Determination of the Density of An Unknown Liquid

Objective:

1. To determine the density of a liquid using the pycnometer method

2. To identify the unknown liquid

Introduction:

Density is an elementary physical property of matter. It is defined as the ratio of mass to volume of a substance as shown in the following:

Numerically it represents the mass per unit volume of matter. The SI unit of density is always expressed in the unit of kg m^-3 or g cm^-3. As the temperature increases, the density of a substance will be decreased, this will be discussed later in discussion. When thermal expansion occurs, the volume of the substance increases while its mass is remained. By referring to the formula given, the density of the substance decreases as the temperature increase. The density of a substance is varies with the change of temperature at surrounding.

In this experiment, the density of unknown liquid is determined by using pycnometer method. Density determination by pycnometer is a very precise method to determine the density of an unknown liquid. This experimental determination requires two important measurable quantities which are mass and volume of the selected piece of matter. The mass of the dry and empty pycnometer is represented by M and M_W be the mass of the pycnometer filled with water at t°C. The mass of water in the pycnometer at this tempearature will be determined by using mass of pycnometer, M_W minus mass of empty pycnometer, M. Let d be the density of water at t°C, so that the volume of pycnometer, V at this temperature can be expressed in the term as:

The value of d can be refers to the Table 1 given.

 

Table 1 Density of pure water at different temperatures

Temperature, °C	Density, g cm-3
18	0.99862
20	0.99823
22	0.99780
24	0.99732

Let M_L be the mass of pycnometer filled with the unknown liquid whose density is to be determined in this experiment. The mass of liquid is (M_L – M). From equation 1, we can obtain the volume of empty pycnometer. The volume of pycnometer is equals to the volume of unknown liquid occupied. Thus, once we obtain the volume and mass of unknown liquid, the density of the unknown liquid, d_L can be determined precisely. The density of unknown liquid, d_L can be derived by substituted Equation 1 into Equation 2.

Thus, the density of unknown liquid could be by using the formula as shown below:

 

A pycnometer (shown in Fig. 1) is a glass flask that has a close-fitting ground glass stopper with a capillary hole through it. This fine hole allows the release of a spare liquid after closing a top-filled pycnometer and the escape of air bubbles in the liquid. Besides, the fine hole allows for obtaining a given volume of measured and/or working liquid with a high accuracy.

Materials:

Distilled water, unknown liquid

Apparatus:

Pycnometer, weighing balance

Procedure:

1. A dry and empty pycnometer was weighed and the weight was noted.

2. The pycnometer was filled up with distilled water and the air bubbles were allowed to rise before the stopper is being inserted.

3. The filled pycnometer was leaved at room temperature for about 5 minutes.

4. The neck of pycnometer was being picked up with papers between the fingers to avoid the expansion of pycnometer due to heat of the hand.

5. Make sure the outside of pycnometer dry, the pycnometer with distilled water was weighed again.

6. The V, volume of pycnometer was found.

7. Steps 1-7 were repeated with unknown liquid.

8. The experiment was repeated once.

Results and calculation:

Experiment	First time	Second time
Weight of dry and empty pycnometer	22.9880 g	22.9873 g
Weight of pycnometer + weight of water	74.4976 g	74.4965 g
Weight of pycnometer + weight of unknown	63.6783 g	63.6709 g

The experiment was conducted at room temperature of 24°C.

Average weight of dry and empty pycnometer

= (22.9880 + 22.9873) g / 2

= 22.9877g

Average weight of pycnometer + weight of water

= (74.4976 + 74.4965) g / 2

= 74.4971g

Average weight of pycnometer + weight of unknown liquid

= (63.6783 + 63.6709) g / 2

= 63.6746g

In order to calculate the density of unknown liquid, the formula below can be used:

where D_L = density of unknown liquid

d = density of water at room temperature

= 0.99732 g cm^-3

M_L = weight of unknown liquid

= 40.6869g

M = weight of empty pycnometer

= 22.9877g

M_w = weight of water

= 51.5094g

D_L = 0.99732 g cm^-3 (63.6746g – 22.9877g) / (74.4971g – 22.9877g)

= 0.788 g cm^-3

Thus, the density of unknown liquid is 0.788 g cm^-3. By referring to the table 2 given as below, the unknown liquid is ethanol which possesses the nearest reading of density to 0.789 g cm^-3.

Table 2 Density of pure organic compounds

Liquid	Density, g cm-3
Ethanol	0.789
Methanol	0.791
Chloroform	1.490
Benzene	0.877

Precaution steps:

1. Handle the pycnometer with extra care because it is very expensive and fragile.

2. Ensure the bulb and stopper of the pycnometer are both engraved with the same number.

3. The pycnometer must be empty and dry before conduct the experiment.

4. Make sure there is no air bubbles present in the pycnometer after filling.

5. Make sure that the pycnometer is completely dry before weighing.

Sources of pigments

Usually, pigments are always being found in the animals and plants in which these pigments are known as biological pigment or biochromes. In the living organism, most of them have the biological structures which consist of pigments such as skin, hair, eyes, fur that present in animals. The most common animal pigment is refers to melanin and heme groups (haemoglobin). Melanin is naturally present in the human body especially in the skin laye while heme groups are found in the blood . Besides, leaf and flowers of plants contain their own biological pigment. Plant pigments include a variety of different kinds of molecule which includes porphyrins(chlorophyll), carotenoids, anthocyanins and betalains. Table 1 below shows the categories of plant pigment as well as their examples.

Table 1 Types of plant pigment and their examples

Plant pigments	Examples
Porphyrins/ Chlorophyll	Chlorophyll a, chlorophyll b
Carotenoids	Carotene I. Alpha carotene II. Beta carotene III. Lycopene IV. rhodopsin Xanthophylls I. canthaxanthin II. zeaxanthin III. lutein
Anthocyanin	Aurantinidin, cyaniding, luteolinidin, malvidin, peonidin, petunidin, rosinidin
Betalains	Betacyanin, betaxanthin

Both animals and plants have the pigments which can absorb and reflect sunlight at certain wavelength and hence the colours of living organism can be presented naturally as their physical appearances.

The organic and inorganic pigments are always being synthesized chemically from the reaction between organic compounds and from the reaction by using transition elements respectively. Most of them are artificially made in industry. However, some inorganic pigments occur naturally in the form of metal oxides and sulfides. For example, the natural inorganic pigment can be found in limonite, amorphous, hematite, ocher, umber, sienna and others.

Uses of pigments in living organism and non-living organism

Uses of pigments

1.In human being and animals

The natural pigment in the living organism plays a very crucial role in their daily life. Most common pigment presents in human and animal’s skin is known as melanin. Melanin is used to protect human being and animals from sunburn due to the ultraviolet radiation from sunlight. Eumelanin (one types of melanin) is the pigment which is mainly concerned with the protection of skin by absorbing the incoming ultraviolet radiation. The concentration of eumelanin will increase after more exposure of skin towards ultraviolet light. As a result, the skin becomes darker because it tends to protect the skin from damage. Some individual humans and animals have very little or lack of this pigment in their bodies is known as albinism (skin disease). In the diagram 3, the process explains the formation of melanin and the effect of lack of this pigment in the body.

Diagram 3 Formation of melanin process and the effect of lack of melanin

In addition, the biological pigment present in some of the animals especially prey are used to protect themselves from the hunting of predators. The biological pigment can imparts the colour of their physical appearances similar with the colour of surrounding environment such as rock, sand and leaf in order to make them difficult to be spotted by predators. For example, grasshopper and porcupines are using their pigment to protect themselves. This phenomenon that happens within the body living organism is known as camouflage (Diagram 4).

Diagram 4 Camouflage of grasshopper

Besides, the pigments present in animals are also used in mimicry in which one animal mimics the other one’s physical appearance in order to protect themselves from the predators. Normally, a palatable or harmless species will tend to mimic an unpalatable or harmful species. Mimicry between eastern coral snake and scarlet king snake in Carolinas is a good example to explain the importance of biological pigment in animals. Both species live in Carolinas, but the kingsnake is also found in regions without poisonous coral snakes. The predators that inherit an avoidance of coral snake’s colouration and hence they will attack less often towards the scarlet king snake in the regions where coral snake could be found.

Diagram 5 Mimicry between eastern coral snake and scarlet king snake

The biological pigments that present naturally in the animals are very important in mate selection in animals. This is because some of the female animals will choose the male animals as their partners based on the colourful appearances of the male animals. In the mate selection of peacocks, the female peacocks are always attracted by the peacocks possesses a colourful feather and tail.

Diagram 6 Peacock is wooing a peahen

2. In plants

For plant, the natural pigment is playing a very important role for their survival. Chlorophyll is the green pigment that is very essential for all the plants to carry out the photosynthesis. This is because chlorophyll can absorb the energy at certain wavelength from the sunlight in which it induces photosynthesis to takes place. Photosynthesis is a process that absorbs energy from sunlight to convert carbon dioxide and water to produce glucose and oxygen. Hence, pigment is very important for all the plants to survive.

Moreover, the presence of pigment in plants such as anthocynin(appears as red, blue or purple in plants) makes them to have colourful appearances. In order to attract pollinators, the colour of the flowers must be presentable and attractive and so that pigment is very determinative for plants. Pollination is a process of pollen transferring in plant reproduction and therefore enabling fertilization and sexual reproduction in plants. In flowers, the bright red and purple characteristic is adaptive for attracting insects or animals for pollinations. Anthocynin is also important in the fruits. This is because colourful skin of the fruits can attract the attention of animals and then the seeds could be dispersed into other area once the fruits are eaten by the animals.

Diagram 8 A European honey collects nectar while the pollen is collected by its body

Last but not least, the pigment found in the plants can act as an antioxidant in order to protect the plants from oxidation of free radical. The pigment is called carotenoids. Sunlight tends to induce the formation of free radical in the atmosphere in which they will destroy the plant structure by oxidizing the protein, DNA and photosynthesis structure of plants. The carotenoids participate in the energy transfer process in order to protect the plants from auto-oxidation. In non-photosynthesizing organisms, such as humans, carotenoids have been linked to oxidation-preventing mechanisms. For example, carotenoids tend to absorb or quench the energy of excited molecules such as singlet state oxygen and triplet state chlorophyll, thereby preventing the destruction of protein, DNA and the photosynthetic apparatus.

3. In application of painting industry and food dye

Pigment is also being widely used in the painting industry to apply on the textile, fabrics, cosmetic, plastic, paper and various materials. The pigment could be the biological pigment extracted from plants, organic pigment and inorganic pigment that synthesized chemically from metallic elements or found naturally. Pigment is one of the components of paint. Pigment serves two functions:

I. Providing colours

II. Protective function for surface underneath of paint and binder from destroyed by ultraviolet light

Pigment is a coloring material usually a finely ground powder which does not dissolve and it is suspended in a liquid solvent to become the coloring material in paint. In organic pigment, the colours are caused by the light energy absorbed by the delocalization of π electron conjugated system. However, the π electron will not absorb all the wavelength of light, therefore the unabsorbed light will be appears as the colours of paints. Inorganic pigments are being applied in paints due to their selectively wavelength absorption properties. During the formation of metal complexes, the transition of electron allows absorption of light energy at different wavelength to occur and so that inorganic pigments can also exhibit in many colours. Hence, colourful organic pigments and inorganic pigments are widely used in painting materials.

The extracted biological pigment in animals and plants can be applied in the food colourings. Some of the artificial pigment which is harmless to human body sometimes can be used in food colourings. In order to produce colourful and attractive food products, pigments are being used in the beverages, baked goods, dairy products, pet foods, and a variety of other products. Pigments can be used to increase the attractiveness and physical appearances of products and hence the products can have a higher market value. For example, the bright purplish-red pigment of the red dragon fruit is an excellent source of natural betanin.

4. In medical uses and health supplement

Nowadays, most the pigment extracted from the plants can be used in the manufacture of nutrient supplement in the current market. This is because the pigment that found in the plants such as carotenoids (beta-carotene and lycopene) can act as an anti oxidizing substance. This antioxidant can against the free radical that present in the atmosphere. Beside that, beta-carotene that found abundantly in carrot can be used to promote healthy eyesight of humans. For those people who have night blind can take this supplement in order to improve their eyesight at night.

Other that that, a derivative from chlorophyll which is named as chlorophyllin can be used in medicine for wound healing. The chlorophyllin is used to promote wound healing process and hence this can reduce the risk of patient to getting infection or inflammation which causes fatal.

Characteristics of pigments

Characteristics

Pigment is a substance that changes its colour when exposed to visible light by reflecting or absorbing different wavelength of visible light. Pigments appear as the colours they are due to their wavelength-selective absorption properties. White light is the equal mixture of the entire spectrum of visible light with a wavelength range from about 400 nanometers to about 780 nanometers. When the pigment encountered by light, parts of the visible light are absorbed by the chemical bonds of the conjugated systems and other components of the pigment. At the same time, some of the visible light at other wavelength are reflected or scattered.

Other than that, most of the pigments are exist as polar compounds. They are not soluble in water like dyes but they are tend to soluble in the oil or other polar solvent.

Pigments can be made up from a few to even more than ten benzene rings which attached with many different substituents in a single pigment molecule. Some of the pigments are made up from a porphyrin with a metallic element present in the centre. These pigments are called heme or porphyrin-based pigments. Diagram 1 shows the simplest structure of porphyrin which is one of the components in pigments. Examples of heme or porphyrin pigment are haemoglobin, myoglobin and hemocyanin.

Diagram 1 structure of porphyrin, the simplest structure of porphyrin

Furthermore, some of the biological pigments contain a long chain of hydrocarbon which attached to porphyrin-based. For example, chlorophyll is one of the examples that have a long chain hydrocarbon attached to a porphyrin-based. Diagram 2 shows the pigment structures of chlorophyll a and chlorophyll b.

Diagram 2 structure of chlorophyll

Organic pigments are considered as the substances that can imparts change of colour when expose to sunlight. This is because the presence of π electron conjugated system in organic compounds can absorb different wavelength of light to form many colourful dyes. Functional groups in organic pigment that absorb visible light is called chromophores such as -N≡N-, -C=C-, -C=O, -C=S, -C=NH, -N=O, -NO₂. For inorganic pigments, the transition of electron from lower energy state to higher energy state induces the absorption of sunlight at certain wavelength and thus the complexes show colourful compounds. Hence, organic and inorganic pigments also possess a characteristic to exhibit different colours as their appearances.