Purity and purifications of solids using melting points.
Tatyana Aleksandrova
CHE 337, Section 001
Department of Chemistry
Portland State University, Portland, OR
Abstract
Melting points of Naphthalene/Biphenyl mixtures differing in their percent compositions were observed and plotted on a graph that indicated eutectic point of the mixture to be at 50 mole percent Naphthalene. Using melting point technique to identify mixture Unknown H was determined to be 3-ethoxy-4-hydroxybenzaldehyde.
Introduction
Melting point is a technique used by chemists to identify unknown substance. Compounds depending on their chemical structure have
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Based on the fact that mixture with 3-Ethoxy-4-hydroxybenzaldehyde had relatively close range of (76.1 °C - 78.1 °C) with 2.5 °C difference in ending values, which can be considered as a sharp point, unknown H is 3-Ethoxy-4-hydroxybenzaldehyde.
Conclusion
Melting point is a technique used to identify pure substances by observing ranges of melting points as it was done in mixtures of Naphthalene and Biphenyl. Eutectic point, however, should be kept in my when testing for pure substances, for eutectic compositions can mislead results of the test if nothing else is considered. Biphenyl’s on this technique unknown H tested was determined to be 3-Ethoxy-4-hydroxybenzaldehyde based on its sharp MT.
Questions:
1. a) Examples in which a pure substance could give a broad melting range:
5 mole percent of Naphthalene to 95 mole percent of Biphenyl (which is good percentage of purity) has a broad melting range.
b) Example of a situation in which an impure substance melts sharply:
Mixture of 60 mole percent of Naphthalene and 40 mole percent of Biphenyl. Mixture has eutectic composition.
2. Eutectic mixture – mixture in which proportions of its constituents allow a uniform melting point for that mixture. MP range is sharp, despite the fact that mixture is impure.
3. Effects of the impurities on melting behavior of benzoic acid: MP Benzoic Acid – 122 °C a. Fragments of crushed glass – MP of crashed glass is around 1500 °C,
I. LIQUID - Identification of an Unknown Liquid: Using the physical properties of Solubility, Density, and Boiling Point.
Melting Point Data Table Compound Aspirin Caffeine Salicylamide Actual MP (ºC) 93 - 98 260 - 262 96 - 102 Expected MP (ºC) 135 236 140 Percent Error (%) ~30% ~12% ~30%
The primary goal of this laboratory is to correctly identify an unknown substance. To achieve this task, one may use various tests that reveal both chemical and physical properties of a substance. By comparing the results of a known substance and the unknown substance, one may eliminate alternative possibilities and more accurately predict the undisclosed compound. Furthermore, by performing these tests, data can be collected and verified regarding chemical and physical properties of the unknown. Understanding the chemical properties of a known substance aids one’s understanding of the unknown based on comparative analysis of the results of the tests.
In determining the melting point range of the aspirin, a capillary tube (sealed at one end) was one-third-filled with the dried aspirin. The capillary tube and a thermometer were immersed in an oil bath. The temperature at which the solid started to melt and the temperature when the entire sample was completely liquefied were recorded as the melting point temperature range.
4- chlorobenzoic acid which was the aqueous layer has a theoretical melting point of 240-243°C, the organic layer, 4-chlorobenzyl alcohol has a theoretical melting point of 68-71°C. During our experiment we were unable to collect any data for the organic
Abstract: One mixture of two unknown liquid compounds and one mixture of two unknown solid compounds were separated, isolated, purified, and characterized by boiling point. Two liquid unknowns were separated, isolated, and purified via simple distillation. Then, the process of an acid-base extraction and washing were used to separate two unknown compounds into two crude compounds: an organic acid and a neutral organic compound. Each crude compound was purified by recrystallization, resulting in a carboxylic acid (RCO2H) and a pure organic compound (RZ). The resulting mass of the pure carboxylic acid was 1.688g with a percent recovery of 31.80%, the boiling range was 244-245 °C, and its density was 2.0879g/mL. The resulting mass of the pure organic solid was 2.4902g with a percent recovery of 46.91%, the boiling range was 52.0-53.4°C, and its density was 1.5956 g/mL.
No correction had to be made to the melting points because the standard melted in the range labeled on the bottle. The melted point observed is the correct melting point.
Discussion: As seen in the melting point determination, the average melting point range of the product was 172.2-185.3ºC. The melting points of the possible products are listed as 101ºC for o-methoxybenzoic acid, 110ºC for m- methoxybenzoic acid, and 185ºC for p- methoxybenzoic acid. As the melting point of the sample
Identifying this organic acid was an extensive task that involved several different experiments. Firstly, the melting point had to be determined. Since melting point can be determined to an almost exact degree, finding a close melting point of the specific unknown can accurately point to the identification of the acid. In this case the best melting point
To apply thermal analysis to the two-component system, naphthalene-biphenyl at atmospheric temperature. The analysis will be represented by a solid-liquid phase diagram (freezing point diagram).
CONCLUSION The goal of the experiment was to use extraction to separate and identify the components of a mixture of acid, base, and neutral. This was completed, identifying the components to be benzoic acid (36.4% yield), meta-nitroaniline (52.7% yield), and naphthalene (52.7% yield). These identities were confirmed after separation and isolation using melting point analysis. The melting point observed for the benzoic acid was 121.2-122.5ºC, the meta-nitroaniline’s was 111.5-112.8ºC, and the neutral naphthalene’s melting range measured 78.7-81.2ºC.
The melting point that was observed for N-(4-butoxyphenyl)acetamide was 109.7˚C, and is lower than the literature value range of 131˚C. (Royal Society of Chemistry. 2018). This may be due to the presence of water and that impurities may be still present in the product that was formed. Solid substances that contain soluble impurities typically melts at a lower temperature than the pure compound (Kirsop. 2017). This is because the impurity disrupts the repeating pattern of forces that holds crystalline structure of the solid. A smaller amount of energy is required to melt the part of the solid surrounding the impurity, which supports that the compound that was produced was an impure compound.
A titration was then carried out so that the molecular weight could be determined for data analysis purposes. The titration was carried out successfully and it was found that the unknown carboxylic acid used 10.2 mL of NaOH for neutralization. Thus, knowing in neutralization that the NaOH and carboxylic acid are in a 1:1 molar ratio and that the amount of carboxylic acid used was 0.142 grams, the molecular weight was calculated to 139.215 g/mol. This then allowed for the determination that the unknown must be an isomer of methylbenzene, however, 2, 3, and 4-methylbenzene all have a molecular weight of 136 g/mol. Therefore, in order to determine which specific methylbenzene the product is, the unknown melting point must be obtained, however, one was not taken in this experiment.
2-naphthol has a molecular weight of 144.17 g/mol, with a density of 1.22 g/cm3. The melting point of 2-naphthol is in the range of 121 to 123 °C. The stock solution of 2-naphthol has a concentration of 1.9844E-3 M.
Brittain, C. G. (2009). Using Melting Point to Determine Purity of Crystalline Solids. Retrieved from http://www.chm.uri.edu/mmcgregor/chm228/use_of_melting_point_apparatus.pdf