In the Laboratory
Is It Necessary To Dry Primary Standards before Analysis? Jeffrey M. Spraggins, II and Theodore R. Williams* Department of Chemistry, The College of Wooster, Wooster, OH 4469; *
[email protected] In most analytical procedures involving gravimetric or volumetric techniques, it is customary to dry primary standards or gravimetric precipitates at 105–110 ⬚C for several hours or overnight. In a few cases the chemicals are also to be kept in a desiccator after the drying process. Classic textbooks such as Kolthoff et al. (1) as well as modern books such as Qualitative Chemical Analysis by Harris (2) recommended this procedure. An example of this can be found in Lab 29-5, Preparing Standard Acid and Base in Harris’s text where he suggests drying potassium hydrogen phthalate for 1 hour at 100 ⬚C (2). Harris also suggests drying Na2EDTA⭈2H2O for 1 hour at 80 ⬚C and then cooling it in a desiccator before EDTA titration in the analysis of Ca2+ and Mg2+ in Natural Waters (2). In recent years the purity of many chemicals, including primary standards, has greatly improved. In addition, laboratory air quality has improved. In particular, the relative humidity in modern analytical laboratories is usually lower than in the past. In order to assess the water present in primary standards and other widely used chemicals, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to determine the volatile components in these materials. The goal of this study was to determine whether it is still necessary to use traditional timeconsuming drying techniques of standard substances for gravimetric and volumetric experiments. This study should provide this information. The procedures described are not for a student laboratory experiment.
the first procedure, the temperature was initially raised from 25⬚ to 110 ⬚C. After holding the temperature at 110 ⬚C for one hour, the temperature was raised to 300⬚ at a rate of 40 ⬚C per minute. The second procedure was similar to the first except that the temperature was raised to 300 ⬚C at a rate of 10 ⬚C per minute. Sample weights ranged between 5 mg and 10 mg. The sample pans were cleaned between measurement using a propane torch.
Experimental
Results
TGA Experiments A PerkinElmer Pyris 1 TGA, which was equipped with the Pyris data analysis software, was used for the thermal gravimetric procedures. Two different methods were employed to determine the weight change in the samples. In
TGA Results The TGA results (Table 1) show the percent weight loss due to water in all cases to be less than 1%. For example, TGA of Na2CO3 and KCl showed only a 0.37% and 0.086% weight loss due to volatile components, respectively. One can
DSC Experiments DSC studies were done using a PerkinElmer Pyris 1 differential scanning calorimeter with a PerkinElmer liquid nitrogen cooling system and Pyris data analysis software. Sample masses were between 5 mg and 10 mg. The method chosen included holding the sample temperature at ᎑25 ⬚C for 30 min and then raising the temperature to 25 ⬚C at 5 ⬚C per min. These experiments were done to determine whether any phase changes occurred in the temperature range studied. Chemicals and Standards Samples were exposed to laboratory air conditions (Severance Hall, The College of Wooster Chemistry Building) for extended periods of time (up to two hours) before analysis. Severance Hall is an air-conditioned building that has a relative humidity ranging from 25% to 70%. No preparations or drying procedures were done to the samples except for potassium acid phthalate, which was dried for two hours and then allowed to cool in a desiccator.
Table 1. Percent Weight Loss Determined by TGA Reagent
Supplier
Purity
Trials
Na2CO3
Weight Loss (%)
Fisher Chemical
99.95%
7
0.37% ± 0.07
KCl
Fisher Chemical
99.80%
4
0.086% ± 0.067
Pb(NO3)2
Fisher Chemical
99.60%
5
0.034% ± 0.033
Ag(NO3)2
Fisher Chemical
99.70%
4
0.047% ± 0.045
CaCO3
Fisher Chemical
99.00%
6
0.29% ± 0.15
KHC8H4O4 (KHP)
Acros Organics
99.95%
8
0.36% ± 0.42
Tris(hydroxymethylaminomethane)
Life Technologies/ GIBCO BRL
99.90%
4
0.26% ± 0.22
Na2C2O4
Baker
99.50%
5
0.064% ± 0.040
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In the Laboratory
contrast the stability for several standard materials by observing the TGA for these compounds with those of KHP. When this standard was taken from the bottle and dried at 110 ⬚C the weight loss was 0.06%; for two hours, samples, which were exposed to air for seven days had a loss of 1.7% of the initial weight. This change in weight represents the moisture that was added to the sample over this period. This is expected since this compound is known to lose water above 110 ⬚C. In the case of Na2EDTA⭈2H2O, the weight loss is due to loss of its waters of hydration at 110 ⬚C; this explains why Harris suggests drying this standard at 80 ⬚C.
DSC Results The DSC data showed that the quantity of water present in the samples analyzed was below the limit of detection of the instrument. In most cases no peak due to the phase transition of water at 0 ⬚C was observed. In all of the cases where a small peak was present, the peak was so small that it could not be analyzed. If unbound water was present, a signal around 0 ⬚C would have been observed as a result of the transition of water from solid to liquid. Discussion The results of this study include TGA data that suggest the quantity of volatile components in primary standards to be less than 1% of the initial weight and DSC data that show that water present in the same chemicals is below the limit of detection of the instrumentation. This suggests that the 1–2 hour drying period for many primary standards may not be needed before analysis. Exposing samples to laboratory air conditions overnight did not seem to affect the quantity of water in the standards. The long-employed practice of drying samples may have been necessary before laboratories were air-conditioned; however, we theorize that modern air-conditioned laboratories provide the humidity conditions necessary to maintain dry chemical standards. Another reason why drying the standards may be unneeded is the increased purity of modern chemicals. Most of the samples used had reported purities of 99.5% or better.
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In many experiments in which these standards are used, the inherent student errors due to the volumetric procedure are much more significant than those student errors that result from not drying samples. It is not uncommon to have errors that exceed 1%. The typical error in most analytical experiments, such as titration, exceeds this error. All samples would lead to errors less than 1% owing to volatile components. Conclusions The results of this study indicate that there are no significant quantities of volatile components present in the primary analytical standards tested. The volatile materials, which were measured with TGA, were generally consistent with the purity values reported by the vendor. No attempt was made to use new, unopened standards. Samples of Na2EDTA⭈2H2O lost the waters of hydration when heated above 110 ⬚C. For student analytical work the drying procedure is not often a significant factor in the results. Only when very accurate results are required should these samples be dried. We encourage chemists to test the quantity of water in standard materials before assuming that they need to be dried. Often with student analytical work the drying of the sample is not a significant factor in the results. The humidity of the laboratories varies with location. If one is uncertain as to whether or not to dry the samples, a simple test should be performed. The instructor needs only to dry the sample for several hours and note the weight change. If a TGA unit is available, one need only to heat the sample to temperatures of 120 ⬚C and check the weight change. Literature Cited 1. Kolthoff, M.; Sandell, E. B.; Meehan, E. J.; Bruckenstein, S. Quantitative Chemical Analysis, 4th ed.; The Macmillan Company: London, 1969. 2. Harris, D. C. Quantitative Chemical Analysis, 6th ed.; W. H. Freeman and Company: New York, 2003.
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