Expt. 1 . Complexometric Titration of Magnesium
completed before commencing
1. Read the information relevant to this experiment
2. Read this complete experiment, particularly the material in the boxes.
3. Read the safety information at the end of this experiment.
Exercises to be completed before commencing the laboratory
The high formation constant for many metal – EDTA complexes allows titrimetric methods to be used to determine metal ions. In this experiment you will prepare a standard disodium salt of ethylenediaminetetraacetic acid solution to determine the magnesium content of a magnesium supplement capsules. You will then conduct a statistical analysis of your results.
BEFORE COMMENCING THE EXPERIMENT, CLEAN ALL GLASSWARE THOROUGHLY. Your laboratory supervisor will demonstrate the correct use of pipettes and burettes. RED/BLUE COLOUR BLIND STUDENTS SHOULD IDENTIFY THEMSELVES TO
Chemical description of the magnesium supplement capsules.
Clinician’s magnesium capsules contain a magnesium complex designed to support cardiovascular, immune, hormonal and muscular-skeletal health. Active ingredients in each gelatine capsule are 58%(w/w) magnesium diaspartate, 25%(w/w) magnesium oxide and rest is inactive ingredients.
1. Dissolution of Sample
Weigh one magnesium supplement capsule using an analytical balance. The analytical balances are used for precise weighing up to 4 decimal places. Place a funnel on top of a 100 mL volumetric flask. Hold the capsule above the volumetric flask with the funnel and gently rotate the two halves of the capsule until they rotate freely. Pull two halves apart carefully and transfer contents of the capsule into a 100 mL volumetric flask through the funnel. Reweigh the empty capsule. Add 20 mL of 2 mol L-1 HCl (provided, with automatic dispenser, in the fume cupboards) into the flask making sure to wash any powder remaining on the funnel into the flask. Gently swirl to dissolve the contents. Make the solution up to the mark with deionized water. Make sure the solution is homogeneous by inverting the flask several times and shaking vigorously.
Record the product name, manufacturer, batch number and label claim (the manufacturers stated amount in each tablet) in your laboratory book.
Titrimetric Determination of Mg 1-1
2. Preparation of Standard EDTA(disodium salt of Ethylenediaminetetraacetic acid)
Analytical reagent grade EDTA sodium salt, Na2H2EDTA.2H2O, is suitable as a primary standard if the surface moisture is removed by drying at 80oC. Dried analytical reagent grade Na2H2EDTA.2H2O is available in a desiccator by the top-loading balances.
Use a top-loading balance to weigh about 2.3 g EDTA sodium salt into a clean dry weighing bottle.
Using an analytical balance, precisely weigh (ie record the weight to 4 decimal places in your laboratory book) the weighing bottle and its contents. Using a clean funnel, transfer the EDTA sodium salt from the weighing bottle to a 250 mL volumetric flask and reweigh the weighing bottle and any remaining EDTA disodium salt (again, recording to 4 decimal places directly into your lab book). The difference in the weights before and after transfer of the EDTA disodium salt from the weighing bottle gives the weight of EDTA disodium salt transferred to the flask.
Add about 150 mL deionised water (available in the wash bottles on your bench and from taps at the sinks at the ends of each bench) to the EDTA and swirl vigorously to dissolve the crystals. Initially this addition of deionised water should be done through the funnel through which the EDTA was added. (this ensures that all the EDTA put into the funnel gets washed down into the flask). When the EDTA crystals have dissolved make the solution up to the mark with more deionised water. Make sure the solution is homogeneous by inverting the flask several times and shaking vigorously.
Ensure your burette is clean, and then rinse with a small amount of your standard EDTA solution. Clamp the burette so it is vertical and fill (a funnel is provided) to the 0.00 mL mark with your standard EDTA solution. Remove the funnel from the top of the burette. Ensure your 10.00 mL pipette is clean then rinse it with a small amount of your magnesium sample solution. Pipette a 10.00 mL aliquot of your magnesium sample solution into a clean (not necessarily dry) 100 mL conical flask and add approximately 20 mL ammonia pH 10 buffer (provided, with automatic dispenser, in the fume cupboard). Swirl and add a few specks of solid Eriochrome Black T indicator (take care not to add too much indicator – just a few specks are required to give a red/pink colouration).
You are now ready to begin titration. The table below illustrates how you may organize the recording of your titration volumes in your lab book.
Titrant number Initial volume (mL) Final volume (mL) Titre volume (mL) Comment
#1 #2 1.84 3.30 25.68 26.86 23.84 23.56
#3 #4 #5 #6 etc “ “ “ “ “ “ “ “ “ “ “ “ “ “ “
* Note your volumes will be different from this illustration
Record the initial volume of the standard EDTA solution in the burette reading the burette to two decimal places (ie to ± 0.02 mL, see example above) and then titrate with the standard EDTA solution to the red to blue end-point. You may wish to keep the final blue solution obtained from your first titration as a reminder of the endpoint colour. Repeat the titration as many times as you are able within the time available, with the goal of obtaining 5-6 concordant titrations.
Using your data, information from Cecil and your lectures notes (series 1) answer the following questions. Additional information can be found in Harris 7th edition (Chapters 3, 4 and 12).
Titrimetric Determination of Mg 1-2
(b) (c) (d) (e)
Reject any non-valid results, giving brief reasons (eg spilt, burette leak, overshot endpoint). Note: these reasons must be entered into your lab notebook at the time of carrying out the experiment in the laboratory.
List the valid titres in numerical order and identify the (sole) outlier if any. Use the Grubb’s test at the 95% confidence level to decide whether this one outlying result can be rejected.
Calculate (1) the mean, (2) the standard deviation, (3) the range, and (4) the median titre for the accepted valid titres.
Write the molecular formulae and draw the structural formulae for EDTA, disodium salt of EDTA and magnesium diaspartate.
Calculate the molar mass and associated uncertainty (Mr) for disodium salt of EDTA with two water molecules using the IUPAC standard atomic weights (page xv in the Accuracy, Precision & Errors section at the front of this manual)
Use your titration data to calculate the mass of magnesium present in the original capsule. Give a detailed quantitative description of the uncertainties associated with this experiment. (This involves the calculation of the absolute and/or % relative uncertainty, as appropriate for each stage of the calculations, as detailed in the “Accuracy, Precision and Errors” section at the front of this manual).
1. What measurement gave rise to the greatest uncertainty in your determination of the amount of magnesium in your capsule? Justify your answer.
2. Does your determination of the magnesium content of the product agree with the label claim, within your experimental uncertainties?
3. No blank titration was conducted in this experiment. Briefly describe how you would conduct a blank titration, and how you might expect it to impact on the uncertainty of your results (Harris 5th Ed: pp 150, 6th Ed: pp 129, 7th Ed: pp 122).
Each capsule has 58%(w/w) magnesium diaspartate and 25%(w/w) magnesium oxide as the active ingredients in this formulation. Magnesium diaspartate used in this formulation has the molecular formula (C4H6NO4)2Mg.nH2O where n can be 0, 1, 2 or 4.
(4) Using the weight of the content you transferred into 100 mL volumetric flask and the given percentage of magnesium oxide calculate the mass of Mg present as MgO.
(5) Using the values you calculated in (f) and (4) above calculate the mass of magnesium present as magnesium diaspartate in a capsule.
(6) Hence calculate the mass of magnesium diaspartate in a capsule.
(7) Calculate the theoretical mass of Mg diaspartate using the weight you transferred and the given
percentage of 58%.
(8) Using the mass of magnesium diaspartate you calculated in (6) and the theoretical mass of
magnesium diaspartate calculated in (7), calculate the number of water molecules (n) in magnesium diaspartate used in this formulation.
pH 10.0, ca 1 mol L-1 Description: Colourless solution
2 mol L-1
Description: Colourless solution
Titrimetric Determination of Mg 1-3
Risk: Irritating to eyes. Harmful if swallowed Precautions: Rinse immediately with water if splashed
Disposal: Solution to drain, flushed well with
Risk: Corrosive. Harmful if swallowed Precautions: Avoid contact with skin, eyes. Rinse
immediately with water.
Disposal: Solution to drain, flushed well with
Description: White solid
Risk: Harmful if swallowed
Precautions: Avoid contact with skin, eyes. Rinse
immediately with water
Disposal: Solid to solid waste; solution to drain,
flushed well with water
Eriochrome Black T Indicator
Description: Black solid
Risk: Irritant. Harmful if swallowed Precautions: Avoid contact with skin, eyes. Rinse
immediately with water
Disposal: Solid to solid waste, solution to drain,
flushed well with water
Titrimetric Determination of Mg 1-4
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EXPERIMENT 1 – Complexometric Titration of Magnesium
AIM: The aim of this experiment was to determine the amount of magnesium in magnesium supplement capsules by complexometric titration with EDTA solution
PROCEDURE: The experiment is done by following the procedure given for the EXP. 1: Complexometric Titration of Magnesium
Product name: Clinicians Magnesium, Magnesium 625
Batch #: 78192 (expires feb 2018)
Label: 125 mg of Mg per capsule
Weight of EDTA + weighting bottle = 6.5890 g ± 0.0001 g
Weight of emptied bottle = 4.3245 g ± 0.0001 g
Weight of EDTA transferred to 250 ml ± 0.24ml flask = 2.2645 g ± 0.0002 g
About 100 mL type 1 water is added to the flask, stoppered, and shook until all EDTA had dissolved. Made up to the mark with type 1 water, stoppered and shook.
Weight of capsule = 0.7321g ± 0.0001 g
Weight of empty capsule = 0.0961 g ± 0.0001 g
Weight of mg capsule = 0.7321 g – 0.0961 g = 0.6360 g
Mass of the sample = 0.6360 g ± 0.0002 g...