test 14 total hardness

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Test

14

Water Quality with Computers 14 - 1

Total Water Hardness

INTRODUCTION

When water passes through or over deposits such as limestone, the levels of Ca2+

, Mg2+

, andHCO3 Ð ions present in the water can greatly increase and

cause the water to be classified as hard water. This termresults from the fact that calcium and magnesium ions inwater combine with soap molecules, making it ÒhardÓ toget suds. In Test 13, Calcium and Water Hardness, anIon-Selective Electrode was used to determine calciumhardness, in mg/L as CaCO3. In this test, total hardnesswill be determined. Total hardness is defined as the sum of calcium and magnesium hardness1, inmg/L as CaCO3. In addition to total hardness, the test described here will allow you to determinethe concentration of Mg2+, in mg/L.

High levels of hard-water ions such as Ca2+

and Mg2+

cancause scaly deposits in plumbing, appliances, and boilers.These two ions also combine chemically with soap molecules,resulting in decreased cleansing action. The American Water Works Association indicates that ideal quality water shouldnot contain more than 80 mg/L of total hardness as CaCO3.High levels of total hardness are not considered a healthconcern. On the contrary, calcium is an important componentof cell walls of aquatic plants, and of the bones or shells of aquatic organisms. Magnesium is an essential nutrient for plants, and is a component of chlorophyll.

Expected LevelsTotal hardness in freshwater is usually in the range of 15 to 375 mg/L as CaCO 3. Calciumhardness in freshwater is in the range of 10 to 250 mg/L, often double that of magnesiumhardness (5 to 125 mg/L). Typical seawater has calcium hardness of 1000 mg/L, magnesiumhardness of 5630 mg/L, and total hardness of 6630 mg/L as CaCO3.

Total water hardness, in mg/L as CaCO3

1Even though Fe

2+, Fe

3+, Sr

2+, Zn

2+, and Mn

2+may contribute to water hardness, their levels are typically much

less than Ca2+

and Mg2+

. Their levels are not usually included in total hardness measurements.

Hard-Water Cations

• Calcium, Ca2+

• Magnesium, Mg2+

• Others: Fe3+

, Sr 2+

, Zn2+

, Mn2+

Total Hardness(mg/L as CaCO3)

• Soft: 0-30

• Moderately soft: 30-60

• Moderately hard: 60-120

• Hard: 120-180

• Very hard: > 180





TWHTOTAL WATER HARDNESS

Materials Checklist

___ sampling bottles ___ Hardness 1 Buffer Solution

___ 100-mL graduated cylinder ___ two Calmagite3

indicator powder pillows___ 250-mL Erlenmeyer flask ___ 0.01 M EDTA

4titrant (TitraVer Standard Solution)

___ 25- or 50-mL buret ___ water bottle with distilled water

Collection and Storage of Samples

1. This test must be conducted in the lab. Collect at least 300 mL of sample water.

2. It is important to obtain the water sample from below the surface of the water and as far awayfrom the shore as is safe. If suitable areas of the stream appear to be unreachable, samplersconsisting of a rod and container can be constructed for collection. Refer to page Intro-4 of the Introduction for more details.

Testing Procedure

1. Obtain a clean buret and rinse it with a few mL of the 0.0100 M EDTA titrant (TitraVer Standard Solution). Fill the buret a little above the 0-mL level with the EDTA solution. Draina small amount of the solution so it fills the buret tip and leaves the EDTA solution at the 0-mL mark (or just below it). Record the buret level on the Data & Calculations sheet, to thenearest 0.01 mL.

2. Prepare the water sample for titration.

a. Use a graduated cylinder to measure 50 mL of your water sample into a 250-mL

Erlenmeyer flask. b. Add 1 mL of Hardness 1 Buffer Solution to the Erlenmeyer flask using the 1-mL

calibrated dropper. Gently swirl the contents of the flask to mix.c. Add the contents of one Calmagite3 powder pillow to the Erlenmeyer flask. Gently swirl

the contents of the flask to mix. The solution should now be red in color.

3. Titrate the sample you prepared in Step 2.

a. Slowly add 0.01 M EDTA titrant to the sample in the Erlenmeyer flaskÑstart with1-mL additions. Swirl the sample after each addition of titrant.

b. Near the equivalence point, the red color will start to fade, and become more violet(mixture of red and blue). You should reduce the addition volume to 1 drop at a time.Note: The reaction at the equivalence point is very slow (~1Êsecond), so take your time.

c. When the last tinge of red starts to disappear and turn to a pure blue color, you havereached the equivalence point. Record the final buret volume (to the nearest 0.01 mL) onyour Data & Calculations sheet.

d. If you are unsure whether you have reached the equivalence point, simply record the buretvolume, then add another drop of EDTA titrant. If the color becomes a truer blue color,use the new buret reading; if not, use the previous buret reading.

3Calmagite is contained in ManVer 2 Hardness Indicator Powder Pillows.

4Hach TitraVer Hardness Titrant is labeled as 0.01 M EDTA, and also as an equivalent concentration of 0.02 N.



Test 14

14 - 4 Water Quality with Computers

TWH4. Perform a second titration trial. To do this, discard the first titration mixture, as directed by

your instructor. If you have enough titrant left for a second titration, simply record thecurrent buret level; if not, refill the buret and record the buret level. Rinse the 250-mLErlenmeyer flask twice with distilled water, then repeat Steps 2 Ð 3.

Calculations1. To determine the titrant volumes used, subtract the initial volume from the final volume for

each trial. Record the titrant volumes for the two trials on the Data & Calculations sheet.Important: If the titrant volumes for the two trials are not in close agreement, you shouldrepeat StepsÊ1 Ð 3 of the Testing Procedure until you have two trials that are close.

2. Calculate the value for total water hardness.

a. For each trial, determine total water hardness using the equation

total water hardness as CaCO3 (mg/L) = (titrant volume) 5 20.0

b. Record the total water hardness value on the Data & Calculations sheet.

Optional Calculations

3. Calculate magnesium hardness (mg/L as CaCO3) and magnesium concentration (mg/L Mg2+).

a. On the second table of the Data & Calculations sheet, record the calcium hardness valueyou obtained in Test 13, and the total water hardness value from the first part of this test.

b. Calculate magnesium hardness (mg/L as CaCO3). Use the equation

magnesium hardness = total hardness Ð calcium hardness

c. Calculate Mg2+ concentration, using the formula

Mg2+ = (magnesium hardness as CaCO3) 5 (24 g Mg2+

/ 100 g CaCO

3).

d. Record the values for magnesium hardness and Mg2+ concentration on the Data &Calculations sheet.





TWHDATA & CALCULATIONS


Stream or lake: ___________________________ Time of day: __________________________

Site name: ______________________________ Student name: ________________________

Site number: ____________________________ Student name: ________________________

Date: __________________________________ Student name: ________________________

Column A B C D

Total waterhardness

Initialburet reading

(mL)

Finalburet reading

(mL)

Titrantvolume

(mL)

Total waterhardness

(mg/L as CaCO3)

Example 0.10 mL 9.22 mL 9.12 mL 182 mg/L

12

Column Procedure, total water hardness:

A. Initial buret reading (mL)B. Final buret reading (mL)

Average totalwater hardness

(mg/L as CaCO3)

C. Titrant volume (mL) = B – AD. Total water hardness (mg) = C 5 20.0

Column A B C D

Magnesiumcalculations(optional)

Calcium hardness(from Test 13)

(mg/L as CaCO3)

Total hardness(from 14)

(mg/L as CaCO3)

Magnesiumhardness

(mg/L as CaCO3)

Magnesiumconcentration

(mg/L as Mg2+)

Example 132 mg/L 182 mg/L 50 mg/L 12 mg/L

Column Procedure, magnesium concentration:

A. Record value from Test 13B. Record value from Test 14 (above)

C. Magnesium hardness (mg/L as CaCO3) = B – AD. Magnesium concentration (mg/L as Mg2+) = C 5 0.24

Field Observations (e.g., weather, geography, vegetation along stream)_________________________

_____________________________________________________________________________

_____________________________________________________________________________

Test Completed: ________________ Date: ______



Test 14

14 - 6 Water Quality with Computers

TWHADDITIONAL INFORMATION

Tips for Instructors

1. If students pass the equivalence point on the first trial, have them keep the resulting bluesolution to use as a color comparison for the next titration. Having a solution with the actual

blue color is very helpful in subsequent trials.2. If you do not have a Vernier Calcium Ion-Selective Electrode, Test 14 can be done without

having previously performed Test 13.

3. The following reagents can be ordered directly from Hach Company (5600 Lindbergh Drive,Loveland, CO, Tel: 800-227-4224, www.hach.com):

• Hardness 1 Buffer Solution (118 mL, 100 tests) ........................................... 424-37• ManVer 2 Hardness Indicator Powder Pillows (100 tests)............................851-99• TitraVer Hardness Titrant (0.010 M EDTA, or 0.020 N EDTA, 946 mL)....205-16

A reagent kit can also be ordered from Hach Company that includes all three reagents:

• Hardness (Total) Reagent Sets (~100 tests)...............................................24476-00

4. EDTA, ethylenediaminetetraacetic acid or H4C10H12 N2O8, is a weak acid that can lose four H+ ions on complete neutralization. In this section, we will represent EDTA in a simplifiedform, H4X, where X = C10H12 N2O8. Hach Titraver Standard Solution contains the disodiumsalt of EDTA, Na2H2X, which dissociates according to the equation

Na2H2X(aq) → 2 Na+(aq) + H2X2 Ð

(aq)

The titration is carried out near a pH of 10 in an organic amine-amide buffer system(Hardness 1 Buffer Solution). The buffer system keeps the EDTA mainly in the HY3 Ð

form by the reaction

H2X2 Ð

(aq) + OH Ð

(aq) → HX3 Ð (aq) + H2O(l)

where it complexes with Group 2 ions very well, but does not tend to react as readily with

other cations such as Fe3+

that might be present in the water sample. The chelating reactions between this form of EDTA and calcium or magnesium ions during the titration are

Ca2+(aq) + HX3 Ð (aq) → CaX2 Ð

(aq) + H+(aq)

Mg2+(aq) + HX3 Ð (aq) → MgX2 Ð

(aq) + H+(aq)

The 1:1 mole ratio between either Mg2+ or Ca2+ and HX3 Ð will be represented in calculations

below using the following mole ratio

1 mol CaCO3 / 1 mol EDTA

5. In Step 3 in the Calculations section, students are instructed to calculate total water hardnessof the sample by multiplying the EDTA titrant volume by a conversion factor of 20.0. Thisconversion factor is derived from the following equation

g1

mg1000massmolar Molarity

moles

moles

Vol

VolHardnessWater Total

3

3

3

CaCO

CaCO

CaCOEDTA

EDTA

EDTA××××=

g1

mg1000

mol1

g100

L1

mol0.0100

EDTAmol1

CaCOmol1

mL50

VolHardnessWater Total 3EDTA

××××=

20.0VolHardnessWater Total EDTA×=

test 14 total hardness

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