Experiment #9: Tap Water testing

Last updated 11-18-13

Testing Water & Soil
Experiment #9: Tap Water Testing

The water we use at home is probably one of the most important safety concerns we face. It's unfortunate, that polluted water can look the same as pure water. Cloudy or muddy water may just contain harmless sediment. (This image is from an article that talked about high levels of arsenic in a town's drinking water)

Your kit has all items needed for this experiment.

#1 &2: You will be using the plastic pipettes and the 10 milliliter graduated cylinder. These are in the pockets in the lid of your kit.

#3. You will be using the small 50 mL beaker for holding the tap water.

#4: You will be using five of the test tubes that have test papers in them. They will be pH paper 4.0-7.0, pH paper 6.5-10 (or as backup pH 1-14), Hardness, Chlorine, and Nitrate/nitrite.

#5: You will be using the Color Chart for Test Papers and Test Strips.

#6: This meter was added to the kit in the Fall of 2011. It provides a fast way of measuring the total dissolve solids (mostly salts) that is in water and beverages.

INSTRUCTIONS FOR EXPERIMENT

Fill the small 50mL beaker to about 40 mL.

Get these test papers to test the pH (acidity) of the tap water. Tap water is probably close to being neutral (pH 7), so we will use the two test papers that include pH 7 in their range. pH 4.0-7.0 will be good if the water is somewhat acidic. pH6.5-10 will be good if the water is somewhat alkaline.

Let's say we start off with the pH 6.5-10 paper. (If you happen to not have pH 6.5-10, use pH 1 to 14.) Dip the paper into the beaker with the tap water for just a couple of seconds. Then take it to the chart.

For my water, it looks like the pH is between 6.8 and 7.1. So it's looks pretty much neutral. We can now try the other test paper (pH 4.0-7.0) for confirmation.

This is the pH paper for the 4.0-7.0 range. Dip in the tap water for a couple of seconds and take it to the chart.

The 4.0-7.0 pH paper turns dark blue which indicates that it was close to the 7.0 pH reading. So it appears that the two different pH test papers both point to a pH close to the neutral pH of 7. This is usual for tap water unless there is something wrong with the water supply. (Write down your best estimate of the pH of your water)

Let's now do the Nitrate/Nitrite and Chlorine tests. I want to save the Hardness and Alkalinity tests for last.

Chlorine is heavily used by water treatment plants to sterilize the water. Usually some chlorine stays in the water so that the antimicrobial benefits of chlorine is present in the water all the way to people's homes.

Take the chlorine test strip and "swirl" it in the water 3 times. Swirling allows it to contact more of the chlorine that may be in the water. Then take it to the chart. Once you pull it out of the water, you are suppose to read it at 10 seconds. My water shows that it has very little chlorine in it. I'd feel a little more comfortable if it had maybe 0.5 to 3 parts per million (ppm) chlorine. (Write down your best estimate of the chlorine level in your water)

Nitrates and nitrites can get into the water supply from contamination from sewers, cattle yards, and agriculture fertilizers. For babies, this can be deadly. Babies' stomachs are less acid than normal allowing bacteria in the stomach to convert nitrate (NO3) into nitrite (NO2). Nitrite gets into the blood system and prevents the hemoglobin in red blood cells from absorbing oxygen. This results in the "blue" baby syndrome.

Now take the Nitrate/Nitrite test paper and dip it in your tap water for 2 seconds. Take it over to your chart and wait one minute before reading it. Hopefully, there won't be much nitrate or nitrite levels.

Since there are no pink or red colors on the test strips, I would say that thankfully my tap water has no nitrate or nitrite. (Write down your best estimate of the nitrate and nitrite levels in your water)

HARDNESS: I saved Hardness last because I know Mesa water, like Phoenix water is very hard. By hardness they mean that there's a lot of calcium dissolved in the water. When hard water dries, you see a lot of calcium salts left behind. When it dries on a window, it's all spotted. The salts are mostly calcium carbonate (chalk), calcium chloride (deicing salt), and sodium chloride (french-fries salt).

A hardness test is mostly measuring the amount of calcium in the water. An alkalinity test is measuring the amount of carbonate in the water. So both tests are targeting calcium carbonate; one measuring calcium and the other measuring the carbonate. So they both are very similar. In this lab we will just use the Hardness test (calcium test).

Take the hardness test paper and dip in tap water. Remove immediately and wait 15 seconds to read the value on the chart.

Note: The alkalinity test strips don't store well and I discontinued using them. They also measure about the same as what the Hardness strips measure, so they are somewhat redundant.

This is the result of the hardness test. The hue seems to match the 180 ppm, but the water makes it brighter. The hardness might be over 180 but we don't know because 180 is the highest level shown. One way around this is to dilute the tap water with pure water.

Use the plastic pipette and transfer the tap water to the smaller 10mL graduated cylinder. You will only transfer 2 millilters.

The graduated cylinder will get filled to the 2 mL mark with the tap water. You will now use your Purified Water to bring the water up to the 6 mL mark. This will make the hardness of the water three times less. Perhaps, the reading will show up lower on the chart.

When you add the purified water to the graduated cylinder, it's a good idea to hold it up to eye level to see the water level more accurately. Again, fill the graduated cylinder up to the 6 mL mark using your purified water. (Take a picture of yourself at this point in the lab.)

We will want to use the small beaker to hold our diluted tap water that's in the graduation cylinder. So pour out the tap water (it's easy to get more of that).

Dry the beaker.

Pour the tap water that has been diluted with pure water into the clean 50mL beaker.

Dip another Hardness test strip into the diluted tap water quickly and take over to the chart. At 15 seconds after dipping, check the colors.

The color of the test strip has definitely changed after the tap water was diluted with pure water. But I have to admit, it's a little hard to read. The hue of the test strip seems to be between the 60 and 120 ppm levels. The water on the strip makes it brighter than the colors on the chart, but the hue (tint) can be compared. The 180 ppm is a bit more purplish than the test strip and the 30 ppm is more turquoise than the test strip. So I would say the hardness is between 60 and 120. Now since the tap water had been diluted to three times its original volume (2 mL to 6 mL), the readings we get now are 3 times less than they should be. So the calculated hardness is between 3x60 (180 ppm) and 3x120 (360 ppm). When using other brands of test kits, I have seen the hardness around 360 ppm. (So do your best estimate from the chart and then multiply by three and record it.)

With hardness this high, there will always be problems with salt (also called mineral/scale) build up on faucets, showers, windows, and dishes. It isn't unhealthy because hardness is mostly due to calcium, which is good for your teeth and bones. It's just not good for cleaning. Here you see some of the cleaning products I use because of the hard water. Lime-A-Way is made of phosphoric acid which dissolves the calcium carbonate residue. Scrubbing Bubbles dissolves the soap scum that calcium causes when mixed with soap. The two smaller bottles are for the dishwasher. They contain phosphates which capture calcium ions and keep them from combining with the detergent or forming calcium carbonate (scale)

A water softener helps, but as a later experiment shows, you still have the same amount of dissolved salts. The calcium gets replaced by sodium, which works better with soaps and doesn't form scale that is hard to wash away. The sodium, however, is not that healthy to drink.

The hardness test strips only measure the calcium and magnesium ions in the water. This meter measures all salts in the water. It measures Total Dissolved Solids or TDS for short.

Applications of Total Dissolved Solids

River, lake, and stream testing.

Pure water has nothing dissolved in it. So pure water has zero total dissolved solids. However, when minerals, salts, and pollutants dissolve in water, then the total amount of these dissolved solids gives an indication of the water's quality. The Environmental Protection Agency, for example, would measure total dissolved solids (TDS) in lakes, rivers, and streams to monitor water quality.

Swimming pool and spa maintenance.

High TDS indicates hard water, meaning there are a lot of dissolved minerals that will form scale (white crusty mineral deposits made mostly of calcium carbonate) on the sides of the swimming pool or spa and the insides of pipes. Monitoring TDS can allow intervention before scale forms.

Agriculture and hydroponics (hydroponics is the science of growing plants without soil).

Moisture in soil that has high salt levels will not move into the plants' roots, causing drought symptoms even when there is plenty of water present. A TDS meter can see if the water for the plants is too salty.

Using a meter that measures TDS is also useful in keeping track of the level of nutrients in the water. Most of the nutrients for plants increase the TDS levels (more nutrients are dissolved). So in these cases, a high TDS level indicates plenty of nutrients are present. This meter can also check the quality of water that is being brought in to water the plants. In looking at water coming in, high TDS might indicate too many minerals (hard water). So the grower may want to lower the TDS by removing the excess minerals before adding nutrients.

The TDS meter is on the left. An electrical conductivity (EC) meter is on the right. The EC meter is basically measuring the same thing but reports values using different units of measurement.

Aquarium maintenance

TDS (total dissolved solids) is a measurement that can help track the levels of dissolved waste and dissolved minerals. When TDS levels are too high, the aquarium water is pumped through various filters to remove the dissolved waste and dissolved minerals. These filters are often reverse osmosis membranes that allow water to pass but little of anything else.

Aquaculture

Aquaculture is the farming of fish, oysters, or seaweed in controlled environments.

TDS levels are monitored because high levels of TDS can kill young fish.

Water treatment plants and home water use

Water that has high TDS values will taste salty, metallic, or bitter. The Environmental Protection Agency (EPA) sets the maximum level of total dissolved solids for drinking water to be 500 milligrams (half a gram) of dissolved solids for every liter of water.

These dissolved solids are removed using reverse osmosis membranes. As mentioned earlier these membranes allow water to pass through but block large atoms, larger compounds, and microscopic particles that make up dissolved solids. These membranes also block toxic metals and other toxic substances.

Reverse osmosis systems are used in the home, water plants, and at places that dispense drinking water (usually at 25 cents a gallon)

Principles of Total Dissolved Solids
Source and make up of dissolved solids

Rain water has no dissolved solids. So it has zero TDS. However, when it contacts the ground, the rain will dissolve fertilizers, salts and minerals, animal waste, pesticides, plus other chemicals that may be on the ground from cars and industrial pollution. Even decaying plants have chemicals that get dissolved. Water can also pickup more solids as it passes through copper pipes. So these dissolved solids can be a multitude of chemicals.

The most common chemicals counted in TDS tests are salts like sodium chloride (table salt), calcium chloride (salt placed on icy roads) and fertilizers like ammonium nitrate, various phosphates, and various potassium salts (potassium carbonate, potassium chloride, potassium sulfate). There are also dissolved minerals like calcium carbonate (limestone) or magnesium carbonate and calcium sulfate (gypsum/drywall material) or magnesium sulfate (Epsom salts).

There are thousands of other chemicals in our water, but TDS looks at all of them as one group (one reading).

Lab 9: Finding TDS using a TDS meter

TDS meters are popular because they are easy to use. There's just a few things to keep in mind in order to get accurate readings.

This is the TDS meter which is in your kit. The readout screen shows TDS in parts per million (meaning grams of dissolved solids in a million grams of water). Note, that the same numbers can also be read as milligrams per liter.

The top blue button turns on the meter. Use the same button to turn it off. If left on too long, the meter will turn itself off.

The bottom blue button freezes the readout so that you can remove the meter from the water and the readout doesn't change.

The meter needs to be submerged between the green line and the red line. Do NOT submerge the meter any lower in the water than where the red line is. The TDS meter is not water-proof.

The solution needs to cover the metal posts that measure the conductivity. If the meter is dipped to the green line, then the metal posts are covered.

Here is a view of the metal posts that are at the bottom of the meter. Again, that's were the conductivity of the water is measured. Recall that the conductivity is converted to a TDS reading in parts per million (also read as milligrams per liter).

To take a reading, simply dip the probes into the water being measured. Stir meter gently so that any bubbles clinging to the metal posts are dislodged. If you can see the screen, then you can get the reading for Total Dissolved Solids. If it's hard to see the screen, press the lower blue button once to freeze the reading on the screen, then bring the meter up close to you to read the screen.

water level is 50 mL in beaker for TDS meter

When testing, you need a little over 50mL in the larger (250 mL) beaker in order for the probes on the TDS meter to be covered. Remember, you don't ever want it deeper than 150 mL. So the water level should be between 50 mL and 100 mL. Read the mL values on the right side.

The first thing to test is your tap water.

Fill the larger beaker (250mL beaker) between 50 and 100 mL with tap water from the cold water side.

Place the TDS meter into the beaker, and place the thermometer from the kit along side the TDS meter. Turn on the TDS meter (top blue button) and record the TDS reading. Now record the temperature.

The TDS meter is calibrated to be accurate at 25°C or 77°F. If your tap water is different from 77°F, then there needs to be an adjustment for the different temperature. In this example, the reading was 492 mg/L. The temperature was 86°F (30°C). That is over 77°F, so we need to compensate for the temperature difference by subtracting the adjustment value. The temperature difference is 86°F-77°F, which is 9°F. Multiply that by 0.01 to get 0.09. Multiply 0.09 times 492 mg/L to find the adjustment value of 44.28. Since 86°F is over 77°F, we subtract the adjustment value (492-44.28) to get 448 mg/L.

Compensation x temp difference x reading = adjustment value

0.01 x 9°F x 492 mg/L= 44.28 to be subtracted from 492
1 °F

492 mg/L - 44.28 = 447.71 mg/L which is rounded to 448 mg/L as corrected TDS value.
Remember, if your temperature is below 77°F, then you would add the adjustment value.

Report the TDS value that you read off of your meter.
Report the temperature of your water.
Report the TDS value corrected for the temperature of your tap water.

Compare the corrected reading you calculated with the readings in the below chart. What category does your tap water fall into?

Ideal Drinking water from reverse osmosis, distillation, deionization, microfiltration, etc..	0-50 PPM
Often considered acceptable range for carbon filtration, mountain springs or aquifers.	50-140 PPM
Average tap water.	140-400 PPM
Hard water.	170 PPM or above
Less desirable	200-300 PPM
Unpleasant levels from tap water, aquifers or mountain springs.	300-500 PPM
The EPA's maximum contamination level.	500 PPM
Salty tasting water that exceeds EPA's maximum contamination level.	> 500 ppm (mg/L)

Now check the TDS level of a beverage like a soda, juice, tea, coffee, or beer. Warning: If you choose a carbonated beverage, you will probably notice the readings jumping up and down. What do you think would cause that?

If you said, "Bubbles," then you are correct. The carbonation is releasing bubbles of carbon dioxide. These stick to the metal probes. Electricity will not pass through these bubbles, so the conductivity is lowered. As the bubbles pop, the conductivity will go up. So the reading will be erratic. You have to wait until the beverage loses its carbonation. As you know, as these carbonated drinks warm up or get shaken, then the carbonation will be lost. So do that before you take a TDS and temperature reading if you are using a carbonated beverage.

Rinse out the 250 mL beaker and fill it between 50 mL and 100 mL with a beverage of your choice. Put the thermometer in the beaker too. Record the temperature and the TDS value, which is in mg/L (ppm). To get the reading corrected for temperature, do the same calculations mentioned above and also report the TDS value corrected for temperature.

If your beverage was cold, you will notice the TDS values going up as the liquid temperature goes up. That doesn't mean the TDS values are changing because the dissolved solids in the beverage are not changing. It just means the warmer TDS values will have less adjustment for temperature.

To be accurate, record the temperature and the TDS values close to the same time.

In this example, the TDS value is 487. The temperature was 52°F (11°C). This is below the 77°F standard temperature, so we need to add an adjustment value. First find the difference in degrees. 77°F- 52°F= 25°F

Multiply 25°F x 0.01 to get 0.25. Then multiply 0.25 x 487 mg/L to get 121.75. Since our liquid is colder, we need to add this adjustment value of 121.75. So 121.75 + 487 equals 608.75 mg/L, which is rounded to 609 mg/L. So that's the true TDS value which has be found by compensating for the temperature difference. Your TDS values and temperatures will be different, but this shows how the math is done.

What was the TDS value on your meter? What was the temperature of the beverage? What is the true TDS value after corrected for the temperature difference?

Take a picture of yourself holding the beverage that you used. You can use a digital camera or the camera on your phone. Make sure your face is visible. Also, report what kind of beverage you tested.

Email the results you got to me along with the one picture. chm107pc@chemistryland.com

Since March 29, 2004

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