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BORIC ACID AND MANNITOL


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Q. I am required to determine the boric acid concentration in our nickel sulfamate plating bath. Can you provide a procedure for doing this? A.M.

 

A. Here is a procedure for determining the concentration of boric acid in a sulfamate nickel plating bath.

Procedure

Pipette 5 mL of the nickel sulfamate plating solution into a 250-mL Erlenmeyer flask.

Add 50 mL of deionized water to the flask.

Add 5 g of mannitol.

Add 5 drops of bromocresol purple indicator to the flask.

Stir thoroughly and titrate with 0.1-M sodium hydroxide to a light blue end point.

The volume of sodium hydroxide (X) needed, in mL, is used in the following equation.

X mL x 1.24 = g/L of boric acid.
 

Boric acid is a “weak” acid and gives a poor “end point.” Adding mannitol to the flask changes the weak acid into a stronger acid, which gives a good, clear endpoint in the titration with sodium hydroxide.

For those of you there who are not organic chemists, mannitol is an organic molecule that is a combination of a sugar and an alcohol. A drawing of the structure taken from Wikipedia is shown below.

formula

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