write 5

Hardness in water is caused by the presence of multivalent ions in water, with the most common being CA2+ and Mg2+, other contributors to hardness are Al3+ and Fe3+ in some source waters. (Mihelcic and Zimmerman 2010) Hardness is commonly reported as mg/L of CaCO3 (calcium carbonate). Alkalinity in water is an important factor due to its buffering capacity, that is the ability of the water to resist a change in PH when an acid or base is added. (Viessman et al. 2009) Alkalinity is also commonly reported as mg/L of CaCO3. Hardness is composed of both temporary hardness and permanent hardness. Temporary hardness is the amount of hardness in the water paired with carbonate or bicarbonate ions in the water. Permanent hardness is the amount of hardness paired with non-carbonate ions such as SO4, PO4, CL, and NO3 in the water. (Davis 2010) Hardness in water may interfere with soaps and detergents and causes scale formation in pipes. It is imperative that in any water treatment facility the alkalinity and hardness are known so that the proper determinations of chemical additions or treatment processes may be ascertained. In this lab the Hach digital titrator methods were used to determine the alkalinity, calcium hardness, and total hardness. From this information were able to report the hydroxide alkalinity, carbonate alkalinity, bicarbonate alkalinity, calcium hardness, and total hardness.

The objective of this lab was to measure the alkalinity and hardness of three water specimens, de-ionized water, Grand Forks tap water and Red River water.

To calculate the concentration:

Digits used × digit multiplier = mg/L as CaCO3

CO_2(aq) +H_2 O ↔ H_2 CO_3 (10.1.1) H_2 CO_3 ↔ H^+ + HCO_3^- (10.1.2)

H_2 CO_3^- ↔ H^+ + CO_3^(2-) (10.1.3) CaCO_3(s) ↔ Ca^(2+)+CO_3^(2-) (10.1.4) Mg(HCO_3 )_2+2Ca(OH)_2 → 2CaCO_3↓ + Mg(OH)_2↓ (10.1.5)

As expected, measured phenolphthalein (P) alkalinity, hydroxide alkalinity, and carbonate alkalinity were all 0 mg/L as CaCO3 for the de-ionized water specimen. Similarly, all other samples also measured 0 mg/L for P-alkalinity, hydroxide alkalinity, and carbonate alkalinity. Bicarbonate alkalinity was present in all samples. This was not expected in the DI water as it was assumed to contain no alkalinity. In the other samples this makes sense as the total alkalinity must be equal to the sum of the individual alkalinities.

The total hardness of the tap water was determined to be 123.2 mg/L as CaCO3, with 80.8 mg/L of that as calcium hardness and 42.4 mg/L of that as magnesium hardness. The total hardness of the river water was determined to be 282.8 mg/L as CaCO3, with 165.2 mg/L of that as calcium hardness and 117.6 mg/L of that as magnesium hardness.

Figure X - Meq/L Bar Graph for Carbonate and Non-Carbonate Hardness of Tap Water

Table X and Figure X show the distribution of carbonate and non-carbonate hardness in the tap water specimen. Based on the data obtained, the dosage of lime required to soften the water is 2.984 Meq/L or 149.2 mg/L as CaCO3 and the required soda ash dosage is equal to the non-carbonate hardness of .164 Meq/L or 8.692 mg/L of Na2CO3.

Figure X - Meq/L Bar Graph for Carbonate and Non-Carbonate Hardness for River Water

Table X and Figure X show the distribution of carbonate and non-carbonate hardness in the river water specimen. Based on the data obtained, the dosage of lime required to soften the water is 3.846 Meq/L or 193.2 mg/L as CaCO3 and the required soda ash dosage is equal to the non-carbonate hardness of 2.072 Meq/L or 109.82 mg/L of Na2CO3.

The objective of this lab was to measure the alkalinity and hardness of three water specimens, de-ionized water, Grand Forks tap water and Red River water. continue

Davis, M. (2010). Water and Wastewater Engineering, McGraw Hill, New York

Mihelcic, J. Zimmerman, J. (2010). Environmental Engineering Fundamentals Sustainability Design, Wiley, New Jersey

Viessman, W. Hammer, M. Perez, E. Chadik, P. (2009). Water Supply and Pollution Control, 8th Ed., Pearson, New Jersey