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The Purity and Purification of Solids Essay

Recrystallization is a commonly used tool in laboratories when dealing with impure solids. “An effective recrystallization is one in which 1) a high degree of purity is achieved as well as 2) a maximum recovery of sample”.1 Solubility, the tendency of a substance to dissolve in a liquid, is an important physical property of a substance. An effective recrystallization is highly dependent on differences in solubility at different temperatures. “Most, but not all, substance are more soluble at higher temperatures”. 1

Crystallization is a delicate procedure that involves the slow formation of a crystalline solid. Rapid formation of an amorphorous solid is called precipitation, which can contain many impurities. “The purest crystals are obtained when crystallization occurs slowly from an undisturbed solution”.2 In general, crystallization is the easiest process to purify a solid substance to an acceptable quality for basic laboratory work.

EXPERIMENTAL SECTION

The experimental procedures can all be found in the PSU Organic Chemistry Fall 2008 laboratory manual. 1

The ‘General Procedure for Recrystallization’ as outlined in Appendix B was used to recrystallize two samples. First, each empty collection jar was weighed and recorded. In the first recrystallization, 1.0 gram of adipic acid without the use of decoloring charcoal was recrystallized. After securing the yielded sample in one of the weighed jars, the next recrystallization was performed. As previously done, 1.0 gram of adipic acid was recrystallized. However, .10 gram of the pelltized decoloring charcoal was added to the sample. The yielded adipic acid was also collected in one of the weighed jars

Both samples were left without caps on the jars and allowed to dry for one week. The samples were weighed at which point the respective percent yields was determined. The calculation for percent yield used:

(Experimental mass yielded)

% yield= ————————————– x 100

(Theoretical mass)

The samples were then subjected to a melting point test to determine their purity.

RESULTS AND DISCUSSION

Part A- Without Charcoal

Weight of adipic acid= 1.0087 g.

Weight of jar 1 without cap= 19.0900 g.

Theoretical weight of substance and jar= 20.0987 g.

Experimental weight of substance and jar= 18.8673 g.

Weight of purified adipic acid= .7773 g.

Adipic acid (yielded) – .7773 X 100= 77%

Adipic acid (started) – 1.0087

Percent recovery- 77%

The recrystallization of adipic acid yielded a percent recovery of 77%. The substance remained a transparent blue color throughout the experiment, resulting in a light blue powder. The recovered acid was pulverized and subjected to a melting point test which indicated a melting point range of 151-152.6 °C. Compared to the known melting point of adipic acid (152 °C) 3 , the experimental adipic acid came very close indicating a fairly pure substance.

Part B- With Charcoal

Weight of adipic acid= 1.0800 g.

Weight of jar 1 without cap= 22.5000 g.

Theoretical weight of substance and jar= 23.5800 g.

Experimental weight of substance and jar= 23.0600 g.

Weight of purified adipic acid= .5600 g.

Adipic acid (yielded) – .5600 X 100= .5185 ~ 52%

Adipic acid (started) – 1.0800

Percent recovery- 52%

The recrystallization of adipic acid yielded a percent recovery of 52%. The substance began as a transparent blue color in water, however, as it continued to boil the liquid turned clear. This resulted in a white powder. The recovered acid was pulverized and subjected to a melting point test which indicated a melting point range of 152-154 °C. Compared to the known melting point of adipic acid (152 °C), the experimental adipic acid came very close indicating a fairly pure substance.

Melting point summarization-

Melting point of pure adipic acid- 151.0-152.6 °C

Mp of adipic acid (no charcoal)- 151.0-152.6 °C

Mp of adipic acid (charcoal)- 152-154 °C

Mp of impure adipic acid- 134-145 °C

CRC mp of adipic acid- 152 °C

CONCLUSION

The determination of melting points often aids in purity analysis and in the identification of substances. The narrow range of 100% naphthalene and the broadened and depressed range of the compounds mixed with biphenyl demonstrate the behavior of melting points. The experiment concluded that the eutectic point of naphthalene and biphenyl was around 40% naphthalene at 30°C. The experiment also used melting points to determine the identity of “unknown D” to be succinic acid.

REFERENCES

1PSU Organic Chemistry 337 Lab Manual, Fall 2008

2CRC Handbook, CRC Press: New York, 1999

Fessenden; Fessenden; and Feist, Organic Laboratory Techniques, 3rd Ed,
Brooks Cole:

California, 2000


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