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Technical Information Table of Contents Introduction If you have Adobe AcrobatTM set in your browser as a helper application for .pdf files, you can click here to view an acrobat .pdf version of this document.
Introduction Inhibitors are a unique group of products designed to control color, corrosion and scale formation in waters of all kinds. For a small investment they return large dividends in the form of lower capital and operating costs, higher quality, improved plant performance, increased safety, and reduced health risks. Innophos inhibitors, based on phosphorus products, are no exception. They fall into three broad categories each with their own important benefits. These are:
This bulletin describes these products and their uses in a variety of water treatment applications. Further information can be obtained by reading the Product Data Sheets and the Material Safety Data Sheets for these products or by contacting the Innophos Customer or Technical Service Departments.
Innophos Inhibitor Products
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| Cooling water treatment | Drinking water treatment |
| Boiler water treatment | Industrial water treatment |
for:
| Corrosion control | Sequestering | Red water control |
| Scale control | Lead and copper control | Black water control |
| Softening | pH control |
Orthophosphates
Orthophosphates are one of a number of oxo anions of the element phosphorus. All phosphorus oxo anions contain P-OH groups where the hydrogen atom is ionizable. Orthophosphates contain three such groups and have the structure shown below.
Mono, di and tri sodium phosphates are all orthophosphates and result from the progressive neutralization of phosphoric acid (H3PO4) with a sodium based alkali. For example,
Monosodium Phosphate
2H3PO4 + Na2CO3 -> 2NaH2PO4 + H2CO3
(33% neutralized acid)
Disodium Phosphate
H3PO4 + Na2CO3 -> Na2HPO4 + H2CO3
(67% neutralized acid)
Trisodium Phosphate
2H3PO4 + 3Na2CO3 -> 2Na3PO4 + 3H2CO3
(100% neutralized acid)
Innophos offers:
- Monosodium phosphate anhydrous (MSP)
- Disodium phosphate anhydrous (DSP)
- Trisodium phosphate dodecahydrate (TSP)
Table 1. Common orthophosphates - selected properties.
Properties
The orthophosphates of sodium are soluble in water. Their solutions vary in pH, relative to their concentration in water and to the degree of neutralization of the phosphoric acid (see chart 1). They form salts with metals such as iron, lead and calcium which exhibit very low solubility in water.
Figure 1. Titration of orthophosphoric acid with sodium hydroxide(1g. H3PO4 /100 ml. solution);glass electrode with high pH correction. (Kirk Othmer Encyclopedia of Chemical Technology, Volume 10, 1953.)
Uses
Orthophosphate inhibitors are used in boiler, cooling and potable water applications, for corrosion, pH and lead control.
The orthophosphates can act as anodic corrosion inhibitors in the presence of oxygen (e.g. surface and aerated waters). For example, dissolved oxygen attacking a metal such as iron, forms alpha Fe2O3. The ferrite film gradually spreads, corrosive attack occurring at the breaks in the oxide film. These gaps are then plugged by the formation of insoluble iron phosphates resulting from the reaction of sodium orthophosphate with the anodic corrosion product .i.e.
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1. At the anode, Fe0 --> |
Fe+2 + 2e- |
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2. Fe+2 + H2PO4 + 2H2O --> |
FePO4.2H2O + 2H+ + e- |
| (precipitated) |
This type of corrosion control is called passivation. Passivation results initially from chemical precipitation which provides a barrier between the metal and the corroding liquid. In the case of orthophosphates a thin film which is very insoluble and resistant to dissolution is put down. It is a form of conversion coating. Such coatings result when films capable of preventing the migration of ions from the solid state to the solution state are formed. These thin films retard the flow of electrons through the galvanic cell which is driving the reaction, reducing its effects and controlling corrosion.
In boiler water treatment orthophosphates can be used to control caustic corrosion by maintaining the orthophosphate on the free acid side (under neutralized). In this manner local excesses of caustic occurring through evaporation in porous deposits within the boiler can be neutralized by the free acid of the orthophosphate. This may continue until the orthophosphate has been completely neutralized (converted into tri sodium phosphate), after which local excesses of caustic may occur. i.e.
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Na2HPO4 + 1NaOH |
--> Na3PO4 + H2O |
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(66.7% neutralized) |
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Control of this process requires the monitoring of the sodium/phosphate ratio i.e. [Na]/[PO4]. In a solution containing only sodium orthophosphate and caustic soda, the [Na]/[PO4] ratio would have to be maintained under 3 (usually in the 2.3 to 2.6 range) to ensure protection. In practice sodium ions will be present from other sources (sodium bisulfite for oxygen scavenging for instance) and these must be taken into account. In addition, since the solubility of orthophosphates varies with the temperature, care must be taken not to exceed the solubility of the species of orthophosphate present under the boiler operating conditions.
Orthophosphates are able to play an important role in controlling the occurrence of lead in potable water. The detection of lead in drinking water has prompted rules under the Safe Drinking Water Act to reduce the lead contamination of drinking water at the faucet. Such lead is usually the result of the corrosion of lead service lines forming part of the drinking water distribution system, or is derived from lead containing solder used in the brass faucet fixtures and plumbing during building construction. The American Water Works Association has recommended the use of deposition and passivation corrosion control techniques as part of a program to alleviate this problem. In this application the pH and the calcium carbonate content of the treated water would be adjusted to neutralize any excess acid in the system and to allow the controlled deposition of calcium carbonate in the service lines. The deposited calcium carbonate provides a barrier between the water and the lead containing transmission system. The calcium carbonate barrier is however porous to a degree and the effectiveness of the system is improved by the presence of the orthophosphate ion, which will react with the lead and other metals present to form a much less soluble and much less permeable barrier. This technique is discussed more fully in the Innophos Technical Information Bulletin entitled "Potable Water Treatment, Lead Control".
Polyphosphates
Condensed phosphates or polyphosphates, as the names suggest, contain more than one phosphorus atom in the molecule or ion, connected to each other through an oxygen bridge. The chains may contain as few as two phosphorus atoms (pyrophosphates) or an infinite number (polyphosphates). They may be linear or cyclic (metaphosphates). They are prepared from orthophosphates by dehydration and other techniques.
Polyphosphate structures are numerous. Some of the most important as inhibitors are:
Sodium polyphosphate glassy, sodium acid pyrophosphate, tetra sodium pyrophosphate and sodium tripolyphosphate are all polyphosphates. They represent differing degrees of condensation and neutralization of the orthophosphate molecule.
Innophos offers:
- Sodium polyphosphates glassy (SHMP), Vitrafos TM
- Sodium acid pyrophosphate (SAPP)
- Tetrasodium pyrophosphate, anhydrous (TSPP)
- Sodium tripolyphosphate, anhydrous (STPP)
Table 2. Polyphosphates - selected properties.
Properties
Polyphosphates represent a large class of phosphorus chemicals. Generally their salts are soluble in water including the salts of lead, iron and calcium. In solution they tend to rehydrate slowly, reverting to the equivalent orthophosphate. They are sequestering agents.
Uses
Polyphosphate inhibitors are used in boiler, cooling, process and potable water applications for softening, sequestering, corrosion control, lead control and descaling.
Polyphosphates are cathodic corrosion inhibitors. Divalent cations such as calcium (Ca+2) at levels of at least 10ppm are needed along with the polyphosphates for them to be effective. Positively charged colloidal complexes are formed which migrate to the cathode forming an amorphous polymeric film. This film is self limiting. Analysis of the film shows ferric pyrophosphate and iron/calcium metaphosphate to be the primary constituents. The pH is best maintained at the level of 6.5 to 7.0 since as the pH drops, the rate of reversion of the polyphosphate to the orthophosphate increases. In these applications, the polyphosphate is acting both as a reservoir of potential orthophosphate and as a cathodic corrosion inhibitor. More soluble and less likely to be precipitated in its polymeric form, polyphosphates can be retained longer in the system than orthophosphates. Much of the polyphosphate will eventually revert to the orthophosphate condition, at which time it may react with elements such as calcium, lead and iron, to be precipitated and to form protective conversion coatings.
The property of sequestering exhibited by polyphosphates finds application in boiler, cooling, industrial and potable water treatment for descaling, softening and the elimination of red and black waters. Sequestering offers two major benefits when it is used in water treatment. First, it reduces scale formation by preventing the precipitation of calcium salts and second, it prevents discoloration of the water by inhibiting the precipitation of iron and manganese hydroxides. Polyphosphates reduce scale formation by interfering with the formation of calcium carbonate crystals, thereby reducing the rate of film formation. There is a "threshold effect" in using polyphosphates in this way. Once the threshold is reached polyphosphates can prevent scaling at calcium carbonate concentrations far above the saturation concentration. In addition to preventing scale formation polyphosphates can be used to remove scale.
Polyphosphates form colloidal dispersions with metals such as iron, manganese and calcium. Stable negatively charged particles are formed in which the metal is coated with polyphosphate ions. This prevents the particles from coalescing and maintains them in solution. The actual mechanism is not precisely defined at present and it probably varies with each cation. The result is the prevention of scale and of red and black water. It should be noted that if the polyphosphate reverts to the orthophosphate condition, aggregation and precipitation may occur.
Ortho/Polyphosphate blends
The properties of the orthophosphates and the polyphosphates are enhanced when blends of the two are used. Because orthophosphates are very reactive in solution and form insoluble salts with calcium, iron and lead, it may be difficult in some circumstances to keep a sufficient concentration of orthophosphate in the treated water to maintain its effectiveness. To compensate for this, the dose of orthophosphate must be increased, which can lead to additional expense and to areas of excessive deposition in the system, while other areas are still under treated. Polyphosphates on the other hand are unlikely to be precipitated unless they undergo reversion.
Since orthophosphates are anodic corrosion inhibitors and polyphosphates are cathodic inhibitors, and since polyphosphates in solution slowly revert to the orthophosphate condition, by formulating blends of these two phosphate forms we can achieve both anodic and cathodic corrosion inhibition, ensure the orthophosphate ion availability over a longer period of time and control calcium, iron and lead deposition, all at the same time. In this way, blended phosphate products can offer better protection than either ortho or polyphosphate can alone. As an added benefit, blends are effective in reducing copper corrosion, which neither type does very well by itself.
Ortho/polyphosphate blends find application in boiler water, cooling water, potable water and industrial water treatment for corrosion control, lead and copper control, scale control and sequestering. They are generally effective in the pH range of 6.0 to 8.5. Blends improve the range and reliability of phosphate based inhibitors and represent the most vigorous growth opportunities.
For more information on the availability of Innophos Phosphate Based Corrosion Inhibitors, contact the Innophos Sales Representative or Innophos Technical Service.
Table 3. The uses of Innophos inhibitors in water treatment.
TIR-18 NOV. 1996
See your Innophos Sales Representative or call our Order and Customer Satisfaction Team at 1-609-495-2495 for more information about products discussed in this Technical Information Report.
Innophos believes all inform given is this report is accurate. It is offered in good faith, but supplied without consideration or guarantee. Innophos assumes no obligation or liability for the accuracy or sufficiency of the information given or the results obtained, all such information being given or accepted at user's risk. The use(s) referred are listed for purposes of illustration only and the user is urged to investigate and establish the suitability of application of such use(s) in every case. Nothing herein contained is to be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents or as advising or authorizing practice of any patents or patent applications owned by Innophos or others.
P.O. Box 8000
Cranbury, NJ 08512-8000, USA
Customer Service (tel) 609-495-2495
Phosphorus Products (tel) 609-495-2495
Phosphorus Products (fax) 609-860-9563
