A comparison of Methods for Analysis of Potassium (K)




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huddersfield university


A Comparison of Methods for Analysis of Potassium (K)




Analytical Chemistry MSc

Instrumental Methods (SMA0002)

Dr. Peter Clemenson


Student Name: Moayad Khashoqji

Student Number: U1069214




Submit on: 3rd May 2011


A Comparison of Methods for Analysis of Potassium (K)

Contents




  1. Introduction

    1. General 3

    2. Occurrence 5

    3. Properties: Physical and Chemical 5

    4. Biological Importance 6

    5. Isotopes 6

    6. Uses of Potassium 7

  2. Levels in Different Samples 8

  3. Review of Methods for Analysis of Potassium

    1. ICP-MS 9

    2. ICP-AES 10

    3. AAS 11

    4. Tandem MS 11

    5. SEM 12

    6. AFS 12

    7. HPLC 13

    8. PIXE 13

    9. XRD 13

    10. XRF 14

  4. Recent Developments in Methods of Potassium Analysis 15

  5. Articles discussion 19

  6. Conclusion 20

  7. References 21



Introduction- General:



Symbol

Chemical Element

Atomic weight & Number
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The most common form of potassium is potash (KOH). Potassium can be derived from potash using electrolysis. This was the first method employed to obtain potassium. To prevent oxidation, potassium must be stored in a mineral oil.

After lithium, potassium has the lowest density among metals (Density (293 K): 0.862 g/cm3). It has a silvery appearance and, even though it is a metal, it is soft enough to be cut by a knife. Potassium is a vital component of cells and is essential for all living beings. It has a melting point of 63.65 0C (336.8 K, 146.57 0F) and a boiling point of 774.0 0C (1047.15 K, 1425.2 0F).

Potassium burns with a lilac coloured flame. It is extremely reactive. It reacts violently with water, for example, to produce hydrogen gas and potassium hydroxide (environmentalchemistry.com, 2011; Webelements.com, 2010; lenntech.com, 2009; mrteverett, 2011).



Atomic Structure of Potassium (K)



Potassium in Alkali metal group (S-block)


Occurrence:

Potassium was discovered in 1807 by Sir Humphrey Davy. This alkali metal is the seventh most abundant metal on the earth’s surface, roughly accounting for 2.5% of the earth’s crust. Since potassium is highly electropositive and reactive, it doesn’t occur freely in the atmosphere but in the form of its salts.

Minerals mined for their potassium are typically pinkish; some examples of the mineral mined for potassium are sylvite, carnallite, and alunite. Before the First World War, Germany had a monopoly on the mining of potassium. Today, most potassium minerals come from Canada, USA, and Chile (Bergman, 1987; chemistryexplained, 2011; periodic-table, 2005).


Physical properties:

It has a cubic body centred shape. It is a strong base and belongs to Group 1, Period 4 and Blocks of the periodic table. It has an electronegative rating of 0.82 on the Pauling Scale (chemistryexplained, 2011; periodic-table, 2005; mrteverett, 2011; Tiecke, 2010; Green, 2006).

Physical Properties

State of matter

Solid at 298k

Melting point

336.53 K (146.08 °F)

Boiling point

1032 K (1398 °F)

Molar volume

45.94 ×10-6 m3/mol

Heat of vaporization

79.87 kJ/mol

Heat of fusion

2.334 kJ/mol



Biological Importance

The most used form of potassium is potash, which is a major component of certain fertilizers. Plants take up potassium from the soil for proper growth.

Potassium is an essential mineral for the proper functioning of the human body. It accounts for 0.2% of the body weight. The average human consumes up to 5-7 grams a day, and has a store of some 110-140 grams in the human body. It assists in muscle contraction and in maintaining fluid and electrolyte balance in body cells. Potassium is also important in sending nerve impulses, as well as releasing energy from protein, fat, and carbohydrates during metabolism.

Humans need potassium for a healthy nervous system and brain function. Ashypokalemia is a fatal condition that is caused by a potassium deficiency in the body. Hyperkalemia, on the other hand, is caused by higher levels of potassium in the blood. The most effective way to ensure proper levels of potassium is by maintaining a balanced diet (pharmacorama, 2011; spectrumanalytic, 2010; gardeningfromthegroundup, 2009; buzzhealthy, 2010).


Isotopes

Potassium has over 20 known isotopes. The most stable of these is 39K, which accounts for 93.258% of all the potassium. 40K with 0.0117% abundance is unstable and decays to form 40Ar or 40Ca. This is used in carbon dating and as tracers to study weathering. The second most abundant form of potassium 41K (6.72%) is stable (chemistryexplained, 2011; mrteverett, 2011; chemistry.about, 2011; tracesciences, 2008; chemicool, 2010).



Isotope

Half Life

K-39

Stable

K-40

1.28x109 years

K-41

Stable

K-42

12.4 hours

K-43

22.3 hours















Uses of Potassium

Potassium is vital for plant growth. Plants use it, for example, to make proteins, hence the greatest demand for potassium compounds is in fertilizers. 

Potassium hydroxide is a strong alkali and an important industrial chemical. It is used in the manufacturing of soft soaps and as an electrolyte in alkaline batteries. An alloy of potassium and sodium is used as a heat transfer medium. Potassium salts have many commercial uses.

Potassium chloride is used as a healthier alternative to table salt. Toughened glass can be made by immersing glass in molten potassium nitrate. Potassium nitrate is the main explosive ingredient in gunpowder (chemistryexplained, 2011; periodic-table, 2005; education.jlab, 2010). 

Levels in Different Samples

Most potassium occurs in the Earth's crust as minerals, such as feldspars and clays. Potassium is one of six elements that make up 99% of sea salts, the others being sodium, chlorine, magnesium, calcium, and sulphur. Potassium is leached from these by weathering, which explains why there is quite a lot of this element in the sea (0.75 g/l), though it is still significantly lower than the sodium levels in sea.

Seawater contains about 400 ppm potassium. It tends to settle and, consequently, end up in sediment. Rivers generally contain about 2-3 ppm of potassium. This difference is mainly caused by a large potassium concentration in oceanic basalts. Calcium rich granite contains up to 2.5% potassium. 

Although it is soluble in water, little is lost from undisturbed soils because, as potassium is released from dead plants and animal excrements, it quickly becomes strongly bound to clay particles, and is retained ready to be reabsorbed by the roots of other plants.

Plants contain about 2% potassium (dry mass) on average, but values may vary from 0.1-6.8%. Mosquito larvae contain between 0.5 and 0.6% potassium, and beetles contain between 0.6 and 0.9% potassium (dry mass).

In the human body, potassium accounts for 0.2% of body weight. Normal levels of potassium in feces range from 5 to 10 mmol per day, per 100 to 200 mL of stool. The normal values for urine are between 25 - 120 mEq/L per 24 hours. Renal excretion is the major route by which potassium is eliminated from the body.

In food, bananas and raisins have high potassium content containing 467 mg and 363 mg respectively. Other high potassium level foods are citrus juices (236mg), cantaloupes (494 mg), yoghurt (398 mg), tomatoes (400 mg), salmon (319 mg), and other meat.

Foods containing low levels of potassium are blueberries (63mg), cucumber (88mg), egg (55 mg), cheddar cheese (28 mg), etc (mrteverett, 2011; peacecraft.tripod, 2011; livestrong, 2011).


Review of the Methods for Potassium Analysis

(Including advantages/disadvantages of each)

ICP-MS



This analytical technique is based on coupling together inductively coupled plasma as a method of producing ions (ionization) with a mass spectrometer as a method of separating and detecting the ions. The following are some advantages of this technique:

  • Detection limits for most elements are equal to or better than those obtained by Graphite Furnace Atomic Absorption Spectroscopy (GFAAS).

  • Higher throughput than GFAAS.

  • The ability to handle both simple and complex matrices with a minimum of matrix interferences due to the high-temperature of the ICP source.

  • Superior detection capability to ICP-AES with the same sample throughput.

  • The ability to obtain isotopic information.

The disadvantages of this method are that certain unstable elements require special facilities for handling the radioactive fume of the plasma. Isobaric interferences occur for equal mass isotopes of different elements. Other interferences include molecular interference due to the recombination of sample and matrix ions with Argon in the cooler regions of the plasma, and doubly-charged ion interferences, due to relatively rare doubly-charged matrix or sample ions with twice the mass of the analyte and hence the same mass/charge ratio (gclctoronto, 2011; Bradford and Cook, 1997).

ICP-AES



This test uses the inductively coupled plasma to excite atoms and ions that emit electromagnetic radiation at wavelengths that match a particular element. The intensity of this emission indicates the concentration of the element within the sample. 

The excellent sensitivity and wide working range for many elements, together with the low level of interferences, make ICP-AES a nearly ideal method, so long as the sample throughput is high enough to justify the initial capital outlay. Other advantages include high resolution, high wavelength accuracy, and the multiplex advantage (simultaneous acquisition of all wavelengths).

One major drawback to ICP-AES is that it does not identify the oxidation state of the element in its original matrix. Others are high price, noise generated by simultaneous acquisition of all wavelengths, and precision constraints (Bradford and Cook, 1997; Manning and Grow, 1997; American Chemical Society, 1998).


AAS



Atomic absorption spectroscopy is used for the qualitative and quantitative determination of chemical elements by absorbing the optical radiation by free atoms in the gaseous state. The electrons of these atoms jump to higher energy levels and, while coming to its ground state, releases energy of a wavelength that is characteristic of each element.

It is an inexpensive and fast method, though not as sensitive as some of the other procedures. Interferences are mainly from the sample matrix. Also, it can run only one element at a time (Cazes, 2004; Ebdon et al, 1998; Menzies, 1960).


Tandem MS

Tandem mass spectroscopy uses two or more mass spectrometric methods, separated by some molecular fragmentation. The mass analysers produce ions from the samples. These ions collide with the gas in the collision cell. Fragmentation occurs and the ions thus formed are of known weight and can be determined.

The advantages of this technique are analytical sensitivity, rapid results, minimum sample preparation, no gel required and detection by selecting certain ion fragments.

It is a very costly technique and requires sequence databases for analysis (JEOL, 2006; Gates, 2004).

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