Hardness buffer

Hardness titrating solution

The steps of the hardness test are as follows:

1. Measure 50 ml of the sample in the graduated cylinder and transfer it to the casserole.

2. With the calibrated dropper, add 0.5 ml of the hardness buffer reagent to the sample, and stir.

3. Add 4 to 6 drops of hardness indicator. If hardness is present, the sample will turn red.

4. Add the hardness titrating solution slowly from the burette, and stir continually. When approaching the end point, note that the sample begins to turn blue, although you can still see a definite reddish tinge. The end point is the final discharge of the reddish tinge. Adding more hardness titrate solution does not produce further color change.

In using this procedure, add the hardness titrating solution slowly because the end point is sharp and rapid. For routine hardness determination, measure 50 ml of the sample, but add only approximately 40 to 45 ml to the casserole at the start of the test. The hardness buffer reagent and the hardness indicator should then be added as directed and the mixture titrated rapidly to the end point. The remaining portion of the sample should then be added. The hardness in the remainder of the sample will turn the contents of the casserole red again. Titrating is continued slowly until the final end point is reached. A record should be kept of the total milliliters of hardness titrating solution used.

To calculate the results in ppm hardness, use the following equation:

ppm hardness = ml titrating solution x 1,000

(CaCO₃ ) (ml sample)

With a 50-ml sample, the hardness in ppm as CaCO₃ is equal to the ml of titrating solution used, multiplied by 20.

Test for Phosphate The calorimetric test for phosphate uses a decolorizing carbon to remove tannin. Carbon absorbs the tannin, and the carbon and tannin are then filtered out. When tannin is not present, carbon improves the test for residual phosphate by making the tricalcium phosphate sludge more filterable.

The equipment required for the phosphate test is as follows:

One phosphate color comparator block of two standards - 30 ppm and 60 ppm of phosphate as PO₄ . (The Taylor high-phosphate slide comparator may be used instead.)

Four combination comparator mixing tubes, each marked 5, 15, and 17.5 ml, with stoppers.

One filter funnel, 65-mm diameter.

One package of filter paper, 11 cm in diameter.

One 20-ml bottle.

One 0.5-ml dropper.

One 1/4-tsp measuring spoon or spatula.

Two plain test tubes, 22 mm by 175 mm (about 50 ml).

Two rubber stoppers, No. 3 flask.

One 250-ml glass-stoppered bottle or flask, labeled comparator molybdate reagent.

The reagents you need are as follows:

One 32-oz comparator molybdate.

One 2-oz concentrated stannous chloride.

One 32-oz standard phosphate test solution (45 ppm of phosphate, PO₄ ).

One pound decolorizing carbon. (This is a special grade of decolorizing carbon tested to make sure it does not affect the phosphate concentration in the sample.)

For test purposes, the stannous chloride is supplied in concentrated form. The reagent must be diluted and should be prepared from the concentrated stannous chloride on the day it is to be used, because the diluted solution deteriorates too rapidly for supply by a central laboratory. If not fresh, diluted stannous chloride gives low test results. Concentrated stannous chloride also deteriorates and should not be used if more than 2 months old.

The procedure for making diluted stannous chloride is as follows:

1. Fill the 1/2-ml dropper up to the mark with the concentrated stannous chloride.

2. Transfer it to a clean 20-ml bottle.

3. Add distilled water up to the shoulder of the bottle, then stopper and mix by shaking.