Gas purity grades explained: what N2.5 4.0, 5.0 and 6.0 mean, how ppm relates to per cent, and which grade each job needs

A gas purity grade is a short way of writing how many nines the purity has. In the notation used across the industrial-gas trade and in the grade names of IS 309:2023 and IS 1747:2023, the first digit is the number of nines and the second digit is the figure that follows them: 2.5 is 99.5 %, 4.0 is 99.99 %, 5.0 is 99.999 % and 6.0 is 99.9999 %. Every step of one in the first digit cuts the permitted impurities by a factor of ten.

The grade is only half of a specification. The other half is the list of individual impurity limits, because a process usually cares about one impurity, such as moisture in welding argon or oxygen in a nitrogen blanket, far more than about the total.

Grade	Purity, % minimum	Total impurities, %	Total impurities, ppm	Example in an Indian standard
2.0	99.0	1.0	10,000	IS 309:2023 oxygen Grade 3 (commercial)
2.5	99.5	0.5	5,000	IS 309:2023 oxygen Grade 2; IS 1747:2023 N 2.5 nitrogen
3.0	99.9	0.1	1,000	—
3.5	99.95	0.05	500	IS 309:2023 oxygen Grade 1 (N3.5)
4.0	99.99	0.01	100	Market grade for welding argon
4.5	99.995	0.005	50	IS 1747:2023 N 4.5 nitrogen
4.8	99.998	0.002	20	Market grade for argon in titanium welding
5.0	99.999	0.001	10	IS 1747:2023 N 5.0 nitrogen
5.5	99.9995	0.0005	5	IS 1747:2023 N 5.5 nitrogen
6.0	99.9999	0.0001	1	Research and semiconductor grade

Conversions: 1 % = 10,000 ppm, 1 ppm = 0.0001 % = 1,000 parts per billion (ppb). Gas specifications use ppm by volume unless they say otherwise.

The Indian specifications for four of KIGL's gases are summarised here; the full tables are on the linked pages.

Oxygen, IS 309:2023

Three grades, each a minimum oxygen content plus five impurity limits: Grade 1 (N3.5) at 99.95 %, Grade 2 (N2.5) at 99.5 % and Grade 3 (N2.0) at 99.0 %. Grade 2 limits water vapour to 2.0 ppm, carbon dioxide and carbon monoxide to 0.5 ppm each and hydrocarbons to 20 ppm. The standard says Grade 1 is for calibration, Grade 2 for laser-cutting assist gas (and liquid oxygen), and Grade 3 for welding and metallurgy. For oxygen made by air separation, carbon monoxide, carbon dioxide and hydrocarbons are tested only by agreement between buyer and seller. Medical oxygen is outside IS 309 and is governed by the Indian Pharmacopoeia; see industrial vs medical oxygen and oxygen standards.

One quirk: Grade 1's listed impurity limits add up to about 112 ppm, which would imply about 99.99 % rather than the 99.95 % its N3.5 label suggests. The assay figure and the impurity table are not fully consistent.

Nitrogen, IS 1747:2023

The 2023 revision abolished the old "technical" and "pure" grades and set four by purity: N 2.5 (99.5 %, gas only), N 4.5 (99.995 %), N 5.0 (99.999 %) and N 5.5 (99.9995 %). Purity is defined as 100 minus the sum of the listed impurities, not as a separately measured figure.

Impurity, ppm v/v max	N 2.5	N 4.5	N 5.0	N 5.5
Oxygen	5,000	10	2	1
Moisture	—	14	2	1
Hydrogen	—	5	1	0.2
Carbon dioxide	—	5	1	0.5
Carbon monoxide	—	5	1	1
Hydrocarbons as methane	—	5	1	0.5
Sum of listed limits	5,000	44	8	4.2

The sums sit inside the totals the grades allow (50, 10 and 5 ppm), which is how the purity definition stays consistent. A grade such as "99.9 % nitrogen" does not exist in IS 1747; it falls between N 2.5 and N 4.5. More on nitrogen standards and choosing PSA nitrogen purity.

Dissolved acetylene, IS 308:2024

IS 308 sets no minimum purity percentage. It caps four impurities: total impurities insoluble in acetone at 4 % by volume, sulphur compounds (as H₂S) at 0.02 %, phosphorus compounds (as PH₃) at 0.01 %, and moisture at 0.25 % by mass. KIGL's industrial acetylene is specified at a minimum of 98.0 % v/v as a KIGL specification, conforming to the IS 308 limits. KIGL's ultra-high-purity acetylene is at least 99.99 % in DMF, also a KIGL specification, with phosphine and hydrogen sulphide each at 10 ppm or less. See acetylene grades explained.

Carbon dioxide, IS 307:2024

Two grades, with minimum purities of 99.7 % and 99.9 % between them, both limiting carbon monoxide to 10 ppm and oil to 5 ppm by mass. The standard's foreword and its Table 1 disagree on which grade carries which purity, so ask for the certificate to state the purity figure itself, not only the grade number.

Argon

The Indian argon specification, IS 5760, has just been revised, so argon grades here are quoted in the market notation used by the trade: 4.0 for welding, 4.8 for titanium and nickel alloys, 5.0 and 6.0 for analysis and electronics, as set out in argon technical data.

Impurity	Why it matters	Method named in the Indian standards
Oxygen in nitrogen or argon	Oxidises welds, food and electronics	Galvanic (Hersch) cell; colorimetric pyrocatechol method (IS 1747 Annex A)
Nitrogen, argon in oxygen	Lowers cutting speed	Gas chromatography on molecular sieve 5A; spectral analyser for 0–100 ppm N₂ (IS 309 B-1, B-6)
Moisture	Porosity in welds, corrosion, column damage	Cavity ring-down spectroscopy; P₂O₅ absorption tubes (IS 309 B-3, B-10)
CO, CO₂, hydrocarbons	Catalyst poisons; oxygen-system fire risk (hydrocarbons)	GC with flame ionisation detector and methaniser (IS 309 B-1)
Phosphine, hydrogen sulphide in acetylene	Odour, toxicity, contamination of the flame and the work	Molybdate titration for PH₃, barium sulphate gravimetry for H₂S; silver nitrate as a screen (IS 308 Annex B)

IS 309:2023 notes that measuring impurities at the 1 ppm level needs Grade 1 argon carrier gas that has itself been further purified: the analysis grade constrains the product grade.