, and 2774-2777
• Differential pH/ORP  electrodes
function similar to the standard (combination) electrodes, but the reference design is modified and there is a third electrode, the solution ground.  The pH and reference electrodes are measured against the solution ground.    The solution ground drains stray currents away from the reference element, hence maintaining a steady signal at all times.  The reference salt bridge slows or stops various chemical elements from leaching into the reference chamber . Chemicals that leach in may dilute the electrolyte but will not react with the glass-encased reference silver chloride element.  The reference electrolyte can be refreshed if it is diluted or depleted.  The temperature element is embedded in the pH/ORP electrode for an extremely quick response.
Signet offers one group of Differential pH/ORP Electrodes:  Models 2764 - 2767Cutaway of 2776 pH electrode Cutaway of 2766 pH electrode
reference salt bridge
3 (front chamber)
temperature element
Signet offers what is called combination pH/ORP electrodes;  a combination of three or four electrodes built into one common body that measures the pH or ORP of the solutions.  These electrodes are the pH/ORP sensing element, temperature sensing element (pH only), the reference, and sometimes a solu-tion ground.  An electrical path between the process solution, reference elec-trode, and the pH/ORP sensing electrode must always be present to complete the measuring circuit.  When the circuit is broken or interrupted, the result is a faulty reading.  There are only a few things in a chemical process that would high temperatures, and particles that can break the glass.  On the other hand, there are many problems that can occur with the reference electrode.  The reference silver chloride sensing element (wire) is exposed to the process liquid via the primary porous reference junction, which is in con-cals in the process.  Many application liquids do not chemically react with the and therefore, a differential style electrode should be used.  There are three advantages of the differential electrode:A general rule of thumb is to use a differential electrode if you have mercury, copper, lead, chlorate, bromine, iodine, cyanide, or sulfide compounds in the  differential electrodes.1. If the process chemicals attack the KCl electrolyte, the reference electrolyte chamber is refillable.2. If the reference junction becomes clogged by chemical reactions between the KCl and the process chemicals, the reference salt bridge is replaceable.  3. If there are stray currents or if there are process chemicals that attack the silver chloride wire in the standard electrodes, it will not attack it in the differential electrode because the wire is encased in a glass electrode.
reference group• It is important that the sensing end of pH and ORP electrodes remain wet, for they may be permanently damaged if allowed to dehydrate.  This is true for both in-line and submersible installation configurations.  However, be careful to keep the electrical interconnection between electrode and preamplifier dry and clean at all times. Moisture in this area can also cause permanent damage.
• pH control is best when performed in a tank.  This is especially true in neutralization applications since it is very important for reagents to mix thoroughly with waste fluids, and to be allowed adequate time for the reactions to occur. Limiting adjustments to fewer than 3 pH units per stage, and sizing tanks to provide at least 10 minutes retention time, will increase the probability of producing safe effluents.
• For bulb-style pH and ORP electrodes, significant natural self-cleaning by turbulent eddies is achieved at velocities of 1.5m/s or more (5 ft/s). Flat surface electrodes get adequate self-cleaning at velocities of 0.3 to 0.6m/s (1 to 2 ft/s). In all cases, exposure to velocities greater than 3m/s (10 ft/s) can cause excessive measurement noise and electrode wear and should be avoided.
• The aging of pH and ORP electrodes (i.e., reference depletion and decreased glass sensitivity) res
ults from a series of chemical reactions.  And as a general rule, the rates of chemical reactions double with every increase of 10°C (50°F).  This means shorter life expectancy for all pH and ORP electrodes as application temperatures increase.
Important Application Tips
• HF acid and strong caustics etch pH glass.  High concentrations, especially at high temperatures, destroy electrodes quickly.  For applications containing trace quantities of HF (<2%), use the Signet 3-2714-HF or 3-2754-HF electrode.  This electrode has a polymeric constituent in the pH glass that resists attack by HF and extends the service life considerably over “normal” electrodes.• In applications where process temperatures will drop below 10°C (50°F), use the bulb-style electrodes in place of the Flat style electrode.  This is a function of the electrical impedance of the glass that increases dramatically as temperature decreases.    • Proper electrode placement within a tank is also very important.  Electrodes should be mounted in well-mixed areas, away from reagent and waste introduction.  It is usually advisable to position the electrode near the discharge outlet of the tank.• In-line pH control is not recommended because it is very difficult to determine the amounts of reagent necessary to achieve a desired reaction if both pH and flow are variables.  However, in-line pH monitoring is very common
and useful.
• Cleaning pH and ORP electrodes and calibrating the systems should be done regularly.  The required frequency is application-dependent, but once/week for cleaning, and twice/month for calibration is recommended.• Isopropyl alcohol may be used for removing mild grease and oils from the pH sensitive glass or from the metallic tips of ORP electrodes.  Use 5% HCl on porous reference junctions clogged with hard water deposits, or other solvents/detergents as necessary.  Always consider the electrode’s materials of construction when selecting a cleanser . ORP Values of Standard pH Buffers Saturated with Quinhydrone • The purpose of calibration is to compensate the system for the deplete and must be replaced.  A good time to determine the condition of an electrode is after cleaning and during calibration.  Note the mV readings in pH values: pH 7 = 0 mV, pH 4 = +177 mV, pH 10 = -177 mV.  Replace output differs more than 50 mV table below: • 12 months at 25°C (77°F).  • Refrigeration will extend this period, but do not allow them to freeze!  Expansion of internal solutions during freezing can cause permanent damage to the electrodes.  • The risk of putting older electrodes into service is the possible electrodes are marked with date codes to identify the date of manufacture.pH4pH7Temperature (°C)202530202530ORP Value (mV)268264258928779

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