Titration is a quantitative analytical method used to determine the unknown concentration of an analyte by reacting it with a precisely known volume and concentration of a titrant. Achieving accuracy is paramount, as the reliability of the final concentration calculation depends on minimizing systematic and random errors. Maximizing accuracy requires careful technique in preparing materials, delivering the titrant, observing the reaction’s completion, and processing the final data.
Preparation of Reagents and Glassware
The foundation of an accurate titration lies in the meticulous preparation of all solutions and volumetric glassware. Contamination or an inexact titrant concentration introduces a systematic error that compounds throughout the analysis. Volumetric glassware, such as burettes and pipettes, must be thoroughly cleaned with detergent, rinsed with tap water, and given a final rinse with deionized water. Glassware is adequately clean when the water forms a uniform, unbroken film on the interior surface, rather than beading into droplets.
Before filling the burette, it must be conditioned by rinsing it two to three times with small portions of the titrant solution. This removes residual water or solvent that would dilute the titrant and alter its concentration. It is also necessary to ensure the complete removal of air bubbles from the burette tip. A bubble released during the titration occupies space, leading to an artificially high and incorrect volume reading.
The titrant concentration must be precisely known, often accomplished through standardization. This involves titrating the titrant against a high-purity primary standard of known composition and stable concentration. Titrants like sodium hydroxide absorb carbon dioxide from the air, changing their concentration, so regular standardization is necessary. When weighing the primary standard, it must be done on an analytical balance and dried in a desiccator to prevent atmospheric moisture absorption. All reagents should be allowed to reach the ambient laboratory temperature before use, as temperature affects the volume of solutions and glassware.
Optimizing the Delivery Technique
The physical manipulation of the burette and the reaction vessel directly impacts the final result. Titrant should be added quickly at the beginning, where the reaction is far from completion, to conserve time. The flask containing the analyte must be swirled continuously throughout the addition to ensure immediate and complete mixing of the titrant with the solution.
As the reaction progresses and the color change persists for a shorter duration, the rate of titrant addition must be drastically reduced. The stopcock should be controlled with the non-dominant hand while the dominant hand continuously swirls the receiving flask. The final milliliters of titrant should be added drop-by-drop, and eventually in partial drops, to approach the equivalence point cautiously.
Adding a “split drop” involves partially opening the stopcock to allow a drop to hang from the burette tip without falling. This partial drop is transferred to the analyte solution by touching the burette tip to the inner wall of the receiving flask. After transfer, the walls of the flask should be rinsed down with solvent from a wash bottle to incorporate all the added titrant. This meticulous dropwise control prevents the overshoot of the endpoint and ensures the recorded volume is accurate.
Precise Endpoint Identification
Identifying the exact moment the titration is complete, known as the endpoint, requires careful technique. The endpoint is the point where a visible change occurs, typically a color change, and it is intended to closely match the equivalence point (where reactants are stoichiometrically equal). The most important factor influencing this accuracy is selecting an indicator whose color change range precisely brackets the pH of the equivalence point.
Visual observation of the color change is aided by placing a white sheet of paper or tile directly beneath the titration flask. This white background provides maximum contrast, making the slightest color change easily visible. Proper lighting is also necessary to prevent misinterpreting the subtle shift in hue that signifies the reaction’s completion.
For a reliable result, the analyst must titrate to the first permanent color change that persists for a minimum of 30 seconds after swirling. Titrating past this point results in over-titration and an inflated volume reading. The initial color change might be fleeting, but the first hint of a permanent, stable color is the true endpoint that should be recorded.
Minimizing Measurement and Calculation Errors
Even with perfect technique, final accuracy can be compromised by errors in reading instruments and processing data. When recording the volume dispensed, both initial and final readings must be taken at eye level to eliminate parallax error. Parallax occurs when the eye is positioned above or below the meniscus, causing the volume to be incorrectly read.
The volume reading must be taken from the bottom of the curved liquid surface (the meniscus) and estimated to the nearest hundredth of a milliliter for a typical 50 mL burette. Using a burette size that is appropriate for the expected volume of titrant consumed helps to minimize the relative error. Aiming for a titrant volume between 20 mL and 40 mL minimizes the relative error associated with the scale’s precision.
To ensure data integrity, the titration should be performed at least in triplicate until three trials yield results within a tight range of agreement. This process establishes reproducibility, and any trial falling significantly outside the range should be discarded as an outlier. The final volume used for calculation should be the average of the most consistent, acceptable trials. When reporting the final concentration, apply the rules of significant figures correctly so the result does not imply greater precision than the least precise measurement.