Chemical Oxygen Demand Calculator

Theoretical oxygen demand (ThOD) for common organic compounds

Chemical Oxygen Demand Calculator: Complete Guide

Chemical oxygen demand (COD) is the amount of oxygen required to chemically oxidise all organic matter in a water sample, measured in mg O2 per litre. It is a key parameter in wastewater characterisation, treatment design and regulatory compliance. COD is determined by the dichromate oxidation method (ISO 6060, APHA Standard Method 5220) or calculated theoretically from the molecular formula of a known compound (ThOD).

Dichromate titration method

The sample is digested with potassium dichromate (K2Cr2O7) in concentrated sulfuric acid with silver sulfate catalyst at 150 deg C for 2 hours (closed reflux method). The remaining dichromate is titrated with ferrous ammonium sulfate (FAS): COD (mg/L O2) = (Vblank - Vsample) x N_FAS x 8000 / V_sample, where volumes are in mL, normality of FAS is in equivalents/L, 8000 = M_O2/4 x 1000 (converting equivalents to mg O2), and sample volume is in mL. Example: blank = 15.00 mL, sample = 8.40 mL, N_FAS = 0.1000 N, sample volume = 2.00 mL. COD = (15.00 - 8.40) x 0.1000 x 8000 / 2.00 = 2640 mg/L O2. Standard interference: chloride (oxidised by dichromate, giving false high COD -- add mercuric sulfate to complex chloride before digestion); aromatic compounds (partially resistant to dichromate oxidation -- recovery typically 95 to 98%).

Theoretical oxygen demand (ThOD) from molecular formula

For a compound CcHhOoNnSs fully oxidised to CO2, H2O, NH3 and SO4^2-: moles O2 per mole compound = c + h/4 - o/2 + s - 3n/4. ThOD (g O2/g compound) = moles O2 x 32 / M_r. Example: glucose C6H12O6. O2 moles = 6 + 3 - 3 = 6. M_r = 180.16. ThOD = 6 x 32 / 180.16 = 1.067 g O2/g glucose = 1067 mg O2/g. Measured COD/ThOD ratio is called the COD recovery -- for easily oxidisable compounds (glucose, acetate) it approaches 1.00; for pyridine and some aromatics it may be 0.90 to 0.95.

COD vs BOD and wastewater treatment

BOD5 (5-day biological oxygen demand) measures the oxygen consumed by microorganisms in 5 days at 20 deg C. COD is always greater than or equal to BOD5 because COD measures total oxidisable material (including non-biodegradable compounds). The BOD5/COD ratio indicates biodegradability: above 0.5 -- readily biodegradable (municipal sewage, food processing); 0.3 to 0.5 -- moderately biodegradable (mixed industrial); below 0.3 -- poorly biodegradable, may require physical/chemical pre-treatment. EU discharge limits: Urban Wastewater Treatment Directive -- COD less than 125 mg/L for populations above 10,000 PE; individual industrial discharge permits set by member states.

Worked example and step-by-step calculation

Worked calculations in organic chemistry rely on consistent application of atomic masses and stoichiometric relationships. The key atomic masses used throughout this suite: H = 1.008; C = 12.011; N = 14.007; O = 15.999; S = 32.06; Cl = 35.45; Br = 79.90; F = 18.998; P = 30.974; Si = 28.085. For molecular formula problems, always verify by summing atomic masses to confirm the calculated molar mass matches the experimental or given value. Round atomic masses to the precision of the given data -- if elemental analysis is reported to 0.1%, round the molar mass to the nearest whole number before deriving the molecular formula.

Common errors and connections to related tools

Frequent mistakes in organic calculations: (1) using integer atomic masses (H=1, C=12, N=14, O=16) instead of precise values -- causes errors of up to 0.5% in molar mass for large molecules; (2) forgetting that combustion of nitrogen-containing compounds produces N2, not NO2 -- nitrogen appears as N2 in the gas and must be accounted for separately; (3) confusing degree of unsaturation with number of pi bonds -- a ring counts as one degree of unsaturation, a double bond as one, a triple bond as two; (4) not accounting for halogen substitution in IHD calculations -- each halogen reduces the hydrogen count by one equivalent. This tool connects to the broader LazyTools chemistry suite: use the Combustion Reaction Calculator for balanced equations and heat of combustion; the Molarity Calculator for solution preparation from purified compounds; and the Beer-Lambert Law Calculator for absorbance-based concentration measurement of coloured organic compounds.

Applications in industry and research

Organic chemistry calculations underpin a wide range of industrial and research applications. Elemental analysis (CHN analysis) is performed by combustion of a 1 to 3 mg sample in a pure oxygen atmosphere, measurement of CO2, H2O and N2 by thermal conductivity or infrared detection, and back-calculation of C, H and N percentages. This is a routine quality control technique in synthetic chemistry, pharmaceutical API characterisation and polymer analysis. Degree of unsaturation calculations guide structure elucidation -- a DBE of 4 suggests an aromatic ring; a DBE of 2 in a C4H6 molecule suggests two double bonds or one triple bond or one ring plus one double bond. COD measurements are used in wastewater treatment to quantify the oxygen demand of effluent before and after biological treatment, with regulatory discharge limits typically set at 125 mg/L O2 for municipal wastewater in the EU (Urban Wastewater Treatment Directive) and at values set by individual permits for industrial discharges.

Step-by-step worked example

An organic chemistry student receives an unknown white solid for structural characterisation. The molecular ion appears at m/z 150 in the mass spectrum. Elemental analysis gives: C 64.00%, H 8.00%, N 0.00%, O (by difference) 28.00%. Step 1 -- find atom ratios: C = 64.00/12.011 = 5.33; H = 8.00/1.008 = 7.94; O = 28.00/15.999 = 1.75. Step 2 -- divide by the smallest ratio (1.75): C = 3.05, H = 4.54, O = 1.00. Step 3 -- multiply by 2 to clear fractions: C = 6, H = 9, O = 2. Empirical formula = C6H9O2. Step 4 -- empirical formula mass = 6 x 12.011 + 9 x 1.008 + 2 x 15.999 = 72.07 + 9.07 + 32.00 = 113.14 g/mol. Step 5 -- multiplier = 150 / 113.14 = 1.33 -- not an integer. Recalculate: try empirical C3H5O, M_emp = 57.07; 150/57.07 = 2.63. Try CH3O2, M_emp = 47.03; 150/47.03 = 3.19. Reconsider rounding -- adjust H to 4 per O unit: empirical C3H4O, M = 56.06; 150/56.06 = 2.68. Check: the molecular formula C6H8O2 (M_r = 112.13, not 150). Correct approach: use the M_r to directly calculate atom counts -- C = 0.64 x 150 / 12.011 = 7.99 ~ 8; H = 0.08 x 150 / 1.008 = 11.90 ~ 12; O = 0.28 x 150 / 15.999 = 2.63 ~ 3. Molecular formula = C8H12O3 (M_r = 152.19 -- small discrepancy, suggesting the M_r from MS may be 152, not 150). DBE = (16+2-12)/2 = 3. The formula, molar mass and DBE together constrain the possible structures significantly.

Connections to the chemistry calculation suite

Organic chemistry structure determination is an interconnected process. The Degree of Unsaturation Calculator gives DBE from the molecular formula -- this constrains whether rings, double bonds or triple bonds are present. The Combustion Analysis Calculator derives the molecular formula from elemental analysis (CHN combustion or percent composition with molar mass). The Double Bond Equivalent Calculator extends this to derive molecular formulas from percent composition data directly. The Chemical Oxygen Demand Calculator uses molecular formulas to calculate the theoretical oxygen demand, linking structural chemistry to environmental engineering. The Crude Protein Calculator converts Kjeldahl nitrogen to protein content, connecting elemental analysis to food and feed analysis. All tools in the LazyTools organic chemistry suite are linked in the related tools sections and work sequentially -- results from one tool feed directly into the next, with copy buttons to transfer results without transcription error.

Practical applications in research and industry

The calculations covered by the LazyTools organic chemistry suite are performed daily across a wide range of applications: pharmaceutical drug discovery (molecular formula verification, DBE-guided structure elucidation, purity determination); food and feed analysis (Kjeldahl protein, CHN combustion, moisture by difference); environmental monitoring (COD and BOD in wastewater, ThOD for effluent modelling); petrochemical analysis (CHN of crude fractions, theoretical oxygen demand); polymer characterisation (elemental analysis of copolymers, C:H ratios for characterisation); forensic chemistry (elemental analysis of unknowns for compound class identification); agricultural chemistry (soil organic matter nitrogen, Kjeldahl-based fertiliser nitrogen analysis). In every case, the calculation follows the same systematic approach: identify known quantities, select the appropriate formula, apply it consistently with matching units, and verify the result against independent data where possible.

Frequently asked questions