Reliability of Volumetric Analysis
Wet-test laboratory methods include gravimetric (by weight) and volumetric (solution-based) determinations. Of the two methods, gravimetric analysis is the more accurate. In many instances, modern instrumental methods have supplanted both wet-test methods.
-
Gravimetric Technique and Error
-
A gravimetric procedure may involve adding a slight excess of reagent to a solution containing a known substance of unknown concentration, resulting in formation of a precipitate, which is then collected, washed, dried, and accurately weighed.
Most laboratory scales weigh to the ten-thousandth of a gram. Weighings are most frequently less than a gram.
On the assumption that a gram is weighed and that the balance is accurate to +/-0.0001 (0.0002 total), the error is,
0.0002/1.0000 x 100% = +/-0.02% error.
Of course that error is increased if the weight is less than a gram. Some additional error may also be introduced by poor laboratory technique, such as incompletely gathering the precipitate, failure to thoroughly wash it. Drying may be incomplete, resulting in too high a weight; however, with care, great accuracy is maintained.
Volumetric Technique and Error
-
Volumetric technique includes the weighing of a solid, with its error, that is then put into solution using a volumetric flask, which introduces additional error. One famous commercial provider of certified sodium hydroxide solution attests it is accurate to +/-0.50%.
The solution is then transferred to a burette for titration. That will introduce yet more error on the order of +/-0.30%. Before and after the titration is carried out, the burette should be read with care, keeping the eye at the meniscus level (curved top of the liquid) in the burette, reading the liquid level from the bottom of the meniscus.
Volumetric Reliability
-
Despite the somewhat higher introduced error than is experience with gravimetric analysis, volumetric analysis offers relatively high accuracy, producing satisfactory results for most purposes. Where greater accuracy is required, gravimetric analysis, if practical, may be employed.
Newer Analytical Methods
-
There are analytical methods other than these two historic techniques. GLC (gas-liquid chromatography), HPLC (high-performance liquid chromatography), TLC (thin-layer chromatography), and spectrophotometry are accurate and rapid instrumental methods well suited for analytical determinations. Initial investment cost is higher for these methods.
In Conclusion
-
Which method is to be used depends on accuracy required, speed needed for results and available equipment. In addition, the best analytical technique doesn't make up for poor technician performance.
-
Related Searches
References
Resources
- Photo Credit Image by Flickr.com, courtesy of Hey Paul
