How Healthy is Your Water? Objectives: To learn how to analyze water for total dissolved solids, alkalinity and phosphates

pH of a water sample Acid Rain CO2 dissolves in rain water Low pH water dissolves more minerals (inc Al3+, Pb2+ …) Dissolves the CaCO3 in the shells of crustaceans Disturbs ion balance in fish which lose Na+ through the gills and may ultimately die Little Echo Pond in Franklin, New York. Little Echo Pond has a pH of 4.2

pH tolerance

pH of drinking water Humans and mammals can have a bigger tolerance to the pH of water Soda drinks have pH 2-4. Public water pH 4-9 Public Health Service Act states water needs to be in the range 6.5-8.5 Low pH increase the solubility of Pb, Cu, Zn and Fe metals in pipes!

Alkalinity 1 drop (0.05mL) of 0.1 M HCl added to 250 mL of distilled water changes the pH from 7 to 4 Natural waters are protected from drastic changes in pH by natural buffers The capacity of water to neutralize acid is called its alkalinity

Alkalinity continued … carbonates, bicarbonates and hydroxides are anions responsible for alkalinity Originate from calcite (CaCO3), magnesite, (MgCO3), dolomites, and brucite (Mg(OH)2) Typical neutralization reactions will be

Day 1 4 tasks to complete today Determine the pH of the water Filter the water to remove bacteria Measure the Alkalinity Initiate the determination of the total dissolved solids in your water

Procedure for Determining pH Use a pH sensor with the Science Workshop Interface, along with phmeters.sws in the Chem 1B folder on the desktop Calibrate with pH 4 buffer and then pH 7 buffer Determine the pH of your 50-75 mL of your unfiltered sample in a 100 mL beaker

Filtering your Water Sample Before continuing with the analysis you will need to remove any bacteria that could interfere with the subsequent tests This is done in 2 stages Pre-filter about 300 mL with a Buchner funnel first Then filter this 300 mL using a 0.45μm filter

Total Dissolved Solids (TDS) The total amount of dissolved chemical species in water = total dissolved solids Good measure of the concentration of ionic substances present Seawater = 30,000 – 40,000 mg/L Freshwater < 1500 mg/L If an organism is placed in water with low salinity (distilled water) the cells absorb water (osmosis) If an organism is placed in water with high salinity the cell loses water (osmosis) – if it loses too much it may die. High TDS makes water cloudy, inhibits photosynthesis, heats up the water Humans contribute to TDS via runoff from industry, construction, agriculture, logging, sewage treatment discharge

Gravimetric Determination of TDS Label a clean 150 mL beaker with the sample number and your name Place it in a drying oven for 1 hour Then place beaker in dessicator until cool weigh beaker Place 50 mL of your filtered water in the beaker and heat just below boiling reducing the volume from 50  10 mL Place beaker in over at 103-105oC till Monday

Measuring Alkalinity Best done by titrating the water with a strong acid (H2SO4) To make life simple assume alkalinity due mainly (solely) to CaCO3 Titrate 100mL of your water with 6 drops of bromocresol green with 0.01M H2SO4 Titrate till indicator = light green pH = 4 (pH of carbonic acid)

Day 2: Determining Phosphate Concentration Ammonium molybdate (NH4)2MoO4 reacts with phosphates to form molybdophosphoric acid Which in turn reacts with ammonium metavanadata to form the yellow vanadomolybdophosphoric acid complex H3+nPVnMo12−nO40: PVn The intensity of the yellow is proportional to the phosphate concentration

Determining Phosphate Concentration Standardize the Spectrometer Using the 20 ppm (mg/L) phosphate standard provided make 25 ml of 1.6, 4.8, 8.0 and 11.2 ppm solutions of the phosphate, (these require 2.00, 6.00, 10.00 and 14.00 mL of the 20 ppm phosphate which can be dispensed from one of the two burettes set up in the lab Put 25.00 mL of each solution into a 50 mL volumetric flask add a drop of phenolphthalein if pink at 1 drop 6M HCl Add 10.00 mL of the vanadate-molybdate solution and fill to the mark with DI water If pink add 6M HCl drop by drop till clear, then fill up to the calibration mark with DI water Wait 10 mins for the color to develop Measure the absorbance at 420 nm Construct an absorbance vs. concentration curve in Excel and fit a linear trendline Compare with your sample, by following these steps with 25 mL of your water instead of 25.00mL of the phosphate standard

calcium carbonate (ppm) Day 2 Water Hardness Water hardness is measured as the sum of calcium and magnesium ions The [Ca2+]and [Mg2+] can be determined by titration with EDTA Water Hardness calcium carbonate (ppm) designation 0-43 Soft 43-150 Slightly Hard 150-300 Moderately Hard 300-450 Hard 450 Very Hard

Determining Water Hardness Water hardness can be determined by titrating our water with EDTA The acid can lose 4H+ and becomes a chelating agent that is hexadentate, complexing with Ca2+ and Mg2+ The endpoint of the titration is when all of the Ca2+ and Mg2+ are complexed with EDTA To see this we use a special indicator Eriochrome Black T, which forms a very stable wine-red complex, MgIn–, with the magnesium ion At the end the Mg2+ can no longer complex with the indicator and the solution turns blue-violet

Determining Water Hardness The titration is carried out at pH = 10 to ensure that we have EDTA4+(aq) in solution EDTA4+(aq) + Ca2+(aq)  CaEDTA2+(aq) EDTA4+(aq) + MgIn-(aq)  MgEDTA2+(aq)+ In-(aq) But for the indicator to work there is a little Mg in the indicator to start with, so the end point for us is when it turns violet, not sky blue End-point

Determining Water Hardness 2 titrations Titrations should agree to within 5% if not do a third titration Moles EDTA = Moles Ca2+ 𝑀 𝑒𝑑𝑡𝑎 × 𝑉 𝑒𝑑𝑡𝑎 = 𝑀 𝐶𝑎 ×5.00𝑚𝑙 𝑀 𝐶𝑎 = 𝑀 𝑒𝑑𝑡𝑎 × 𝑉 𝑒𝑑𝑡𝑎 5.00𝑚𝑙 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠= 𝑀 𝐶𝑎 ×100.0869 𝑔 𝑚𝑜𝑙 EDTA 250 mL 5.00 mL your water 50 mL DI water 3.0 mL pH 10 buffer 1 mL 1:1 Mg:EDTA 6 drops indicator

Ion Exchange Pour a large test tube of resin into a 250mL beaker Add 50 ml 6M HCl Stir for 5 mins Decant liquid into waste container Add 100ml DI water stir and decant Check pH of water with pH paper in it is pH < 4 repeat last step, keep washing till the pH of the decanted water is about 4 Place 2 cm of glass wool in bottom 25 mL burette Transfer contents of beaker into burette washing with plenty of DI water (you may want to have tap open to prevent over filling of the burette. Make sure you always have 1cm or more of water above the resin Your resin should be about 15 cm in length when settled Wash with 30 mL DI water – the water’s pH > 4 Pipette 5.00mL of your water into the burette and leave it 5 mins with the tap close Collect water in a clean dry 250 mL Erlenmeyer flask, when it has drained to surface of resin, start adding DI water, and allow a total of 50 mL to drain through into the flask Titrate the water in the flask with NaOH and phenolphthalein to determine [H+]