General Chemistry: Water, Acids, Bases, Buffers and Radioactivity

Let us make an in-depth study of the general chemistry. After reading this article you will learn about: 1. Introduction to General Chemistry 2. Water 3. Acids and Bases 4. pH 5. Buffers 6. Calculating pH 7. Colloids 8. Osmosis and 9. Radioactivity.

Introduction to General Chemistry:

If we look at our body we can see that it is made up of flesh and bones. But definitely, a question comes to our mind as to what is this flesh and bones made up of? The answer to this question is given by biochemistry. In anatomy you study, as to how the different organs of our body are arranged and in physiology you study the functions of these organs, but biochemistry shows what they are made up of and how do they function.

Human body can be divided into a number of divisions or parts, the least particle being the electron, which cannot be divided further. The electron is negatively charged particle. Similarly, there is a positively charged particle called proton and neutral particle known as neutron which also cannot be divided further. The electron, proton and neutron collectively form the atom (if named in particular is called as an element).

A second question arises in our mind as to how and wherefrom did these particles come into living beings. The answer to this question from the developing science is that before the formation of the earth, a bunch of fire got separated from the sun containing, solemnly these particles, viz., electrons, protons and neutrons. On gradual cooling of this fire the electrons, protons and neutrons combined in different numbers to give rise to a substance called atom or element. Up till now 93 different kinds of elements are found in earth’s crust.

The periodic table of elements is given below:

Structure of an atom:

Where there are many numbers of electrons, protons and neutrons, there are a number of possibilities of their being combined with each other. Let us start with number one. Supposing there is one electron, you know it is negatively charged, so it will combine with a positively charged proton to form an atom. To recognize this atom it is given a particular name.

The atom or the element with one electron and one proton is known as hydrogen. Likewise an atom with a combination of two electrons, two protons and two neutrons is known as helium atom. Lithium has three numbers of each, etc. etc.

Atomic number:

The atomic number of an atom is the number of electrons in that atom (or net positive charge on that atom).

Atomic weight:

It is the mass of an atom. The elements most commonly found in our body are — hydrogen, oxygen, carbon, nitrogen, phosphorus, sulphur, etc. The final question is: Why and how do these elements form the different organs of our body? So the answer is: All the atoms except ²He, ¹⁰Ne, ¹⁸Ar, ³⁶Kr, ⁵⁴Xe and ⁸⁶Rh (inert elements/noble gases) are unstable, hence in order to attain stability they combine with each other and thereby also attain the noble gas configuration. In order to attain stability they either gain, loose or share electrons from, to or with other atoms respectively.

1. Sharing of electrons:

Hydrogen having one electron shares an electron with another hydrogen so that both of them can now have two electrons each which is a stable configuration as that of helium. The aggregation of two or more atoms is known as a molecule. In the hydrogen molecule, a force of attraction develops due to sharing of electrons, which holds the two hydrogen atoms together. This force is known as a chemical bond. A bond formed by mutual sharing of electrons is known as a covalent bond.

(a) Single bond:

If one electron from each of the sharing atoms are contributed for the bond formation, then a single bond results (C—C).

(b) Double bond:

If two electrons from each of the sharing atoms are contributed for bond formation, then a double bond is formed (C = C).

(c) Triple bond:

If three electrons are shared (C≡C).

2. Unequal sharing of electrons or coordinate bond:

Here both the electrons for sharing between the two atoms are contributed by one atom only. For example in the formation of ammonium ion (NH⁺₄) from ammonia and proton (hydrogen ion) two electrons are contributed by NH₃.

There is a second type of coordinate bond wherein the sharing electrons are pulled more towards one of the atoms. For example, in water molecule H—O—H, the electrons are more towards oxygen atom than towards hydrogen atom. Hence the bond formed due to unequal sharing of electrons is known as co-ordinate or native or semi-polar bond.

3. Transfer of electron (gain or loss of electrons):

Sodium (Na) contains an electron more than its neighbour inert gas (neon), and chlorine (CI) contains an electron less than its neighbour inert gas (Argon). Hence the Na atom donates an electron to CI to form NaCl (sodium chloride). A bond formed by the complete transfer of one or more electrons of an atom to another atom is known as ionic bond.

The following are a few important molecules found in biological systems—

i. The combination of carbon and hydrogen is known as “hydrocarbon” (—CH—).

ii. The group of carbon compounds containing an —OH are called as “alcohols” (—CHOH).

iii. The group of compounds containing ‘C’ double bond ‘O’ with one hydrogen and one carbon are called “aldehydes” (—CHO). (C = O with 2H also).

iv. The group of compounds containing ‘C’ double bond ‘O’ with two ‘C’ substitutes are called “ketones” (C—CO—C).

v. Compounds containing carboxylic group are called “carboxylic acids” (—COOH).

These molecules along with a few other elements combine to give rise to carbohydrates, proteins, lipids, nucleic acids etc. which in turn contributes to the structure and life of an organism.

Water:

Water is the major component of our body. It constitutes to about 70% of the total body mass. Most of the reactions in the cell are carried out in aqueous medium (water). Water is made up of oxygen and two hydrogen atoms. Oxygen has a tendency to pull the electrons more towards itself, thereby becoming electronegative and leaving the hydrogen’s electropositive. This results in the creation of a dipole due to which each water molecule is surrounded by four other water molecules.

The bond between ‘H’ of one water molecule and ‘O’ of the other is known as hydrogen bond. Properties:

(1) It has a high boiling point when compared to other liquids.

(2) It has a high specific heat of vaporization.

(3) High melting point.

(4) The pH of water is 7.

Specific heat of water is one calorie, it is, therefore, best suited to maintain constant temperature of the body with varying environmental temperature. The heat of vaporization of 540 cal/gram at 100 °C is helpful in maintaining the body temperature, i.e., large amount of body heat is lost with only a small amount of water being vaporized from the surface of the skin.

Acids and Bases:

An acid is a proton- donor and a base is a proton- acceptor (Bronsted-Lowry theory). Acids:

HCl → H⁺ + Cl^–

CH₃COOH → CH₃COO^– + H⁺

Weak acids are those which have a slight tendency to give up protons, e.g., acetic acid. On the other hand, strong acids give up protons readily, e.g., HCl.

Bases:

NaOH → Na⁺ + OH^–

H₂O → H⁺ + OH^–

There are strong and weak bases similar to that of acids, e.g., NaOH is a strong base which releases hydroxyl ions very easily, and water is a weak base as it is a poor source of hydroxyl ions.

pH:

pH is defined as the negative logarithm of the hydrogen ion concentration in a media.

pH = -Log₍₁₀₎[H⁺]

In simple terms it is a value that gives the amount of hydrogen ions present in a solution. This value is expressed in a reverse or negative form, i.e., higher the pH value lower is the hydrogen ion concentration and lower the pH value higher is the hydrogen ion concentration. The pH of all the solutions ranges between 0 and 14 only. pH of value 7.0 is neutral and pH ranging from 0 to 6.9 is acidic and 7.1 to 14 is basic or alkaline.

The water molecule dissociates as:

H₂O → H⁺ + OH^–

The hydrogen ion concentration in pure water was found to be 0.0000001 moles/litre.

This can also be written as 1/10000000 or 1 x 10^-7

Taking log of the above number we get = -7.

Negative logarithm of the above number = 7.

The negative logarithm of the hydrogen ion concentration is known as pH. Therefore, the pH of water is 7.

To calculate the pH of any weak dissociable acid, the following equation is derived:

This is known as the “Henderson-Hassel Balch equation”.

The normal pH of blood plasma ranges between 7.35 and 7.45, average being 7.4. The intracellular pH of the tissues is 7.25 to 7.35 averaging to 7.30 and pH of extracellular fluid is 7.30 to 7.40 with an average of 7.35. A decrease in the pH of blood is termed as acidosis and an increase in the pH of blood is termed as alkalosis. Alkalosis is more fatal than acidosis.

Buffers:

A buffer solution is one which resists the changes in pH of a solution upon the addition of small amount of acid or alkali.

Buffer solutions are a mixture of:

(a) Weak acid and its salt (or its conjugate base).

(b) Weak base and its salt (or its conjugate acid).

(c) Weak acid and weak base, e.g., weak acid ch3cooh and its base CH₃COONa.

There are two important chemical buffers that act in the biological system, they are:

1. Bicarbonate Buffer:

Contains a mixture of carbonic acid (H₂CO₃) and bicarbonate (HCO₃^–). This maintains the pH of blood and extracellular fluid.

2. Phosphate Buffer:

Consists of a mixture of HPO₄^-2 and H₂PO^–₄. It maintains a pH of 6.86, hence it is more active intracellulary. In addition to these two chemical buffers, the human body has proteins (albumin, haemoglobin, etc.) that maintain the pH of the biological system.

Calculating pH:

Using Hydrogen Ions to Calculate pH:

Calculate the concentration of hydrogen (H⁺) ions by dividing the molecules of hydrogen ions by the volume, in liters, of the solution. Take the negative log of this number. The result should be between zero and 14, and this is the pH. For example, if the hydrogen concentration is 0.01, the negative log is 2 i.e. the pH. For a 0.1 M solution of an acid: 0.1 M = 10^-1 M. The negative log of 10^-1 = 1 i.e. the pH.

pH + pOH = 14. pOH is log of [OH^–], or the negative log of the hydroxide ion concentration. If you were told that the pOH was 9.3 and asked to calculate the pH you should 14 – 9.3 = 4.7. The pH is 4.7.

Question 1:

Calculate the pH of 6.9 x 10^-4 M HNO₃

Answer:

There is only one mol of [H⁺] and the hydrogen ion concentration given is 6.9 X 10^-4. To find the pH take the antilog of [H⁺] = .9984124777 i.e. rounded pH is 1.0

Using Hydroxide Concentration to Calculate pH:

The pH of a solution can also be determined by finding the pOH. Determine the concentration of the hydroxide ions by dividing the molecules of hydroxide by the volume of the solution. Take the negative log of the concentration to get the pOH. Then subtract this number from 14 to get the pH. For example, if the OH^– concentration of a solution is 0.00001, take the negative log of 0.00001 which will be 5. This is the pOH. Subtract 5 from 14 and you get 9. This is the pH.

Question 2:

Calculate the pH of 4.5 x 10^-4 M Ba(OH)₂

Answer:

There are two mols of OH^– and the concentration of each mol is 4.5 x 10^-4. So multiply the concentration by 2 and we get = 9 x 10^-4. The value of OH^– is now 9 x 1 0^-4, Take the -log of OH- to obtain the pOH. -log(9 x 10^-4) = 3.045757491

The value of pOH is 3.045757491

To Get the value of pH by setting pH and pOH equal to 14

14 = 3.045757491 + pH

14 – 3.045757491 = pH

10.95424251 = pH

pH is 10.95424251

Calculation of pH of a Buffer Solution:

Write the chemical reaction for the dissociation of your buffer solution. Determine the dissociation constant for the relevant acid or base. The dissociation constant is the ratio of dissociated ions to initial compound present at equilibrium, and is represented as Ka for an acid or Kb for a base.

Take the negative log (base 10) of your dissociation constant. If the solution is at equilibrium, this is the pH of your buffer solution. For example, hypobromous acid has a dissociation constant of 2 X 10^-9. log(2 x 10^-9) ~= -8.699, so the pH of a hypobromous acid buffer would be approximately 8.699.

Calculating pH Using a Calculator:

If you are told to find the pH of an acid whose concentration is 0.0045 M, you should type in 0.0045 then hit the “log” button, then the +/- button. You will get 2.3 for the pH of the acid. If you are told to find the Molarity (concentration) of an acid whose pH is 6.5, then type in 6.5 then the +/- button, and then do the anti-log. On most calculators it is just hitting the 2^nd button then the “log” button. Your answer will be: 3.16 x 10^-7 M.

Colloids:

A colloidal solution is one that contains the solute particle of the size of 1 millimicron to 200 millimicron.

Tyndall Phenomenon:

When light passes through a colloidal solution it looks like a milky yellow solution due to scattering of light by the colloidal particles, this is known as Tyndall effect.

There are two types of colloidal solutions:

(1) Suspensoids or lyophobic and

(2) Emulsoids or lyophilic.

Suspensoids:

Contain particles of insoluble substances like metals and some inorganic salts.

Emulsoids:

Are the solutions of proteins, carbohydrates, etc.

Measurement of Particle Size:

The size of the colloidal particles can be measured by any of the following methods:

1. Measurement of sedimentation constants.

2. X-ray analysis.

3. If the particle is a single molecule of the substance then its size is given by its molecular weight, e.g., Proteins.

Osmosis:

If two solutions of different concentration are separated by a semi-permeable membrane then the solute particles (permeable) move from the solution of higher concentration to that of the lower concentration or/and the solvent moves from the solution of lower concentration to the higher concentration, this phenomenon is known as osmosis. The pressure that should be applied to prevent the osmosis is known as osmotic pressure. The osmotic pressure of the blood plasma is termed as oncotic pressure.

Transport Across Membranes:

Most of the substances are transported across the membranes in the intestine or other parts of the body due to osmotic difference. However, the biological membranes are impermeable to most molecules that help in retaining ionized metabolites within the cell and prevent them from diffusing out.

Cells require nutrients for their activity, so their intake and output are sometimes against the gradient (osmosis) and are mediated by specific active transport systems. The entry of some substance like ATP into organelles in the cell (like mitochondria) also requires active transport. It is called mediated or facilitated transport.

Protein molecules in the membrane play a crucial role in the process of transport. The specific protein serving the role of transport is called transport system, carriers or translocases. Mediated or facilitated transport may be active or passive. The transport increases with the concentration of substrate till a certain level after which the capacity of carrier molecule becomes saturated and no further increase in transport occurs.

1. In active transport the substance is usually transported against the concentration gradient.

2. The transportation requires expenditure of energy usually breakdown of ATP.

3. The transport is unidirectional. e.g., in the erythrocytes, sodium ion is transported out of the cell and potassium ion into the cell.

Mechanism of Transport:

Specific carrier protein (P) contains specific sites for specific substance (S). ‘P’ is positioned on one side of the membrane that can take up ‘S’ to form ‘P-S’ complex. ‘P’ diffuses across the membrane to the other surface or undergoes rotation or conformation change, so that the binding site faces the other side of the membrane. The ‘S’ gets discharged. This is passive transport and occurs in either direction, depending on the concentration of the substance on either side of the membrane.

Radioactivity:

Certain atomic nuclei are less stable, and in order to achieve stability they emit radiations and hence are said to be radioactive. The radiations are due to throwing off a stream of electrons. In order to attain stability the nucleus gives of protons and neutrons which ultimately results in the loss of electrons. This process of disintegration of nucleus is known as radioactive decay. Due to the loss of electrons the atomic number of the element is changing, i.e., it is being converted to entirely a new element. This process of conversion of an element to another is known as transmutation.

There are three types of radiations:

(1) Alpha rays

(2) Beta rays, and

(3) Gamma rays.

1. Alfa rays:

They are composed of 2 protons and 2 neutrons (helium nucleus). Uranium, a radioactive element, gives off alpha rays.

₂₃₈U⁹² → ₂₃₂Th⁹⁰ + ₄He²

Thorium is again unstable and hence it also gives off radiations and gets converted to another element and finally a stable element lead (₂₀₇Pb⁸²) is formed.

2. Beta rays:

They are composed of electrons.

3. Gamma rays:

Almost similar to X-rays.

4. Cosmic rays:

These are the fourth type of rays which originates from the sun and most of which is prevented from entering the earth’s atmosphere by the ozone layer (O₃). Some of the cosmic rays that escapes the ozone layer and enters the earth’s atmosphere is disintegrated by the gases to electrons, protons, neutrons, positrons and the nucleus.

Unit of radioactivity:

The unit of radioactivity is curie.

1 curie = 3.7 x 10¹⁰ disintegrations/sec

Half-life:

It is the time required for a radioactive element to reduce half of its radioactivity.

U = 1700 years and C = 5570 years.

Detection and Measurement of Radiations:

1. Scintillation counter:

It consists of a plate made up of fine sulfide phosphor, when radiations hit this plate it will give a tiny flash of light which is magnified and counted by a suitable device.

2. Geiger counter:

A tube called Geiger-Muller tube is used to detect radiations. This tube is filled with a gas which on radiation gets ionized and the ions are conducted through a wire to a detector.

Artificial Radioactivity:

Radioactivity can be induced artificially by bombarding neutral elements with radiations.

₁₄N⁷ + ₄He² → ₁₇O⁸ + ₁H¹ (from uranium) (radioactive oxygen isotope)

₁₁B⁵ + ₄He² → ₁₃C⁶ (carbon isotope)

₁₄N⁷ → ₁₄C⁶ (radioactive isotope of carbon, β-particle elimination)

Uses:

(1) ₁₃₁I⁵³ (normal is ₁₂₇I⁵³) iodine 131 is radioactive and it is used to detect the physiology of thyroid gland. A known amount of NaI₁₃₁ in urine is measured. If it is nearing the amount given, then it can be conferred that the thyroid gland is not functioning properly, and if the amount of ¹³¹I in urine is far less than the amount provided, then the thyroid gland function is normal. Due to its normal functioning, the thyroid gland has absorbed maximum iodine and thereby a lesser amount of iodine is excreted in the urine. In some cases one of the lobes of the thyroid gland may be normal but the other may be abnormal, in such cases the urine analysis for iodine may not serve the purpose. The best way to measure thyroid ¹³¹I uptake is by scintillation counter.

(2) Radioactive ³²P (normal is ³¹P) is used to detect cancerous cells. Cancer cells take more phosphorous. Hence, radioactive phosphate is injected intravenously and after some time a photosensitive plate (or emulsion) is kept in close contact with the tissue. If the tissue is cancerous a dark spot is observed.

(3) In order to assess the normal function (absorption) of the gastro-intestinal tract, radioactive fatty acids, iron (⁵⁹Fe), and vitamin B₁₂ (Co^{57, 58 & 60}) are given orally and blood samples are analysed at regular intervals.

(4) They are also used to trace the metabolic intermediates. ¹⁴C is used to trace the metabolic intermediates in the metabolism of food stuffs. This helps in identifying the position of the carbon during the metabolism of that food stuff. Recent study on the metabolism of food stuffs using ¹⁴C is for the detection of genetically defective enzymes.

The use of radioactive substances is not restricted only to make diagnosis but it is also used to treat certain diseases like hyperthyroidism, cancers, certain cardiac diseases and pulmonary diseases. It is also used for the pasteurization of milk and food.

Harmful Effects:

Radiations stops DNA synthesis, acts as un-coupler of oxidative phosphorylation, reduces ATP synthesis and cytochromes, causes pyknosis, vacuolization of cells, giant cell formation, mitotic delay, chromosomal breaks and altered permeability is also observed.