Masking & Demasking Reagents and Estimation of Magnesium Sulphate, and Calcium Gluconate

Masking and demasking reagents
A substance is transformed into a new form without entering into a reaction, as when it is shaped by making, without the physical separation of that substance or its products. Masking is the process by which masked substances regain the ability to participate in a specific reaction. In some cases, there is an option for masking cations in a mixture to prevent them from reacting with EDTA or the indicator. In addition to cyanide, other masking agents include manganese, alkaline earth, and lead ions, because these form stable cyanide complexes with the cations. The above metals, however, can be detected by masking them with excess potassium or sodium cyanide to determine the presence of cations such as Ca2+, Mg2+, Pb2+, and Mn2+. Cyanide allows the reduction of iron (II) to iron(I) by ascorbic acid. It can then be masked by iron(I), and then by ascorbic acid.

Triethanolamine, iodine, and mercury can mask titanium (IV), iron (III), and aluminum, and ammonium fluoride masks tin (II), iron (III), titanium (IV), and iron (III). There are times when the metal is transformed into a different oxidation state: hydroxylamine or ascorbic acid may reduce copper (II) in an acid solution. As an indicator, one can carry out nickel or cobalt titrations using murexide, for instance, after the ammoniacal has been rendered, as the copper is no longer present as it is reduced to Cu (I). Similar reductions with ascorbic acid are often able to mask iron (III).

Selective demasking
It is possible to demasked zinc and cadmium complexes using formaldehyde acetic acid solution, or even better, chloral hydrate: By using multiple masking agents and selective demasking agents, many metals may be successively titrated. Titrating a solution that contains magnesium, zinc, and copper can be accomplished by following the following formula:

1. As a further step, titrate with excess standard EDTA using solo chrome black, together with excess standard EDTA. The result will give the total amount of metals present.
2. An aliquot portion should be treated with excess KCN (poison!), followed by titration as before. Mg is the only product.
3. By adding excess chloral hydrate, or formaldehyde and acetic acid, you can release Zn from the cyanide complex, allowing you to find the value of the Zn only. The Cu content is then determined by comparing the results.

Estimation of magnesium sulfate
Materials required -
Magnesium sulfate hydrate - The solution of strong ammonia and ammonium chloride is made with 6.75 g NH4Cl in 74.0 ml of strong ammonia solution and then water is added q.s. to make 100 ml of solution along with the mixture of mordant black II and NaCl (0.2 parts mordant black II and 100 parts of NaCl) is 0.1 g...

In contrast, MgSO4.7H2O assay is based on the reactions following these equations:

Mg2+ + [H2X]2–→[MgX]2– + 2H+

or MgSO4.7H2O≡ Mg2+ ≡ Na2H2X, 2H2O

or 120.38 g MgSO4 ≡ 20,000 ml 0.05 M

or 0.00602 g MgSO4 ≡ 1 ml of 0.05 M Disodium Edetate

Procedure
To dissolve with DW, weigh precisely about 0.3 g of magnesium sulfate heptahydrate and dissolve in 50 ml of the solution. Titrate with 10 ml of strong ammonia-ammonium chloride solution, and apply 0.05 M disodium ethylene diamine tetraacetate in the presence of 0.1 g of mordant black II mixture as an indicator until the pink color has disappeared. MgSO4 is equivalent to 0.00602 g of disodium ethylene diamine tetraacetate in each ml of 0.05 M.

The Standardization of A 0.05 M Disodium Edta Solution
Procedure
The following procedure should be followed: Weigh accurately 0.8 grams of granulated zinc and dissolve with 12 ml of diluted hydrochloric acid and 5 drops of bromine water. In a volumetric flask, boil the solution to remove the excess bromine, then cool and add just enough DW to obtain 200 ml. Neutralize the solution with 2 N sodium hydroxide after pipetting 20 ml of the resulting solution into a flask. Add ammonia buffer (pH 10.0) to dilute to 150 ml, add 5 ml in excess to dissolve the precipitate, then dilute to 150 ml with DW. Titrate with disodium edetate solution until the solution is green with 50 mg of Mordant Black II mixture final. Zinc granules are equivalent to 1 ml of 0.05 M disodium ethylenediaminetetracetate for each 0.003269 g.

Calculation
ZnCl2 ≡ Zn2+ ≡ 1000 ml M

or 65.38 g Zn ≡ 1000 ml M

or 3.269 g Zn ≡ 1000 ml 0.05 M

or 0.003269 g Zn ≡ 1 ml of 0.05 M Disodium ethylenediaminetetracetate.

Estimation of calcium gluconate
Materials required -

1. EDTA - The EDTA solution is 0.025 mmol L of ethylenediaminetetraacetic acid. During several hours or overnight, dry 5 g of the disodium salt of EDTA at 80 °C, and allow to cool. Pour 500 mL of distilled water into a volumetric flask and dissolve 4.65 grams of EDTA.
2. Patton reader indicator triturates - An amount of Patterson-Reeder triturate may be available from Outreach at the University of Canterbury; details are on the back of the page.
3. Sodium hydroxide solution - A 100mL solution of sodium hydroxide containing 8 molL-1 of sodium hydroxide is formed by carefully dissolving 32 g of solid sodium hydroxide into a 250mL conical flask of distilled water. NaOH dissolves in cold water; the temperature of the solution can be controlled by placing the bottom of the flask into a basin of cold water.
4. Dilute HCL
5. Dilute H2SO4

Reaction:

Ca-PR + EDTA4− → PR + [Ca-EDTA]2−

Sample preparation procedure
In the presence of calcium, such as tap water or milk, no further preparation is required. To filter out solids like sand and seaweed from seawater, filtering may be necessary. The acid must be used to dissolve solid samples such as eggshells and limestone.

1. About 0.5 g of the solid should be dissolved completely (this may take several minutes) with about 20 mL of hydrochloric acid diluted in water.
2. The pH of the solution should be almost 7 (as measured by a pH indicator paper) after neutralizing the unreacted acid with sodium hydroxide solution. Eggshells do not dissolve in your solution, so the inner membrane needs to be removed carefully.
3. Make up the flask with distilled water to the mark with the solution in the 100 mL flask.

Procedure
In the cases of undiluted seawater, undiluted milk, eggshell, and limestone; no dilution is required.

1. In a conical flask, pipette an aliquot of the sample solution of 10 mL.
2. Adding 8 mol L-1 sodium hydroxide solution to a 40 mL glass of distilled water (see safety notes) will yield 4 mL of the solution. Shake the solution periodically for about 5 minutes, then discard. A small amount of magnesium hydroxide may precipitate during this time. The indicator should not be added until after you have given this precipitate time to form.
3. The Patton-Reeder indicator should be added to the solution along with 0.1 g of water to dissolve it.
4. Prepare the EDTA solution and titrate the sample. After this step, the sample should be blue rather than pink/red. If the titrations do not agree within 0.1 mL, repeat the titration with additional samples.

Calculations

1. Based on the concordant titers, the average volume of EDTA solution used can be determined.
2. Using the formula above, calculate how many moles of EDTA is necessary to complex the Ca2+ ions in the sample.
3. Calculate the Ca2 + concentration in mol L-1 of your sample solution using the Ca2 +: EDTA ratio of 1: 1.
4. The concentration of Ca2+ in your sample solution should be calculated in parts per million or ppm (milligrams per liter).
5. For a solid sample dissolved in acid, you can calculate the percentage of CaCO3 in the solid sample by measuring the concentration of Ca2+ in the solution. The Ca2+ in the sample must have come from CaCO3.

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