Erythorbate

Another group of compounds called oxygen scavengers retard oxidation by reducing the available molecular oxygen. Products in this group are water soluble and include erythorbic acid [89-65-6] C HgO, and its salt sodium erythorbate [6381-77-7] C HgO Na, ascorbyl pahnitate [137-66-6] 22 38 7 ascorbic acid [50-81-7] C HgO, glucose oxidase [9001-37-0] and sulfites (23). 

As a result of having two chiral centers, four stereoisomers of ascorbic acid are possible (Table 1) (Fig. 2). Besides L-ascorbic acid (Activity = 1), only D-araboascorbic acid (erythorbic acid (9)) shows vitamin C activity (Activity = 0.025-0.05). The L-ascorbic acid stmcture (1) in solution and the soHd state are almost identical. Ascorbic acid crystallizes in the space group P2 with four molecules in the unit cell. The crystal data are summarized in Table 2. 

D-Araboascorbic acid (erythorbic acid) (10) L-Araboascorbic acid... 

Chromatographic methods, notably hplc, are available for the simultaneous deterrnination of ascorbic acid as weU as dehydroascorbic acid. Some of these methods result in the separation of ascorbic acid from its isomers, eg, erythorbic acid and oxidation products such as diketogulonic acid. Detection has been by fluorescence, uv absorption, or electrochemical methods (83—85). Polarographic methods have been used because of their accuracy and their ease of operation. Ion exclusion (86) and ion suppression (87) chromatography methods have recently been reported. Other methods for ascorbic acid deterrnination include enzymatic, spectroscopic, paper, thin layer, and gas chromatographic methods. ExceUent reviews of these methods have been pubHshed (73,88,89). 

In apphcations where vitamin C activity is unimportant, often D-erythorbic acid (D-araboascorbic acid) can also be used, providing the same antioxidant and reducing properties as L-ascorbic acid. 

NOTE Where MU requirements exceed 5 to 10% ofLTHW system volume per month, or 5% of the steam output ofLP steam boilers, a separate oxygen scavenger is required. This usually is tannin, diethylhydroxylamine (DEHA), or erythorbic acid (isoascorbic acid). 

Commercial sodium sulfite is acceptable for food plants higher grades are used for pharmaceutical and electronic chip manufacturing, but in these higher grades the catalyst is usually changed to sodium erythorbate (at a level of 0.05 to 0.1%, which also provides lower pressure and temperature passivation). 

Sulfite is not recommended above 850 to 900 psig because it decomposes to form corrosive sulfur compounds. In the absence of erythorbate, it begins to provide a magnetite-passivation effect above 300 to 325 psig above 400 psig, it starts to break down in the boiler (see equation 1), although initially the level of decomposition is not particularly serious. 

NOTE Cupric copper (Cu2+) is a catalyst for the hydrazine-oxygen reaction, as well as a catalyst for sulfite, DEHA, erythorbic acid, and hydroquinone. Cuprous copper (Cu+) acts as a complexing agent in the desirable formation of protective, pasivated copper oxide films. 

Erythorbic acid is supplied as white granular crystals or a powder of sodium erythorbate monohydrate C6H706Na H20, having a fivesided ring and a very reactive ene-diol structure (-COH=COH—) it is a sterioisomer of sodium ascorbate, with a MW of 216.1. It is approved under 21CFR 182.304, GRAS. 

Erythorbic acid is available as a 98+% Food Chemical Codex product (as per Pfizer sodium erythorbate FCC, although as Pfizer is now a pharmaceuticals company, much of the world s erythorbate is now supplied from China and other Asian countries) and in many diluted or blended forms, such as Nalco Sur-Gard . The ONDEO Nalco material is typical of products used within the industry, being based on erythorbic acid buffered with amine. A 1% solution of the product has a pH level of 6.3. 

NOTE Application patents for the use of erythorbates in BW were instituted by Nalco, Betz, and Calgon. The fact that three water treatment patents were issued for essentially the same product application shows the ease of finding alternative application protocols within the industry. 

Erythorbic acid and sodium erythorbate are very safe products, widely used in the food industry as antioxidants and alternatives to vitamin C. In the water treatment industry, they are strong reducing agents that reduce metal oxides and hydroxides to their more soluble ferrous forms and promote the passivation of boiler waterside surfaces (magnetite formation). 

The oxygen reaction with sodium erythorbate is catalyzed by traces of nickel, copper, or iron in solution, so stainless steel or plastic stor-... 

Formulated products tend to use a 10 to 20% neutralized erythorbate, buffered to pH of 5 to 6 with ammonia, morpholine, cyclohexy-lamine, diethanolamine (DEA), or triethanolamine (TEA) to reduce the acidity of erythorbic acid. Similarly, amines are used with sodium erythorbate to improve the reaction rate. 

Various combination oxygen scavenger products exist that use erythorbate with, for example, tannins and sulfite, to obtain safe products having the benefits of low-cost scavenging and good passivation. 

Erythorbates are safe products and there are no harmful breakdown products, although when early formulations utilized ammonia as a PH buffer (and neutralizer for part of the carbon dioxide), copper corrosion problems resulted. However, erythorbates are not steam-volatile,and consequently there is no post-boiler oxygen scavenging potential available. Thus, in the event of complete breakdown of the product at high pressure, oxygen-induced, ammonia corrosion of copper may continue unchecked. 

Erythorbates are claimed to be suitable for pressures of up to 1,800 or even 2,000 psig, with only moderate FW reserves required (200-500 ppb erythorbate). This is not borne out in practice because at these cor-... 

Additionally, control of erythorbate reserves in the boiler is difficult and should thus be generous, as shown below ... 

Practical feed rates vary, but for a formulation containing, say, 10% erythorbate, the requirement is typically between 100 and 120 ppm whole product per 1 ppm 02. Consequently, this type of adjunct product may be expensive because of the high combining ratio required. 

NOTE Formulations based on sodium erythorbate and stabilized with citric acid against premature reaction (thus, they remain safe products, despite the pH level being around 2.0). Formulations based on ammonium erythorbate lose their safe status but have an improved pH of 6.3 to 6.5. 

Where sodium erythorbate is employed, expect the condensate pH to be at least one pH unit lower than with ammonium erythorbate. Sodium erythorbate can be buffered up to pH 4.0 by the addition ofl% DEHA (85% strength product), and this provides some marginal improvement in condensate pH. Where DEHA is added in excess of 1 Wo, the product becomes unstable. 

As a result of the limited solubility of MEKO, it is often formulated with other boiler treatment ingredients such as filming and neutralizing amines, sulfite, or erythorbates, which permit increased concentration, improved stability, and better overall effectiveness. 

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