Basics of electroplating

Electroplating is the process of depositing a layer of metal electrolytically on to a surface.The articles to be plated make the cathode of an electrolytic cell and a rod of the plating metal makes the anode.

Normally pure metals are used. E.g. Cu, Ni, Cr, Au, Ag, Pt, Zn

But there are exceptions such as Alloys (Cu-Zn, Ni-Cr) and metal with polymers or ceramics (metal-PTEF, metal- Ceramic)

Requirements for electroplating  

-Proper bonding between the plating material and the surface.

-Evenness of the plating.

-Cleanliness

-Leak holes must not be left.

-Texture: should have high brightness.

-Resistant to chemicals in the environment that can cause damage.

Essential parts of electroplating

Difference between Mⁿ⁺ and complex ions

Complex ions release metal ions slowly. Therefore its concentration is low. It’s important to obtain a smooth ending.

Additional electrolyte can be used to increase the conductivity of the system but it will not effect the solution’s ions. Sodium sulphate is an additional electrolyte.

Additives

Additives increase the quality of the electroplating.

-Brighteners: Saccaric acid, Thiourea

-Levelers:  Formaldehydes

-stress relievers: organic substances

-wetting agents: Sodium lauryl suphate

Factors affecting the quality of the electroplating

-Nature of the electrolyte

-Concentration of the electrolyte

-Purity of the electrolyte

-Nature of the additives

-Concentration of the additives (should be low)

-pH of the solution

-Temperature

-Current density

-Geometry of the electrode(whether it’s round or flat)

-Shape of the bath

-Flow conditions (stirring is good)

Hull cell
To study the quality of electroplating dependence on the current density.

Haring-Blum cell

To determine the throwing power of the electroplating process. 

Biosynthesis and function of aromatic amino acids in plants

Most aromatic amino acids in plants are formed by three main types of aromatic acids;

•    Phenylalanine
•     Tyrosine
•     Tryptophan

These three important aromatic amino acids are exclusively synthesized by Shikimic acid pathway that is unique to plants and microbes. This pathway got its name by an important intermediate forms,called Shikimic acid.

Shikimic acid pathway starts from the condensation of Erythrose-4-phosphate with Phosphoenolpyruvate (PEP). PEP is provided by the glycolysis while Erythrose-4-P comes from either oxidative pentose phosphate pathway or Calvin cycle. Therefore Shikimic acid pathway is combined with other important metabolic pathways of the cell. 

The condensation produces 3-deoxy-D-arabinoheptulosonic acid-7-phosphate (DAHP). DAHP undergoes another series of reactions including condensation with another molecule of PEP to give out Chorismic acid. Shikimic acid forms as an intermediate in this reaction and regarded as the key intermediate.

Chorismate is a central intermediate giving rise to two products; Prephenate and Anthranillic acid. Shikimic acid pathway is shown simply as below.

The synthesis of aromatic amino acids is important as these amino acids are the precursors for the synthesis of defense and repair compounds.

Phenylalanine

·         Flavonoids: in plant pigments (eg: Anthocyanine), act against pathogens. Antioxidants.

·         Coumarins: Has appetite-suppressing properties.

·         Liginin: In lignicolous fungi

Tyrosine

·         Tocopherol: Antioxidant in cornifers.

·         Plastoquinone: Important in photosynthesis.

·         Cyanogenic glucosides: Phytoanticipants. Important in plant defense against herbivores due to bitter taste and release of toxic hydrogen cyanide upon tissue disruption.

Tryptophan

·         Alkaloides: provides protection as it prevents insects and herbivores eating the plant.

·         Plant growth regulators

Diversity of bacteria according to nutritional requirments, temperature, Oxygen conditions and water activity

Nutritional type	Energy source	Carbon source	Examples
Photoautotroph	Light	Inorganic carbon, i.e. CO2	Some purple and green bacteria (Chromatium)
Photoheterotroph	Light	Organic compounds	Some purple and green bacteria (Rhodospirillum)
Chemoautotroph (Lithotroph, Lithoautotroph)	Inorganic compounds; H2, H2S, NH3	CO2	Many Archaea and few bacteria (Nitrosomanas)
Chemoheterotroph (Heterotroph)	Organic compounds	Organic compounds	Few Arcaea and many bacteria (Pseudomonas)

Type	Minimum (0C)	Optimum (0C)	Maximum (0C)
Psychrophile	Below 0	10-15	Below 20	Contain unsaturated fatty acids in plasma membrane to tolerate.
psychrotroph	0	15-30	Above 25	Able to grow at low T but prefer moderate T
Mesophile	10-15	30-40	Below 45	Most bacteria especially the ones associated with warm-blooded animals.
Thermophile	45	45-70	Above 100	Contain Saturated fatty acids in plasma membrane. High glucose and carbon content as well as high melting point for DNA.
Hyperthermophile	80	80-115	Above 115	Contain phytane and modified proteins in plasma membrane. High glucose and carbon content as well as high melting point for DNA.

Type	Aerobic condition	Anaerobic condition
Obligate aerobe	Growth	No growth
Microaerophiles	Growth; when the O2 is at very low level	No growth
Obligate anaerobe	No growth; O2 is toxic	Growth
Facultative anaerobe/ facultative aerobe	Growth; Not essential to grow but utilized when available	Growth
Aerotolerant anaerobe	Growth; neither essential nor utilized	Growth

Type	Speciality
Halophile	Require NaCl for growth
Halo tolerant	Able to grow at moderate salt concentrations but grow best in the absence of NaCl
Osmophile	Able to grow in high levels of suger
Xerophile	Able to grow in dry conditions

Bonding in transition metal complexes:- Valence bond theory, Crystal field theory, Molecular orbital theory

There are three theories of metal to ligand bonding in complexes.

Valence bond theory

 Coordination compounds contain complex ions, in which ligands form coordinate bonds to the metal. Thus the ligand must have a lone pair of electrons, and the metal must have an empty orbital of suitable energy available for bonding. The theory considers which atomic orbitals on the metal are used for bonding. From this the shape and the stability of the complexes are predicted. The theory has two main limitations. Most transition metal complexes are coloured, but the theory provides no explanation for their electronic spectra. Further, the theory does not explain why the magnetic properties vary with temperature. For these reasons it has largely been superseded by the crystal field theory. However it is of interest for study as it shows the continuity of the development of modern ideas from Werner’s theory.

Crystal field theory

The attraction between the central metal and ligands in the complex is considered to be purely electrostatic. Thus bonding in the complex may be ion-ion attraction (between positive and negative ions such as Co³⁺ and Cl^-).  Alternatively, ion-dipole attractions may give rise to bonding (if the ligand is a neutral molecule such as NH₃ or CO).  This theory has been remarkably successful in explaining the electronic spectra and magnetism of transition metal complexes. Particularly when allowance is made for the possibility of some covalent interaction between the orbitals on the metal and ligand. When some allowance is made for covelencey, the theory is often renamed as the ligand field theory. Three types of interaction are possible. The σ overlap of orbitals, π overlap of orbitals, or dπ – pπ bonding (back bonding) due to π overlap of full d orbitals on the metal with empty p orbitals on the ligands.

Molecular orbital theory

Both covalent and ionic contributions are fully allowed for in this theory. Though this theory is the probably the most important approach to chemical bonding, it has not displaced on the other theories. This is because the quantitative calculations involved are difficult and lengthy, involving the use of extensive computer time. Much of the qualitative description can be obtained by other approaches using symmetry and group theory.

Reference: Inorganic chemistry, J.D Lee