The Chinese national standard GB/T 534-2024 is the normative reference for industrial sulfuric acid sold as concentrated acid or oleum. It is not limited to total acidity; it fixes grade-dependent maximum mass fractions for iron, arsenic, lead, mercury and cadmium. A buyer that requests only 98.0% H2SO4 without the impurity table may receive a qualified-grade product that contains significantly more Fe and Pb than a premium-grade shipment. The 2024 revision supersedes GB/T 534-2014 and inserts cadmium into the required trace-metal matrix while retaining element-specific analytical routes. In practice, the seller’s certificate of analysis should identify the product grade, the declared H2SO4 or free SO3 content, and the measured values for all five specified elements.

The trace-metal content is reported as mass fraction of the element in the liquid. It is not equivalent to mg/L because the density of concentrated sulfuric acid is approximately 1.84 g/mL at 20°C. For example, 0.0001% by mass equals 1 mg/kg, not 1 mg/L. This distinction matters when a receiving specification is converted from a volumetric dosing limit to a certificate-of-analysis unit.

What Changes in GB/T 534-2024 Matter for Export Buyers?

The most visible structural change is the inclusion of cadmium as a separate tabulated element. Under the previous revision, Cd was not commonly listed in the concentrated acid table; export buyers with zinc smelter acid in the supply chain often had to request a separate Cd determination. The 2024 revision also clarifies the grade splits for Pb, Hg and Cd, so a premium-grade claim now carries more specific trace-metal obligations. Buyers should require certificates that cite the 2024 revision date rather than an undated GB/T 534 reference, because older certificate templates may omit Cd and may apply obsolete grade language.

Another change affects conformity evaluation. If any specified element exceeds the limit for the declared grade, the lot cannot be reclassified as a lower grade solely by blending with a cleaner batch unless the contract permits such adjustment. The buyer may accept a blended product if the receiving specification is explicit, but the standard’s conformity statement remains grade-specific.

Grade-Dependent Fe, As, Pb, Hg and Cd Maxima in Concentrated Acid

For concentrated sulfuric acid, the standard organizes the five specified element limits by product grade. The table below lists the maximum mass fractions in the most restrictive grade and the two broader grades. The values are expressed as percent by mass.

To convert to mg/kg, multiply the percentage by 10,000. Thus 0.005% Fe equals 50 mg/kg, 0.0001% As equals 1 mg/kg, and 0.0001% Hg equals 1 mg/kg. Oleum products carry separate free SO3 declarations; buyers should confirm whether the trace-metal limits are applied on an as-sold or acid-equivalent basis in the contract.

Iron is the highest-volume contaminant and is often the first parameter to drift out of specification. It originates from corrosion of carbon steel piping, cooler leaks, and dust carryover from sulfur handling. In a production setting, a 316L stainless steel plate cooler installed in 98% acid service can release soluble Fe when acid temperature exceeds 45°C and wall velocity rises above 1.5 m/s; field observations in such systems show Fe rising from 15 mg/kg to over 60 mg/kg within several operating weeks. Export buyers who intend to use the acid in battery electrolyte, food-grade phosphate production, or fine chemical synthesis often impose a receiving limit below the standard’s premium grade, typically in the range of 10–20 mg/kg Fe. The standard’s premium limit is therefore a conformity benchmark, not an automatic fitness-for-use guarantee.

Arsenic is controlled tightly because it is a catalyst poison in the contact process and a regulated impurity in fertilizer phosphate chemistry. The premium limit of 0.0001% As (1 mg/kg) is low, but it can be exceeded by acid produced from pyrite or from metallurgical off-gas that has not passed through dedicated As removal. Arsenic in sulfuric acid may exist as As(III) and As(V) species; reduction to arsine during sampling or analysis can create losses if the sample is not oxidized. Buyers should specify total As, not As(III), for export certificates.

Lead, mercury, and cadmium limits are relevant for different end uses. Lead affects lead-acid battery electrolyte make-down and can precipitate in phosphoric acid derivatization. Mercury is a volatile element that can contaminate off-gas scrubbers and downstream caustic or food-contact systems. Cadmium is associated with zinc smelter acid and phosphate rock; its presence at low mg/kg levels can create waste-stream complications under the receiving country’s effluent rules. The premium values of 0.005% Pb, 0.0001% Hg, and 0.0001% Cd should be read as upper limits for the declared grade, not typical production averages. Most sulfur-burning export plants produce acid below these values, but metallurgical acid plants require more analytical surveillance. Published data for Cd and Hg variance across different metallurgical acid plant configurations is limited, and site-specific monitoring is normally required in purchase agreements.

If Iron and Arsenic Limits Are Not Interchangeable in Export Specifications

A buyer cannot use the Fe result as a proxy for As, Pb, Hg or Cd. Iron is dominated by corrosion and solid carryover, while arsenic is dominated by feedstock composition and gas-cleaning performance. In a double-contact sulfur-burning acid plant, Fe may be elevated by a corroded drying-tower distributor while As remains below 0.5 mg/kg. In a metallurgical acid plant treating zinc roaster gas, the reverse pattern can occur: Fe may be low because the gas cleaning system uses wet electrostatic precipitation, but As and Cd can be present unless the gas is scrubbed and the weak acid bleed is treated. A specification that only lists Fe and total acidity is therefore insufficient for cross-border sale of acid intended for regulated applications.

Arsenic also introduces an analytical interferent in cold vapour mercury determination when stannous chloride reducing agents are used in a high-arsenic matrix. The laboratory should separate the hydride-forming elements or use atomic fluorescence with matrix-matched standards. Failure to control the interference can bias Hg results low unless a trapping or separation step is used.

Oleum and Dilute Acid Trade Under the Same Standard

Oleum shipments are covered by the same standard but require an additional free SO3 declaration. The trace-metal maxima for oleum are generally evaluated on the same impurity basis as concentrated acid, but the buyer should confirm whether the certificate reports trace elements on the oleum as-sold mass or the equivalent H2SO4 mass. This becomes material when oleum is diluted at destination: the addition of process water introduces Fe, Cd and Hg unless demineralized water is used. A demineralized water source containing 0.005 mg/L Cd and 0.010 mg/L Hg can become measurable in the diluted acid at low mg/kg levels when large dilution ratios are employed.

For concentrated 98% H2SO4, carbon steel tanks can be used, but low-metal export grades require lined storage or anodic protection. Unlined tanks that previously contained spent acid or organic residues can re-mobilize Pb and Cd from sludge layers; the first product drawn after tank cleaning may contain solid iron oxide and adsorbed metals above the premium limit. A loading line with carbon steel filters or carbon steel flexible hoses can also add Fe. Export suppliers commonly use PTFE-lined loading arms and HDPE or lined storage for premium-grade product.

Field sampling systems made from unlined carbon steel are a common source of false high Fe results. A PTFE or polypropylene sampling valve and pre-cleaned HDPE sample containers are specified for trace-metal work. If the sample is taken from a bulk tank that previously held hot 96% acid, the heel should be circulated for at least 30 minutes before sampling; otherwise, settled solids near the bottom outlet produce a non-representative result. For Hg, the sample should be stored in borosilicate glass with a PTFE-lined cap, because polyethylene can adsorb mercury over multi-day holding times. The lot is nonconforming if any specified impurity exceeds the limit in a representative composite sample under the standard’s sampling protocol.

When a receiving specification is stricter than GB/T 534-2024, the contract should name a referee laboratory and a specific analytical method. Without this, two laboratories can produce different results for the same acid because of digestion, dilution, and instrument calibration differences. The five-element panel should always include the detection limit for each element; a reported result of “<1 mg/kg” is meaningful only when the method detection limit is below the grade limit.

Analyzing Fe, As, Pb, Hg and Cd Without Overstating Precision

For Fe, 1,10-phenanthroline spectrophotometry after reduction of Fe(III) to Fe(II) is a long-established route; ICP-OES at 259.940 nm or 238.204 nm provides simultaneous multielement data when matrix-matched standards are used. For As, hydride generation atomic absorption spectrometry or atomic fluorescence spectrometry separates the analyte from the high acid matrix. For Pb and Cd, graphite furnace atomic absorption at 283.3 nm and 228.8 nm, respectively, is appropriate for low-level quantification; ICP-MS can also be used after dilution with high-purity acid and internal standardization. Mercury is best determined by cold vapour AAS or atomic fluorescence because the element is volatile and may be lost during normal ICP-OES sample introduction unless the sample handling system is closed and reducing conditions are maintained.

Method detection limits in diluted acid commonly reach 0.1 mg/kg Fe, 0.05 mg/kg As, 0.02 mg/kg Pb, 0.01 mg/kg Cd, and 0.005 mg/kg Hg depending on the dilution factor. These values are below the premium grade maxima, so a negative result is analytically meaningful only if the detection limit is reported alongside the result. Laboratories that report “0.0001%” for Hg without stating the detection limit may be quoting the specification rather than an actual measured value.

Electronic-grade and semiconductor-grade sulfuric acid are outside the scope of GB/T 534-2024. Buyers for those uses should reference SEMI C28 or equivalent high-purity specifications, where selected metal limits can be below 10 µg/kg. GB/T 534-2024 is an industrial standard and does not provide sufficient trace-metal control for sub-ppb cleaning or etching chemistries.

Export documentation under GB/T 534-2024 should be reviewed at the batch level. The certificate of analysis should state the product grade, the H2SO4 or free SO3 mass fraction, and measured values for Fe, As, Pb, Hg and Cd with their methods. If the receiving specification is stricter than the standard, the contract should define the sampling point, analytical method, acceptance criterion, and referee laboratory. Tank containers returning from non-acid service should be washed until rinse water contains less than 0.1 mg/L of each target metal before loading premium-grade acid; otherwise, residual heel contamination may be enough to push Fe or Pb out of specification after transport. These operational controls are not part of the standard, but they determine whether the delivered batch still conforms when it reaches the buyer’s tank.