Essay: Determination of Sodium & Potassium using Flame Photometry Detection

Abstract:

Among the possible methods for the determination of elements is flame photometry which is a type of atomic spectroscopy whereby a sample is introduced into a flame and undergoes various processes, this leads to the formation of excited atomic species that emit radiation. This method of analysis is suitable for both the qualitative and quantitative determination of metals such as sodium and potassium which are easily excited to higher energy levels. In this study, the contents of an oral rehydration sachet were used to make a 1/50 dilution and standard solutions for sodium and potassium were prepared in order to obtain unknown concentrations of a diluted and undiluted solution. The flame photometry instrument determined the concentrations of the standard solutions and this data was used to produce calibration curves demonstrating the flame photometer readings against the concentration for sodium and potassium. Although the diluted concentration obtained showed a ratio of 3:1 for sodium and potassium which correlated to that of the oral rehydration sachet manufacturers specification, it is clear that an error has occurred throughout the study as the undiluted solutions produced a lower concentration than the diluted solutions. Despite these skewed results, it was not possible to determine the specific source of error. While this method successfully determined the presence of sodium and potassium, this study looks into the limitations of flame photometry detection and what can be implemented to avoid inaccurate results, as well as alternative techniques such as the ion-selective electrodes method which can be used to determine elements. Comparative studies on these analytical techniques are also looked at to gain an understanding of the suitability of different methods in various applications.

Keywords: Flame Photometry; Analysis; Sodium; Potassium; Concentration; Calibration Curve; Analytical Technique; Ion-Selective Electrodes

Introduction:

The Alkali Metals and the Alkaline Earth Metals

Group 1 alkali metals display trends as the atomic mass increases and their reaction with water becomes increasingly rapid as the atomic weight of the alkali increases (Halka and Nordstrom, 2010: 1). Differences in behaviours of the alkali metals are often due to variations in sizes of ions and different heats of hydration (Halka and Nordstrom, 2010: 1).

Table 1: Melting and boiling points of alkali metals

Element Melting Point (°C) Boiling Point (°C)

Li 181 1342
Na 98 883
K 63 760
Rb 39 687
Cs 29 667
(Jain, 2009)

The melting and boiling points of alkali metals decrease down the group (table. 1) due to a decrease in binding energy (Jain, 2009). These metals also have very low densities which means they are light, considering their size. For example; lithium, sodium and potassium have such low densities, they float on water (Lew, 2010: 8).

The alkaline earth metals form group 2 of the periodic table. These metals are the second most electropositive metals of all the elements, with the alkali metals being the most. (Ropp, 2012). These metals have 2 electrons in their outer valence shell and are silver-coloured, soft metals which react readily with halogens to produce ionic salts. They do react with water but not as rapidly as the alkali’s, to form strongly alkaline hydroxides (Ropp, 2012).

As a result of their low ionization enthalpies and high electropositive character, these metals tend to lose valence electrons (Verma, Khanna and Kapila, 2010).

In comparison to the alkali metals, these elements are smaller and have a more closely packed structure, causing them to have higher melting and boiling points (Verma, Khanna and Kapila, 2010). They are also denser and harder than the alkali’s, due to them being held by stronger metallic bonds. This means they are more tightly packed in their crystal lattices, accounting for a higher density and increased hardness (Verma, Khanna and Kapila, 2010).

Sodium and potassium are macrominerals required by the human body in amounts of 100mg per day. These major minerals typically occur in the body as positive ions or cations, mineral salts that dissolve within the body help regulate fluid balance, osmotic pressure and acid-base balance (Rogers, 2013: 44).

Potassium is a vital constituent of cellular fluid and storage within cells depends on the maintenance of ratio with calcium and sodium. This element is essential for muscle and nerve responsiveness, heart rhythm and especially intracellular fluid pressure and balance (Rogers, 2013: 109). A lack of potassium in the body can cause exaggeration of the effects of sodium in decreases and increases of normal metabolic activity (Rogers, 2013: 110). The sodium and potassium content in the average man represent approximately 1.4g/kg and 2.0g/kg, respectively (Crichton, 2012: 178). Thus, they are among the most important metal ions regarding concentration.

Within the environment, potassium is among the 5 most copious elements present in the earths crust, with its relative content being close to 29×103 (2.9%). This is just above that of sodium’s which is 2.6% (Sigel, Sigel and Sigel, 2016: 22). The majority of potassium ions in soils are mostly compromised of potassium-bearing primary minerals such as feldspar or muscovite. The concentration of potassium in soil varies highly with typical concentrations falling within the range of 10-5 to 10-3 (Sigel, Sigel and Sigel, 2016: 22).

A build up of sodium or potassium within plant cells can lower water potential, leading to water entry into the cell. But during adverse conditions such as drought, this can be beneficial as potassium accumulation in plants allows for an improved osmotic adjustment (Sigel, Sigel and Sigel, 2016: 22). In the case of excess sodium however, maintaining high internal potassium concentrations is a key determinant of salt tolerance by helping to retain water and reduce sodium uptake (Sigel, Sigel and Sigel, 2016: 22).

Analytical Techniques and Methods

An analytical technique is a chemical or physical principle that can be used to study an analyte whereas a method is the application of technique for a specific analyte such as flame photometry (Harvey, 2016). This is a traditional method for determining sodium and potassium in biological fluids but there are several other alternative techniques.

Atomic absorption spectrophotometry is another method for determining the concentration of elements such as sodium and potassium. This technique is based on measuring the absorption of radiation in atomic vapour formed by a sample at a wavelength that is characteristic of the inspected element (Elwell et al., 2013: 3). With atomic absorption methods, the main purpose of the flame is to produce atomic vapour of the element. Whereas in emission flame methods, excitation of the atoms is an additional requirement (Elwell et al., 2013: 57). In emission, the outer mantle of the flame absorbs some of the emitted radiation from the hotter centre and this causes a curved calibration graph. However, in atomic absorption, this increased absorption is an advantage (Elwell et al., 2013: 57).

Another analytical technique for the determination of the alkali metals is the ion-selective electrode method, which is now largely used in place of flame photometry as it is more suitable for automation (Elwell et al., 2013: 58).

Flame Photometry Detection

Emission flame photometry was used to determine sodium and potassium in this experiment. This method of analysis is based on the principle that when an alkali metal salt is drawn into a non-luminous flame, it absorbs energy from the flame and emits light of a characteristic wavelength due to the excited atoms decaying to the unexcited ground state (Thangaraj, 2015). The emitted light is detected by a photocell and is converted into a voltage. Sodium and potassium emit light at different wavelengths, so the emission from these elements can be measured using the appropriate coloured filter (Thangaraj, 2015).

When using this instrument, the flame colour can be used to detect the present element. In group 1, lithium, sodium and potassium display flame colours of red, orange and lilac, respectively. The group 2 elements calcium, strontium and barium cause a brick red, crimson red and green colour flame, respectively (Beavon and Jarvis, 2003: 64).

Figure 1: Flame Photometry Detector (McMahon, 2008: 71)

Within the pharmaceutical industry, flame photometry detection can be used to measure the major cations sodium and potassium in infusion solutions such as sodium chloride solution. In the production of blood collection tubes, the end products can be checked for proper chemical composition by means of flame photometry (Kruess, 2018).

In environmental analysis, this method can also be applied for the determination of alkali and alkaline earth metals. For example, the control of discharged wastewater for potassium, sodium and magnesium can be monitored using flame photometry (Kruess, 2018).

The aim of this practical was to successfully determine sodium and potassium using flame photometry detection and to analyse the importance of quality controls, quality assurance and good laboratory practice within the chemical industry.

Method:

The contents of an oral rehydration sachet (fig. 2) were dissolved in 200ml distilled water and a further 1/50 dilution was made in a 50ml volumetric flask.

Figure 2: Oral Rehydration Sachet

• Calibration standards for sodium and potassium were prepared. For sodium; 10ml, 8ml, 4ml, 2ml, 1ml and 0.5ml of 5mM sodium solution was added to 0ml, 2ml, 6ml, 8ml, 9ml and 9.5ml distilled water, respectively. For potassium; 10ml, 7.5ml, 5ml, 2.5ml, 1ml and 0.5 ml of 2mM potassium solution was added to 0ml, 2.5ml, 5ml, 7.5ml, 9ml and 9.5ml distilled water, respectively.
• After selecting the sodium filter on the instrument, the blank control was set to 000 while distilled water was being aspirated.
• The highest sodium standard was then aspirated for 20 seconds and the fine control was set to 100. Colour change of the flame was observed at this stage. Distilled water was aspirated again and the blank control set to 000. This was repeated until the blank reading stabilised to 000 (+/- 002) and the high standard was 98-102.
• Each sodium calibration standard was then aspirated for 20 seconds, beginning at the lowest standard and the unknown solution was then also tested.
• The filter was changed to potassium and the same steps were carried out using the potassium standards. Calibration curves were plotted for both ions to determine the unknown concentrations.

Health & Safety:

A laboratory coat, safety glasses and gloves were worn. Care was taken when using the flame photometry detector so as not to lean over the chimney of the instrument causing risk of being burnt or set alight.

As sodium and potassium chloride are both slightly hazardous in case of skin and eye contact (irritant), ingestion and inhalation, care was taken when handling the solutions (ScienceLab MSDS, 2005).

Results:

Figure 3: Flame Photometry Detection (FPD) Concentration of Sodium

Figure 3 demonstrates flame photometer readings plotted against the sodium concentrations which made it possible to determine the diluted and undiluted concentration of sodium from the oral rehydration sachet.

Figure 4: Flame Photometry Detection (FPD) Concentration of Potassium

Figure 4 demonstrates flame photometer readings plotted against the potassium concentrations which made it possible to determine the diluted and undiluted concentration of potassium from the oral rehydration sachet.

Table 2: Results for Oral Rehydration Sachet

FPD Reading Diluted Concentration (mM) Undiluted Concentration (mM)
Sodium Ion 50 2.135 0.534
Potassium Ion 42 0.703 0.176

Table 2 shows the results of the oral rehydration sachet which were analysed in order to determine the diluted and undiluted concentrations of sodium and potassium.

Discussion:

This practical focused on the application of flame photometry detection in the determination of sodium and potassium and the presence of these elements were identified by observing an orange coloured flame for sodium and a purple coloured flame for potassium.

The calibration curves (fig. 3 and fig. 4) were obtained by analysing the standard solutions which contained known concentrations for both elements. The curves were then used to calculate the diluted and undiluted concentrations for both sodium and potassium.

As shown in the results, the final diluted concentration for sodium was obtained as 2.135mM and the undiluted 0.534mM. For potassium, the diluted concentration was 0.703mM and the undiluted 0.176mM. Clearly, there is a discrepancy in these results as an undiluted solution should have a higher concentration than a diluted solution.

The manufacturer specified 12mM of sodium and 4mM of potassium within the contents of the oral rehydration sachet (fig. 2) in 200ml of a prepared solution. Considering this, the ratio of sodium to potassium should have been 3:1. The ratio obtained from the diluted concentrations of sodium and potassium was 3:1 (2.135mM:0.703mM) and for the undiluted solution the concentration ratio was also 3:1 (0.534mM:0.176mM).

Although the ratio is accurate compared to that of the sachet, it does not account for the fact the diluted solutions showed a higher concentration for both elements.

It is possible that the error in these results is due to either the procedure being carried out incorrectly or an untrue concentration of sodium and potassium being printed by the manufacturer. However, it is not possible to determine the true cause of error in this case and the results are therefore inconclusive.

A further discrepancy worth nothing is that the information displayed in figure 3 does not correlate to the diluted concentration of sodium that was calculated using Y = MX + C. By looking at figure 3, it is visible that at an FPD reading of 50, the sodium concentration is approximately 1.8mM and does not exceed the 2mM mark. This contradicts the 2.135mM concentration that was calculated.

Although flame photometry is a routinely used reference method used in biochemistry that can provide simple analysis, there are other alternatives that may be used to obtain more accurate results such as ion-selective electrodes. This is a method now commonly used in place of flame photometry and a comparison study carried out in 2011 by Subramanian et al analyses both of these reference methods used to measure electrolytes. Flame photometry has certain drawbacks such as low throughput and the requirement of manual operation which may make it an undesirable method of analysis (Subramanian et al., 2011).

Another limitation of this technique is ionisation interferences, whereby the high temperature flame may cause ionisation of some metal atoms (Sudha, 2012: 264). For example, with sodium it can be shown as Na  Na+ + e- where the sodium ion holds its own emission spectrum with frequencies different to those of atomic spectrum of the sodium atom and this reduces the radiant power of atomic emission (Sudha, 2012: 264).

Ion-selective electrodes are a significant class of chemical sensors that have important uses in a number of routine applications used in analysis today. One key factor in their development was their implication in automated clinical analysers for the high throughput determination of electrolytes in physiological samples (Subramanian et al., 2011). Although similar results were achieved in this comparative study, the ion-selective electrode technique compared to that of flame photometry is one that can decrease time consumption and provide rapid results (Subramanian et al., 2011).

In a separate correlation study by Worth, these two methods were assessed for precision and time requirement and the study revealed that flame photometry would be most suitable for smaller scale hospitals whereas the ion-selective electrode technique would suffice for larger hospitals (Worth, 1985).

Further studies may be improved by ensuring the flame photometer is correctly calibrated to avoid inaccurate readings which may have been a source of error in this case. Another factor to consider here is the sensitivity of potassium which can easily cause a spike in results if the settings are used incorrectly, so it is vital that the procedure is carefully followed. Additionally, correct pipetting technique is important when carrying out chemical analysis in order to obtain accuracy.

Conclusion:

In conclusion, the aim of this experiment was achieved by using flame photometry technique to determine sodium and potassium. By comparing this technique to other studies, it was found that flame photometry has some limitations and other methods such as ion selective electrodes may be more desirable in obtaining accurate results.

While there were some discrepancies in the results relating to the concentrations of the diluted and undiluted elements, this method still allowed for the successful determination of sodium and potassium. Additionally, a positive comparison was still achieved where the 3:1 ratio of sodium to potassium stated on the oral rehydration sachet matched the 3:1 ratio obtained in the practical.

References:

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Appendix 1: FPD Concentration Readings

This table contains the raw data that was used to plot the calibration curves (fig.3 and fig.4) for concentrations of sodium and potassium.
Sodium (mM) 5.0 4.0 2.0 1.0 0.5 0.25
FPD reading 100 88 57 34 18 10
Potassium (mM) 2.0 1.5 1.0 0.5 0.2 0.1
FPD reading 100 85 65 38 16 9

Appendix 2: Sodium and Potassium Concentration Calculations

Sodium:

To firstly calculate the diluted sodium concentration, the equation Y = MX + C was used. This equation was rearranged as X = Y – C / M in order to find the missing X value. The Y value was the figure obtained for the FPD reading and the M & C values were taken from the graph equations (fig. 3). The calculation was made as follows:

Y = MX + C
Y = 19.59(X) + 8.1752
X = 50 – 8.1752 / 19.59
X = 2.135mM

To then calculate the undiluted sodium solution, the equation C1V1 = C2V2 was used. As 200ml was the total volume of solution used, this value was inputted for V1. The value for C2 was the diluted concentration previously calculated and V2 was 50ml as a 1/50 dilution was made of the redissolved sachet. The calculation was made as follows:

C1V1 = C2V2
C1(200) = 2.135(50)
C1 = 2.135(50) / 200
C1 = 0.534mM

Potassium:

The same steps were carried out to calculate the diluted and undiluted potassium concentrations. The calculation for the diluted potassium concentration is as follows:

Y = MX + C
Y = 50.497(X) + 6.481
X = 42 – 6.481 / 50.497
X = 0.703mM

The calculation for the undiluted potassium concentration is as follows:

C1V1 = C2V2
C1(200) = 0.703(50)
C1 = 0.703(50) / 200
C1 = 0.176mM

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