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Digestion, Filtration and Titration for Chloride Concentration in Hydraulic Cement Mixtures[edit]

Purpose


This test is to determine the chloride concentration with respect to depth in cementious mixture samples. Titration is performed using the 716 DMS Titrino.

Note that this procedure is only applicable for mixtures composed of sand, cement and water. If sample contains slag, etc., then modifications to this procedure are required.

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Apparatus


1. 100 ml beakers

2. 250 ml beakers

3. Watch glasses

4. Scale (sensitive to 0.0001 g)

5. Glass rod

6. 50 ml graduated cylinder

7. 10 ml graduated cylinder

8. Hot plate

9. Timer

10. Filtration flasks

11. Filtration funnels

12. Pump

13. 9-cm coarse-textured filter paper

14. Magnetic stirring bars

15. Nitric acid (1+1)

16. Distilled water

17. 716 DMS Titrino


Procedure


In all steps, ensure that the multiple samples are carefully labelled. Also, ensure that beakers containing samples remain covered at all times.


1. Digestion[edit]

  • The basic procedure with each sample is mixing it with distilled water and nitric acid. Let the solution sit for 4 minutes before placing on the hot plate. Remove when boiling point is reached.

The steps involved with multiple samples may seem confusing at first. They are given for further assistance. It allows for a cyclic rotation of sample prep to minimize the required time.

1.1. Record the mass of 250 ml beaker.

1.2. Measure 2.00±.05 g of powdered sample into 250 ml beaker, and cover with watch glass. Record the mass of sample. Perform steps 1.1 and 1.2 for all samples.

1.3. Pour in distilled water and nitric acid in 250 ml and 100 ml beakers respectively, as much as required for the samples.

1.4. Measure 35±1 ml of distilled water using 50 ml graduated cylinder, and 7.0±.1 ml of nitric acid using 10 ml graduated cylinder.

1.5. Turn on the timer.

1.6. First pour the distilled water, immediately followed by nitric acid into the beaker which contains powdered sample. Always remember to pour Acid into Water. Stir the solution vigorously for about 10 seconds. Ensure that no clumps remain.

1.7. Clean the glass rod using distilled water on to the beaker. Try to use as little distilled water as possible. Cover with watch glass. Set aside the sample.

1.8. In case of multiple samples you may skip this step and refer to step 1.9 and onwards. After 4 minutes, place the beaker (covered with watch glass at all times) on to the hot plate. Heat until the solution is boiling. Do not let it boil over a few seconds. Remove from hot plate.

  • In the case of multiple samples:

1.9. Measure another batch of 35 ml distilled water and 7 ml of nitric acid.

1.10. When timer reads 2 minutes, quickly pour in the distilled water and nitric acid in to the second sample beaker. Stir. Cover. (Steps 1.6 – 1.7)

1.11. Measure another batch of 35 ml of distilled water and 7 ml of nitric acid.

1.12. At 4 minutes, put the first beaker on to the hot plate. Then immediately prepare the third sample.

1.13. Remove the beaker from the hot plate when the solution boils. Do not let it boil for over a few seconds.

1.14. Repeat steps 1.10 to 1.13 and keep placing beakers on to the hot plate in 2 minute intervals.

  • Following are pictures demonstrating the major steps mentioned above:

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2. Filtration[edit]

2.1. Begin this part of the procedure once all the samples have reached room temperature.

2.2. Set up the filtration equipment (flask, funnel, pump, filter paper). First, filter some distilled water to ensure that the funnel is clean.

2.3. Throw out the distilled water from the flask.

2.4. Clean the underside of watch glass (where steam has condensed after heating) onto the sample beaker.

2.5. Turn on the pump. Pour in the solution from beaker into the middle of the filter paper to ensure no sample is lost. Carefully clean the beaker using distilled water on to the filter paper.

2.6. Once all of it is filtered, turn off the pump; clean the underside of the funnel cup and pour some distilled water down the funnel to ensure all sample is obtained.

2.7. Pour the filtered solution from the filter flask into the same beaker that initially contained the unfiltered solution. Ensure that the hole for the pump in the flask is facing upwards when pouring out the solution. Clean the flask and pour it onto the sample beaker. Cover the beaker.

2.8. After letting the sample settle for about 10-15 min (or more, until sample has cooled down to RT), record the mass of the sample with beaker (excluding watch glass).

  • Following are pictures demonstrating the major steps from above:

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3. Titration[edit]

  • The titrant used is 0.01 N Silver nitrate.
  • Before beginning titration, the titrant must be recycled. This is done with the "recycle100" option in the 716 DMS Titrino. It should be performed 1-2 times.

3.1. Pour the necessary amount of filtered solution into the 100 ml beaker. Note that the amount required is relative for each test. Where high concentration of chloride is expected, pour in less filtered soltion, and vice versa. For example, for high very concentration, pour in 5-7 g. For very low concentration pour in 18-20 g.

3.2. Fill the beaker with distilled water about half way (50 ml). Place the stirring bar in the solution.

3.3. Find the mass of powder in the solution, using all the mass measurements obtained so far: beaker mass (step 1.1), sample mass (step 1.2), filtered mass (step 2.8) and solution mass (step 3.1)

  • Equation:

powder mass = (solution mass * sample mass) / (filtered mass - beaker mass)

3.4. Place the 100 ml beaker on the magnetic spinner in the 716 DMS Titrino. Carefully place the electrode and dispenser inside the solution.

3.5. The black mark of the electrode and dispenser must be completely submerged in solution. Add more distilled water if necessary. Ensure that there are no bubbles.

3.6. Ensure the bar is rotating at the appropriate speed (knob is set to about 9).

3.7. Enter necessary information (ID, powder mass, units) on to the 716 DMS Titrino to start titration.

3.8. Once titration is completed, record the chloride concentration (in ppm).

3.9. Remove the electrode and dispenser. Thoroughly clean them with distilled water.

3.10. Repeat steps 3.1 – 3.9, increasing or decreasing solution mass (in step 3.1) as felt suitable.


  • Clean all equipment primarily with soapy water. Then rinse with distilled water.


  • Following are pictures demonstrating the major steps from above:

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