Determination of bismuth in Pepto-Bismol antacid tablets by inductively coupled plasma optical emission spectroscopy (ICP-OES) and Atomic Absorption Spectroscopy (AAS)
Pharmaceutical’s containing bismuth such as bismuth subsalicylate are used in over the counter medicines such as Pepto-Bismol as an antacid remedy. Pepto-Bismol can be purchased as pink tablets, chewable tablets or in liquid form as a pink suspension. Gorbach writes:
“Bismuth has been shown to be effective against two major gastrointestinal disorders: peptic ulcer disease and diarrhoea. In peptic ulcer disease it is as effective as the H2-receptor antagonists, costs considerably less, and offers a lower rate of relapse. When Helicobacter pylori are implicated, bismuth acts as an antimicrobial agent, suppressing the organism but not eliminating it. In recent studies, bismuth compounds have been used with conventional antibiotics, producing elimination of the organism, histological improvement, and amelioration of symptoms for periods longer than one year. Bismuth subsalicylate has shown modest efficacy in treating diarrhoea and acute diarrhoea in children, and it is effective prophylactically for diarrhoea…. [Neurological toxicity as a possible side effect] …was reported in France in the 1970’s with prolonged bismuth treatment, usually bismuth subgallate and subnitrate. Such toxicity has been rare with bismuth subsalicylate and bismuth subcitrate.”1
The aim of this experiment is to check the dose of bismuth in a batch of pepto-bismol tablets and confirm whether this matches manufacturer specifications. This will be performed using AAS and ICP-OES, with the performance of the two techniques in this analysis critically evaluated.
Atomic-absorption spectroscopy (AAS) uses the absorption of light to measure the concentration of gas-phase atoms and Inductively coupled plasma optical emission spectroscopy (ICP-OES) uses the emission of light for the same purpose. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace (for AAS) or argon plasma (ICP-OES). The atoms absorb ultraviolet or visible light when making transitions to higher electronic energy levels and emit light when transitioning back to lower energy levels. The analyte concentration is determined from the amount of absorption or emission. Applying the Beer-Lambert law directly is difficult due to variations in the atomization efficiency from the sample matrix, and non-uniformity of concentration and path length of analyte atoms. Concentration measurements are usually determined by calibrating the instrument with standards of known concentration.
Agilent 280FS Atomic Absorption Spectrometer (JPW3/68)
The method that follows describes how the experiment would have been carried out to collect that data that you have been provided with. Videos on Brightspace show you the operation of the two instruments.
Hazards and safety precautions
All chemicals should be treated with caution. Absence of specific comments does not mean that a substance is harmless. You must wear eye protection and a lab coat and avoid ingestion of solids or liquids whilst in the lab.
|Pepto-Bismol tablets contain bismuth subsalicylate 262.5 mg||Non-toxic
Hazard category – low, exposure potential – low, use in open lab
|1000 ppm bismuth solution contains nitric acid.||Causes acid burns. In the event of eye contact, rinse with plenty of water and seek medical advice.
Hazard category – medium, exposure potential – low, wear gloves wash hand after use.
|Causes severe burns, irritating to respiratory system. May cause fire if in contact with combustible material. In the event of eye contact, rinse with plenty of water and seek medical advice.
Hazard category – high, exposure potential – low, use in fume hood wear gloves. Wash hands after use.
Remember – always add acid to water not water to acid
Prepare a set of standard bismuth solutions each 100 ml solution should contain 2 ml of concentrated nitric acid. Measure the mass of a whole Pepto-Bismol table on an analytical balance to the nearest 0.1 mg then grind up the tablet using a clean dry mortar and pestle. Precisely weigh approximately 0.5 g of the ground tablet into a clean dry 100 ml beaker to the nearest 0.1 mg. In a fume hood add approximately 30 ml of deionised water then and 10 ml conc nitric acid, and heat on a hotplate in order to dissolve the tablet. Filter the resulting solution into a 100 ml volumetric flask. Cool and make up the volume to the graduation line with deionised water using a Pasteur pipette. Carry out the next section in duplicate: dilute the sample solution 5 ml plus 2 ml conc nitric acid into a 100 ml volumetric flask. i.e. there should be 2 solutions from a single tablet
The ICP-OES requires purging with gas before use. Ask a technician to show you how to switch on the purge gases at least 30 minutes before you are ready to start analysing your samples (see the video).
To prepare calibration standards take 25 ml of the 1000 ppm Bi standard dilute to 250 ml (becomes a 100 ppm Bi) solution. Dilute 10, 20, 30, 40 and 50 ml of this solution to 100 ml; add 2 ml conc nitric acid to each solution before making each solution up to 100 ml graduation mark (10, 20, 30, 40 and 50 ppm Bi solutions). Prepare a blank solution containing 10 ml of conc nitric acid made up to the line with deionised water (0 ppm Bi).
Take all your samples to the instrument. The AAS has an automated routine that will ask for these to be presented in order, blank, then standards in order of increasing concentration, then the analyte solutions. Examine all your solutions using AAS and ICP-OES.
- Brief introduction to the two techniques (see ref. 2)
- For both techniques construct calibration graphs of absorbance or peak intensity against bismuth concentration (0 to 50 ppm Bi).
Is a straight line graph obtained? If not, suggest reasons for the non-linearity.(The best way to check for linearity is to look at the residuals of the linear regression).
- Use the calibration graphs for both instruments to find the concentration of bismuth in the sample solutions and uncertainty in these values.
- Use the sample mass and total mass of the tablet to calculate the mass in mg of bismuth and bismuth subsalicylate in the original Pepto-Bismol tablet. Remember to include the uncertainty in the determined mass. The molar mass of bismuth subsalicylate is 362 g/mol and Bi atomic mass is 208.98 g/mol
- Comparison of measured mass with the expected result quoted by the manufacturer of 262.5 mg bismuth subsalicylate per tablet.
- Compare the results from the two instrumental methods by
i) plotting a graph of AAS vs ICP-OES signal for the calibration data to – see Miller and Miller §5.9 for details of this analysis.
ii) Determine the limit of detection for each method
iii) Are the results from the two methods significantly different? (t-test)
- The Pepto-Bismol tablet also contains calcium carbonate (350 mg per tablet), magnesium stearate, red 27 aluminium lake dye and insoluble silicon dioxide and sodium starch glycolate. Potentially we could have carried out the analysis for Bi, Ca, Mg, Al and Na. In your report discuss which technique, ICP-OES or AAS, would be most appropriate for analysis of these metals and whether there might have been any problems encountered (e.g. spectral inferences or chemical interference)?
- How could the experiment design be improved? (i.e. how could uncertainty in the determined concentrations most effectively be reduced)
Your work will be graded against the 20 point mark scheme on the cover sheet. Please ensure you include all points, you will lose marks for any missing item!
- L. Gorbach, Gastroenterology. 1990, 99, 863 – 75.
- Skoog D.A., Holler F.J. & Crouch S.R., Principles of Instrumental Analysis, 7th, Brooks Cole, 2017
- Miller J.C. & Miller J.N., Statistics and Chemometrics for Analytical Chemistry, 7th ed., Pearson, 2018