Methylene Chloride DCM ACS Reagent (Analytical Grade)

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Methylene chloride, CAS 75-09-2, linear formula CH₂Cl₂, relative molecular mass 84.93 g/mol, is supplied in the ACS Reagent Grade as a clear, colorless, chlorinated aliphatic solvent with a normal boiling point of 39.8–40.0 °C at 101.325 kPa, density 1.325–1.327 g/mL at 20 °C, refractive index 1.4241–1.4245 at 20 °C, vapor pressure approximately 46.5 kPa at 20 °C, and water solubility approximately 13 g/L at 20 °C. The ACS Reagent model is not a single universal catalogue number; it denotes conformance to the current ACS Reagent Chemicals monograph for dichloromethane, requiring assay by gas chromatography at ≥99.5%, residue after evaporation ≤0.001%, water by Karl Fischer titration ≤0.02%, titratable acid ≤0.0003 meq/g, color ≤10 APHA, and a negative free-halogen test. The product is intended for analytical extraction, trace residue methods, general wet chemistry, and glassware cleaning where chlorinated-solvent purity is specified by an official method or an internal standard operating procedure. This grade is not automatically a pharmaceutical excipient, a food-grade solvent, or a low-particulate electronic cleaning fluid; each of those uses requires an additional monograph or application-specific specification.

Because dichloromethane permeates and stress-cracks many polymer classes, primary packaging is selected from amber borosilicate glass with PTFE-lined closures, and transfer lines are constructed from 316L stainless steel, PTFE, or PFA. Typical packaging configurations include 1 L, 2.5 L, and 4 L amber glass bottles, plus 20 L or 200 L steel containers for production-scale laboratories; catalogue product codes are supplier-specific and are not a substitute for lot-level certificate review. Polycarbonate, acrylic, ABS, nitrile, and flexible PVC components are incompatible for prolonged contact. Bulk receivers are typically 316L stainless steel or phenolic-lined carbon steel; the lining must be verified against the specific lot because chlorinated solvent moisture can promote acid formation and pinhole corrosion at the liquid-vapour interface. A 0.2 µm PTFE filtration step is recommended when the ACS Reagent material is routed into ICP-MS autosamplers, HPLC-UV systems, or particulate-sensitive optical cells, because the ACS monograph does not impose a particulate count.

How Does the ACS Reagent Designation Differ from Technical-Grade, HPLC-Grade, and Pesticide-Residue-Grade DCM?

Technical-grade dichloromethane is sold for industrial degreasing, polymer processing, and closed-loop synthesis; its residual acidity, water, and nonvolatile residue are not controlled to the same monograph limits, and its stabilizer package is selected for process compatibility rather than analytical detectability. HPLC/UV or spectrophotometric grades add lot-specific UV transmittance, submicron filtration, and lower particulate background; they may or may not meet the ACS monograph. Pesticide-residue and environmental-analysis grades are frequently lot-screened by GC-ECD and GC-MS after a concentration step to verify the absence of target interfering peaks; this is an application-specific selection, not part of the ACS Reagent tests. Deuterated dichloromethane for NMR spectroscopy is a chemically analogous but isotopically labelled product with separate specifications and is not interchangeable with ACS Reagent grade. The following compliance matrix lists the core ACS Reagent parameters that should be confirmed on the certificate of analysis before lot acceptance.

For UV-visible spectrophotometric work, ACS Reagent dichloromethane is normally clear across the visible and near-UV range, but the solvent absorbs strongly below approximately 233 nm. Methods that read at 220–235 nm therefore require a lot-specific UV transmittance curve or selection of a dedicated UV-grade solvent; the ACS monograph does not define a universal cut-off wavelength. Absorbance, water, and free-halogen values can drift after container opening if the headspace is exposed to humid air or light, so the bottle should be topped with dry nitrogen and stored in a ventilated solvent cabinet at 15–25 °C. Water uptake after opening is a known operational issue in high-humidity laboratories; a lot-specific water re-test is required before use in Karl Fischer or moisture-sensitive derivatization workflows. The residue limit of ≤0.001% corresponds to 10 mg/kg nonvolatile residue; for a 100 mL extraction evaporated to dryness, this is a maximum of approximately 1.3 mg total residue.

In high-performance liquid chromatography, ACS Reagent grade may be acceptable for some normal-phase or preparative applications, but it is not automatically a low-particulate HPLC grade. Dedicated HPLC/UV dichloromethane is generally filtered through 0.2 µm membranes and tested for near-UV transmittance and fluorescence background; these additional tests are outside the ACS monograph. Similarly, ACS Reagent grade is not a substitute for anhydrous dichloromethane in water-sensitive reactions or derivatizations. Anhydrous commercial DCM may specify water below 0.005% or 0.001%, whereas ACS Reagent permits ≤0.02%. Drying ACS Reagent DCM over activated molecular sieves can reduce water but may also introduce fines or concentrate the stabilizer; if a method requires water below 0.005%, a dedicated anhydrous product should be selected rather than drying the ACS grade in the laboratory.

Stabilizer Chemistry and Extract Interference in Amylene-Containing Reagent Lots

Dichloromethane is susceptible to slow oxidative degradation to hydrogen chloride and acidic chlorinated by-products when stored hot, under UV light, or in contact with certain metals; therefore many ACS Reagent lots are stabilized with 50–150 ppm of amylene, 2-methyl-2-butene. The stabilizer is a low-boiling olefin that can co-elute with low-molecular-weight hydrocarbons in headspace gas chromatography or appear as an extra peak in GC-MS scan methods if the solvent is concentrated 100× or more. For US EPA SW-846 Method 3510C separatory-funnel extractions and Method 3520C continuous liquid-liquid extractions, the analyst should run a method blank after concentrating the clean solvent through the same dry-down protocol used for samples. If the blank shows an unresolved envelope or a recognizable amylene response above the method detection limit, an inhibitor-free ACS Reagent lot should be substituted or the lot should be redistilled immediately before use. The inhibitor-free variant is more sensitive to light and air and is supplied in smaller amber bottles under inert gas; it should not be stored for extended periods after opening.

Because dichloromethane density is 1.325–1.327 g/mL, it forms the lower phase in aqueous extractions, and the organic layer is collected from the bottom stopcock of a 2 L PTFE-lined separatory funnel. Emulsions may form with high-particulate or surfactant-loaded matrices; addition of 5–10% sodium chloride or centrifugation at 1500–3000×g can be used to break the emulsion. The extract is then dried through anhydrous sodium sulfate before concentration. In Soxhlet extractions for semivolatile organics, the low boiling point allows cycling at 40 °C, but efficient condenser cooling is required to prevent solvent loss.

Concentration of DCM extracts is typically performed at a rotary evaporator bath temperature of 30–40 °C and a vacuum setpoint that maintains gentle reflux at the condenser. A chiller at −5 °C to 5 °C is required to return the low-boiling solvent; without chilled coolant, DCM vapour loads the vacuum pump and reduces recovery. Sudden bumping is a known batch-to-batch issue in high-organic extracts because the solvent surface tension is low and the boiling point is close to ambient temperature; a 40–60 rpm flask rotation with a fritted anti-bump head or controlled vacuum ramp is used to prevent sample carryover. When evaporating to dryness for gravimetric residue, wall temperature should not exceed 80 °C after the visible solvent film disappears; residual stabilizer and matrix components may decompose and bias the residue mass.

When the Target Analyte List Demands a Lot-Specific Interference Screen Before Lot Acceptance

In pesticide residue, polychlorinated biphenyl, or semivolatile organic analysis by US EPA SW-846 Method 3510C, the extraction solvent is a reagent in the analytical system, not merely a diluent. A new lot should be screened by extracting 1 L of reagent water, drying the extract through sodium sulfate, concentrating to 1 mL, and injecting into the gas chromatograph with the same column, detector, and temperature program used for samples. If the screen generates peaks above 10% of the calibration level for any target compound, the lot is rejected for that method even though it meets the ACS monograph. This lot-screening is necessary because ACS Reagent grade does not define maximum phthalate, organochlorine, or hydrocarbon interference at trace concentration. The same screen is applied in HPLC-MS/MS methods when methylene chloride is used to reconstitute or dilute nonpolar extracts, because plasticizer carryover can mimic matrix interferences at low detection limits.

In volatile organic analysis by purge-and-trap or headspace methods, methylene chloride is a common laboratory contaminant originating from extraction areas. Storing ACS Reagent DCM near a purge-and-trap autosampler, filling volatile organic analysis vials in the same hood, or using unsealed transfer lines can produce background signals that compromise method blanks and calibration integrity. Segregation from VOC instrumentation is therefore an operational control separate from the solvent’s monograph purity. This contamination risk applies equally to ACS Reagent, HPLC, and technical grades and is not resolved by higher assay alone.

Occupational exposure to methylene chloride is regulated under US OSHA 29 CFR 1910.1052 with an 8-h time-weighted permissible exposure limit of 25 ppm and a 15-min short-term exposure limit of 125 ppm; the product requires local exhaust ventilation and sealed transfer. Restricted uses include consumer paint-stripper formulations under REACH Annex XVII entry 59; this restriction does not prevent professional or industrial use where occupational controls are applied. Reactive compatibility is limited to glass, PTFE, PFA, 316L stainless steel, and phenolic-lined steel; contact with strong oxidizers, strong anhydrous bases, alkali metals, aluminum powder, or zinc dust must be avoided unless the specific process has been evaluated. The solvent is not classified as flammable under ordinary ambient conditions, but thermal decomposition can generate hydrogen chloride and chlorinated decomposition products; containers should not be exposed to open flame or high-temperature surfaces. For trace-level work, methylene chloride that has been exposed to air for extended periods should be retested for acidity, free halogens, and water before use.