Choose the method, enter titrant, sample and the volumes used in each replicate to get the mean content with standard deviation and confidence interval, and see whether the volume fell in the good range of the burette.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Content from each titration, with the mean and the 95% confidence interval of the mean.
Class A tolerance divided by volume, with the recommended 20% to 90% of capacity and the replicate volumes.
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The volume used, less the blank, times the actual titrant concentration gives moles of titrant; stoichiometry converts that to moles of analyte and molar mass to mass. Dividing by the sample amount gives the content. In the example, 12.41 mL of 0.01 M EDTA with a 0.9985 factor on 50 mL of water gives about 248 mg/L of calcium hardness as CaCO₃.
Acid-base and EDTA with metals are usually 1:1. In the Mohr method each Ag⁺ precipitates one Cl⁻. In redox it depends on electrons: 1 MnO₄⁻ oxidizes 5 Fe²⁺ and 1 Cr₂O₇²⁻ oxidizes 6 Fe²⁺. In iodometry 2 S₂O₃²⁻ react with 1 I₂, so available chlorine uses 0.5 and dissolved oxygen 0.25 mol per mol of thiosulfate.
Between 20% and 90% of capacity, preferably more than half. A Class A 25 mL burette may be off by 0.03 mL: with 12.4 mL used that is 0.24% of the result; with 3 mL, 1%. Above 90% the titrant may run out before the endpoint. Adjust the sample size to land in the range.
It subtracts titrant used by the indicator, by impurities in water and reagents and by endpoint detection. In the Mohr chloride titration the blank is often 0.2 to 0.3 mL; ignoring it overstates chloride. Run the blank with the same water and reagents as the sample.
At least three. The mean cuts random error, the coefficient of variation shows repeatability and the Grubbs test flags outliers: with 4 replicates, G above 1.481 is suspect at 5%. In the example the CV is 0.35% and no replicate is an outlier. Find the cause before discarding a result.
Total hardness adds calcium and magnesium and is titrated with EDTA at pH 10 with Eriochrome Black T. Calcium hardness is titrated at pH 12 to 13, where magnesium precipitates, with murexide or calcon. The difference is magnesium hardness. For the Langelier index, use calcium hardness.
Titrant not recently standardized, pH outside the method range, overshooting the endpoint, bubbles in the burette tip, reading off eye level and a dirty sample pipet. These errors are systematic and do not show up in replicate scatter. For results with legal or contract weight, use a lab accredited to ISO/IEC 17025 or certified under EPA rules.
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