Phosphonate Testing and Reporting
There are two fundamental approaches to testing for phosphonates in cooling water, each based on the reaction between a metal and a complexing agent (phosphate or phosphonate). Both reactions are accompanied by the development of a distinguishable color, measurable either by a comparator, colorimeter, photospectrometer or by titration.
The “Total Phosphate” method is based on the amount of orthophosphate produced by the degradation or phosphonates. The oxidation can be accomplished photochemically or by heat in the presence of appropriate chemicals. The “Phosphonate” direct reading method is based on titration with an appropriate metal (usually thorium) in the presence of a color indicator (straight or modified xylenol orange) to determine phosphonate content (reported as phosphate or phosphonate).
Total Phosphate – Conversion to Orthophosphate
Since orthophosphate is already present in many waters (treated and untreated) and other phosphorus compounds will degrade to it before these tests are completed, our conversion factors are based solely on the PO 4 content.
Orthophosphate, polyphosphate and organic phosphorus values are determined by running all three of the following tests, while only two are needed if the species of inorganic phosphate is not of interest or if phosphonate levels alone are required (Hydrolyzable plus either UV Test or Persulfate test).
1. Reactive phosphorus
Direct orthophosphate determination is made on an unheated, undigested sample. Orthophosphate reacts with molybdate in an acid medium to produce a yellow complex. Ascorbic acid or an amino acid or stannous chloride then reduces the complex, yielding a good blue color. In some tests, vanadium replaces ascorbic acid to provide a strong yellow color. Any of these tests, and possibly the following one, are necessary to establish the amount of phosphate present in the water supply (raw or pretreated).
2. Acid hydrolyzable phosphorus
Phosphates in condensed forms (meta-, pyro-, or other polyphosphates) must be converted to the reactive form before analysis. Boiling of the sample with sulfuric acid guarantees hydrolysis, the procedure is completed with tests from Part 1. The orthophosphate value determined earlier is then subtracted from the total inorganic value, and the difference is the measure of polyphosphate levels. Organic phosphorus is not, in general, converted by this process, but a small fraction of orthophosphate may be unavoidably contributed. The operator must allow enough time for the reaction to be completed.
3. Oxidation of Phosphonates
Oxidative digestion and subsequent development of a complex of recognizable color allow us to determine the phosphorus content of organic compounds.
a) In the Persulfate Test, hydrolysis takes place in boiling sulfuric acid, accompanied by persulfate oxidation, and the test results are recorded as reactive phosphate. The organophosphorus content is then calculated as the difference between total phosphate and total inorganic phosphate. The total of orthophosphate found in this manner is an algebraic sum of the contribution of the individual phosphates involved.
b) The Ultraviolet Photochemical Oxidation Method is conducted without heat or acid but uses persulfate as an oxygen source. The orthophosphate which develops is again determined by the methods described in Part 1. If an undigested blank has been run, the orthophosphate present in raw water or as impurities in the phosphonate will have been accounted for. A figure for phosphonate results from the subtraction of the original orthophosphate from total phosphate. No significant hydrolysis or degradation of polyphosphates should occur. This process requires expensive equipment and involves the hazards of LTV light. The operator must allow enough time for the reaction to be completed.
c) Another method utilizes cerium, a powerful oxidizing agent in acid media. No heat is required, but a very good operator technique is. One consideration is that polyphosphate will be partially hydrolyzed by this test-not a problem if acid hydrolyzable phosphorus has already been determined. This method works beat for phosphonates (such as HEDP) which do not contain nitrogen, but triazoles and silica are sources of interference.
d) Other methods involve Perchloric Acid, and Sulfuric Acid/Nitric Acid, both methods requiring boiling and both releasing unpleasant vapors. The Perchloric Acid method is very dangerous.
Thorium Titration – Direct Reading Phosphonate Test This is a titration in which a sample containing phosphonate is titrated with a solution of thorium nitrate to form a stable, colorless complex with phosphonates. A xylenol orange indicator in the acid range results in a color described as pink, rose, violet and purple. Under these test conditions, the xylenol orange complex with thorium is less stable than is the phosphonate complex, and will not form until all of the phosphonates has been used up.
Straight xylenol orange indicator salts have to some degree “been supplanted by screened or mixed indicators which allow for a more discernable color change at the endpoint.” These indicators are less subject to “sample turbidity, poor lighting conditions, degradation because of humidity” and “fading … endpoints” from metal interference.’
The desired pH of between 2.5 and 3.0 is obtained with sulfuric acid. Among the reasons for maintaining this pH range are that sulfate may interfere below pH 2.0, and as Monsanto comments in a recent report2:
“Calcium, magnesium and barium ions do not form stable complexes with Dequest at pH 2.5 to 3.0, and do not interfere with titration at this low pH.”
Interference results from other sequestrants such as polyphosphates, from chlorine, bromine, fluorine, iron, suspended solids and from high levels of sulfate. These may be controlled to some extent by either filtration, the addition of specific suppressants, or by using indicator powders described above which contain additives to perform these functions.
In 1968, a Monsanto Report’ stated that Dequest 2000 forms 1:1 complexes with thorium.
The results presented in column “d” of that table assumes that the complexing ability of the phosphonates for thorium is directly related to:
1. The activity percentage of the product.
2. The phosphorus content of the product.
3. The molecular weight of the product-under the assumption that the complex with thorium is a mole for mole relationship.
4. Our desire to standardize on a factor of 2.00 for Dequest 2000.
Comparing the results in column “d” with the conversion factors recommended by a number of test kit manufacturers grouped in column “e” shows that we have a problem and we need to know more about the nature of the complexes which thorium forms with the phosphonates.
In 1980 Jarvio4 commented: “No standard calculations from this method are possible since the complex formed is different for each phosphonate and is never a stoichiometric integer ratio of phosphonate to thorium.”
This should not be surprising to us since field experience has caused us to recommend that HEDP is selected over AM? for handling heavy metals. We have support from the Stability Complexes with Fe +3 (16.2 vs. 14.4) and for CU+2 (19.0 VS. 17.4). We have also seen that HEDP chelates more copper per mole than does AMP at pH 6 ( 1.80 vs. 1.02 moles of copper per mole of phosphonate). We have not found comparable information for thorium.
Also in 1980, CampbeI15 said: “All phosphonates react with multivalent metals to produce mixtures of complexes in solution. Because of the mixtures, there is no simple one atom of metal to 1 or 2 or 3 molecule (s) of AMP relationship. In a titration, there will be a point at which no more metal can be complexed by the sample, but this point is the result of many complex factors such as pH, temperature, and concentration to name a few. A subtle change in any of these makes a comparison of different samples meaningless.”
If this is the case, and it appears to be, we must acknowledge that no simple calculations will yield accurate factors for conversion from one phosphonate source to another. We must assume that each test kit manufacturer has established its conversion ratios carefully and verified and adjusted them by testing a variety of phosphonate products and levels. It may be that the variations between sources result from differing philosophies on the number of materials to be added for the control of chlorine, fluorine) iron, etc. For the present, it is best to adhere closely to the recommendations of your test kit supplier.
Colorimetric Iron – Based Test This method involves the complexation of iron by a sequestrant which produces a blue color.
Polyphosphates or phosphonates act as iron sequestrants, decreasing the amount of iron available to the iron sequestrant. As a result, the intensity of the blue color is diminished. Zinc, copper, and magnesium do not usually interfere with this test, though heavy metals may be a problem with other phosphonate tests.
The Presence of Orthophosphates in Phosphonates It has been found that some organic phosphorus compounds contain orthophosphates in significant quantities, present as impurities or as unreacted ingredients. In general, the orthophosphate is of no positive value, having neither sequestering or crystal modifying properties.
One observer commented that the leading suppliers are offering a product at least 20% better than that available in 1980. If we start with a stoichiometric dosage of phosphorus as phosphoric acid or phosphorus trichloride and monitor the results to be sure that there are only small quantities of phosphorous and phosphoric acid left, we will be sure that the balance is organic. We must still consider Campbell’s” concerns that all of this organic phosphorus is functional.
Ciba Geigy suggested some measured PO, values for our use in comparing actual and theoretical content:
This information was presented a few years ago and it is possible that the controls may have been approved for these suppliers or by others.
Other than for Belcor 575, we do not have a breakdown of these products, but suspect that the surpluses are orthophosphate. Unless they help in corrosion control, the orthophosphates serve no purpose and in fact may interfere with our tests unless their presence is known and adequately dealt with. The following two sections will address these matters.
1. Belcor 575 is 50% active HPA but contains about 60% total solids of which as much as 7% may be phosphorus and phosphoric acids. While the theoretical PO, content is 30.5%, the average amount found is usually closer to 35%. The PO, range is as follows:
|Phosphorous Acid||4.5- 5.8|
|Phosphoric Acid||0.5 Z -1 Q|
Phosphoric acid does not in general titrate in the thorium test while phosphorous acid in a low oxidation state may test as “organic phosphate.” If the test procedure chosen is “total phosphate,” more serious problems could arise since the impurities would then constitute about 15% of the total phosphate and could represent a positive bias of one, two, or more ppm in the readings.
Variations in handling or testing techniques often influence the test results for orthophorous acid. Aeration of a sample may oxidize phosphorous acid to orthophosphate prior to either type of test.
To the extent that these byproducts respond to the thorium test, they mislead us as to the threshold or sequesting abilities of the phosphorus compound used.
2. Discussion on Test Procedures
a) It would be helpful if we knew more fully what forms of phosphorus are included in the products we purchase. If these percentages are not provided, regular tests of incoming materials are indicated. To the extent that information is available from either of these sources, the user can adjust test results to exclude non-functional species.
b) As a supplement or alternate to a) above, efforts should be made to oxidize phosphorous acid to the orthophosphate before tests begin. Aeration may help or a controlled action oxidizer might be incorporated into the reactive phosphate test and into the blank phosphate test.
c) If the oxidation suggested in b) is accomplished, the relative phosphorus test should be performed in addition to a blank phosphonate test to provide complete knowledge as to functional phosphonate.
was written by ROBERT R. CAVANO. Any questions, you can send an email to him.