Federal Circuit Holds ‘About’ Indefinite in pH Range Claim — Enviro Tech v. Safe Foods Precedential Opinion

The U.S. Court of Appeals for the Federal Circuit (CAFC) on 4 May 2026 issued a precedential decision in Enviro Tech Chemical Services, Inc. v. Safe Foods Corp. (No. 24-2160), affirming a district court ruling that the word “about” in a pH-range limitation was indefinite under 35 U.S.C. § 112. Judge Lourie authored the opinion, joined by Judge Prost and District Judge Burroughs (sitting by designation). The decision is a fresh and forceful reminder that approximation language in claims must come with a workable interpretive anchor, particularly in chemistry, biotechnology, and materials patents.

Case Background

The case concerns U.S. Patent No. 10,912,321, owned by Enviro Tech Chemical Services. The ’321 patent claims methods for treating poultry during processing using peracetic acid in a chill tank to increase the weight of the poultry. Enviro Tech sued Safe Foods Corp., a competitor, in the U.S. District Court for the Eastern District of Arkansas for infringement.

The disputed limitation in independent claim 1 was a step of altering pH “to a pH of about 7.6 to about 10” by adding an alkaline source. Safe Foods argued that “about” rendered both the upper and lower bound of the range insufficiently certain to allow a skilled artisan to determine the scope of the claim. Safe Foods also challenged the term “an antimicrobial amount” as indefinite, but the Federal Circuit’s opinion addressed only the “about” limitation and held that the additional challenge need not be decided.

The district court agreed with Safe Foods, found the pH limitation indefinite at claim construction, and held all asserted claims invalid. Enviro Tech appealed.

The Federal Circuit’s Reasoning

The Federal Circuit’s analysis rests on the general principle that words of approximation such as “about” or “approximately” are not categorically indefinite, but they must allow a person of ordinary skill in the art to ascertain the scope of the claim with reasonable certainty. Judge Lourie wrote that “words like ‘about’ and ‘approximately’ may be appropriately used to avoid a strict numerical boundary to the specified parameter,” provided that “the parameter’s range must be reasonably certain based on the technological facts of the particular case.”

The decision turned on Enviro Tech’s own conduct during prosecution. In one office action response, Enviro Tech argued that a peracetic acid solution at the lower end of the claimed range—“pH 7.6”—would not have been obvious over the prior art, conspicuously omitting “about.” In the very same response, when distinguishing prior art with reference to another claim, Enviro Tech retained “about” before the pH value. The Court found that this selective use of “about” suggested Enviro Tech itself treated “pH 7.6” and “about pH 7.6” as having different meanings, while never defining anywhere in the intrinsic record what tolerance “about” actually carried. The specification likewise offered no quantitative anchor—no representative measurement error, no synthesis-to-synthesis variability data, and no working examples illustrating how “about” should be applied.

The opinion reaffirms the Nautilus v. Biosig Instruments (2014) “reasonable certainty” standard, applying it strictly in a chemistry-patent context. The pH range from 7.6 to 10 is already two and a half units wide; the panel concluded that adding ambiguity at either bound left a competitor with no reliable way to determine whether a given practice falls inside or outside the claim.

Practical Takeaways

Three practical takeaways for patent prosecutors and litigation counsel follow from this decision.

First, when using approximation language with numerical limitations in chemistry, biotech, and materials patents, draft a specification that gives the term concrete meaning. A simple express definition (“‘about’ means within ±0.2 pH units” or “within ±5% of the recited value”) goes a long way. Working examples that illustrate typical batch-to-batch variability or measurement tolerance provide the technological context the Court demands.

Second, prosecution-history discipline matters. Whether to include or omit “about” before a numerical value during arguments must be a deliberate choice, applied consistently across the file. Selective use will be read against the patentee. As Enviro Tech illustrates, a single inconsistency embedded in an office action response can ripen into a claim-killer years later in litigation.

Third, because this opinion is precedential, it will be cited in future Federal Circuit indefiniteness cases and likely in district court Markman orders involving numerical-range patents. Patent owners with portfolios in pharmaceuticals (pH, osmolality, particle size, dissolution), food processing, agrochemicals, and specialty chemicals should review existing claim sets for approximation language and assess whether the intrinsic record supports the bounds with reasonable certainty. Where it does not, continuation strategy, reissue, or supplemental examination may be worth considering.

Enviro Tech’s infringement case ended with the affirmance. The opinion’s longer-term significance, however, is the operational guidance it provides drafters and prosecution counsel: approximation cannot survive on its own; it must be anchored, defined, and used consistently.