Hardness of Water

 

The hardness of a water sample is usually reported in “parts per million” (ppm) of calcium carbonate (CaCO₃).  (There are additional metal ions, such as those from iron and magnesium, that contribute to the hardness of water but it is customary to ascribe all of the hardness to the presence of calcium carbonate.)  You can determine the amount of CaCO₃ by titrating a sample with a standard solution of EDTA, a complexing agent that reacts with the metal ions, using an indicator to determine when all of the EDTA and calcium  have reacted (the end point).  Your objective is to determine the hardness of several samples of water.  Those samples will be provided.  Among these samples will be a solution of standard hardness, Plover River (primarily spring fed) water, Wisconsin River (primarily run-off) water[1], Stevens Point tap water, and snow.  You must perform two titrations on at least three of the samples.  Each group must analyze the standard hardness solution (so that I may check your technique).  You may choose the other samples that you analyze, and if you prefer, you may bring in a sample of your own to analyze. 

 

The procedure is as follows.  Rinse and fill a buret with the standard[2] EDTA solution.  Pipet 50.00 mL of the water sample into a 250 mL Erlenmeyer flask, add 10 mL of ammonia buffer, and enough indicator (Eriochrome Black T) to get a definite color.  To avoid interference from other metal ions, add three drops of 1 M sodium cyanide solution.  BE CAREFUL TO ADD THE CYANIDE ONLY AFTER THE BUFFER HAS BEEN ADDED.  We will need to collect your samples after titrating!  Ask your instructor about where to place the contents of your flasks from the completed titrations. 

 

Titrate the above solution with the EDTA from the buret.  You will be looking for a color change from red to purple to blue.  An endpoint is indicated by a pure and uniform blue color.  You should be able to achieve a sharp color change.  Record the amount of EDTA added from the buret.

 

The hardness of the sample can be easily determined because the EDTA and calcium react in a one-to-one mole ratio.  Use our typical format in reporting your findings:  purpose, procedure, data/results, and conclusion.  You may want to set up a table similar to the one below. Notice that results are also included on this table.  Please also show a sample of your calculations.

 

Run  number	Initial volume EDTA - mL	Final volume EDTA - mL	Total volume EDTA - mL	Hardness -ppm CaCO3
Standard #1
Standard #2
Snow #1
Snow #2
etc

 

In your conclusion, comment on the differnce found between Plover River and Wisconsin River water.  Also comment on the differenct between snow and river water.  Considering that one drop is 0.05 mL, calculate the smallest amount of CaCO₃ (in ppm) that this method could detect.  We sometimes refer to this as the limit of detection.