The soluble ash is used for estimation of calcium and phosphorus.
Pipette out 10 ml of the soluble ash into a Pyrex beaker (250 ml capacity). view video
Add enough distilled water to increase the volume. view video
pH Adjustment
pH and Ca precipitation
The quantitative precipitation of calcium is influenced by pH. Complete precipitation of calcium is best achieved between pH 2.5 and 3. In this pH, the interference due to co-precipitation of other metals is avoided. Hence, the pH has to be suitably adjusted before adding the precipitating reagent.
Before the calcium in the soluble ash is precipitated as calcium oxalate by the addition of saturated Ammonium Oxalate, it is necessary to adjust the pH between 2.5 to 3.0.
This is done as follows:
Add one or two drops of methyl red to the aliquot in the beaker. The colour will be pink because of the acids added to the soluble ash. view video
Use glass tube and add drop-by-drop dilute Ammonium Hydroxide (NH4OH 1+3) till the pink colour changes to yellow. view video
Now add by means of glass tube, drop by drop dilute HCl (1+3) till the yellow colour is changed to pink. view video
Now the pH of the aliquot will be between 2.5 and 3.
Precipitation of Calcium
After adjusting the pH, the beaker with its contents is placed over the heater. Gently heat the contents of the beaker. view video
Now, take 10 to 15 ml of warm Ammonium Oxalate (saturated) in a test tube. view video
Add the Ammonium Oxalate (precipitant) to the contents of the beaker slowly, with uniform stirring. view video
The precipitation of calcium as calcium oxalate (CaC2O4) takes place as evidenced by the formation of white granular precipitate. view video
Note
Ammonium Oxalate should be added very slowly, since fast addition causes the formation of powder like calcium oxalate that will pass through the filter paper thus leading to loss of calcium and subsequent erroneous result. One should get a coarse and granular precipitate for efficient recovery of the Ca salt. Slow addition of the precipitant (Ammonium Oxalate) with uniform stirring favours this condition.
If the contents of the beaker changes to yellow, add one or two drops of Hcl (1+3) to bring back the pH range indicated by the reappearance of pink colour. Cool the contents of the beaker to room temperature.
Filtration and washing
Fix a clean funnel to a clean conical flask (250ml capacity). view video
Fix a Whatman (No.42) filter paper to the funnel. view video
Start the filtration by transferring the supernatant liquid from the beaker in small amounts. view video
Take care to avoid over filling the funnel upto the brim. This will cause difficulty in washing.
Transfer and filter all the supernatant liquid from the beaker.
The excess of oxalate is washed by adding small quantities of dilute Ammonium Hydroxide (1:50). view video
Add 10ml of dilute ammonium hydroxide to the precipitate in the beaker mix well and allow settling.
Transfer the supernatant fluid to the funnel (No matter if some quantity of precipitate gets transferred to filter paper in the process of filtration).
Repeat this process till the precipitate is free from oxalate.
Barium Chloride test
To test whether the precipitate is free from oxalate, barium chloride (5%) solution is used. This is a qualitative test. Take about 5 ml (not more than this) in a clean test tube and place it in the test tube rack. When the funnel is full with washing, fix the funnel in the test tube in such a way that filtrate flows through the side of the test tube and joins the barium chloride (Avoid straight dropping of filtrate in the barium chloride). The appearance of white ring is indicative of the presence of oxalate, thus necessitating further washing. view video
Dissolving the precipitate
When the filtrate is free from oxalate, wait for complete filtration of the contents of the funnel. Remove the funnel and place it in the beaker. Empty the conical flask and wash it clean. view video
Replace the funnel with filter paper over the conical flask. Puncture the filter paper with the aid of glass rod, wash the glass rod and the funnel to flush down the precipitate to the conical flask. view video
Carefully, with glass rod remove the folds of filter paper flush down the precipitate by forcing a jet of distilled water. view video
Ensure that all the precipitate in the filter paper is washed in to the conical flask.
Add dilute sulphuric acid (1+4) to the beaker and warm the contents.Any precipitate left in the beaker is dissolved in the added acid.
Transfer the warm contents through the funnel.This will dissolve any adhering precipitate in the funnel. view video
Again wash the funnel and filter paper with distilled water. view video
Watch the contents of conical flask. If there is any turbidity, add some more warm H2SO4 (1+4) to dissolve the remaining precipitate.
Remove the funnel with filter paper to the beaker. (Don’t throw the filter paper.) view video
Titration - 0.1 N KMnO4 is used in this titration.
The content of the conical flask is heated to boiling stage. view video
When the first few bubbles appear, take out the conical flask for titration. This is important since the temperature of the contents should not be below 60 ° C during titration.
Check the initial reading of burette by taking the upper meniscus as in case of all coloured fluids. view video
Adjust the burette in such a way, that successive drops follow each drop of permanganate. view video
The permanganate will get reduced by the oxalic acid and the end point is the appearance of pale pink colour (Avoid fast addition of the permanganate). view video
When the end point is reached, take the filter paper and put it into conical flask. Rotate the flask to ensure thorough mixing. The end point will disappear. view video
Continue the titration as before till you get the pale pink colour again. Note the reading of the burette. view video
Note:
If you have thoroughly washed the filter paper, you will get the pink colour within the addition of two or three drops of KMnO4.
Equivalent:
1ml of 0.1N KMnO4 = 0. 002 g of calcium
Calculation:
Here,
V = Titre value (volume of 0.1 N KMnO4 used),
Z = Volume of soluble ash taken for precipitation,
Warm the contents and add 10 to 15 ml of warm Ammonium Oxalate.
If colour changes, bring back the pink colour by adding a drop or two of HCl (1+3).
Filter through Whatman No.42 filter paper.
Wash the precipitate with dilute NH4OH (
Test with Barium chloride.
Transfer the oxalate free precipitate to a clean conical flask by puncturing and washing filter paper.
Take 10 and 20 ml dilute H2SO4 (1+4) in the original beaker and warm the acid.
Pour through the funnel and dissolve the entire precipitate in conical flask.
Remove the funnel. Shake and mix the contents of conical flask. Gently boil and titrate against 0.1 N KMnO4. End point is the appearance of pale pink colour.
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