Objective 1: Learn the meaning, applications, and measurement techniques for "total dissolved solids".
Basic Meaning: "Total Dissolved Solids (TDS)" is the concentration of the dissolved chemicals in a sample of water. Before dissolving, these chemicals could have been a solid or a liquid.
Objective 2: Determine the TDS (Total Dissolved Solids) value of various samples using both of the below methods.
2a. Find TDS using a gravimetric method.
Gravimetric means "by weighing". Balances require gravity to weigh something. You will weigh the total dissolved solids after water is boiled away. This will be done using just one water sample.
2b. Find TDS using electrical conductivity.
This is rather easy to do. Just dip the TDS probe into the water and the TDS meter will measure how well the water conducts electricity. It then converts that to concentration 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 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 sides of pipes. Monitoring TDS can allow intervention before scale forms.
Agriculture and hydroponics (hydroponics is the science of growing plants without soil).
Using a meter that measures TDS is 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.
![reverse osmosis plant]](reverseOsmosis2.jpg)
Water treatment plants and home water use
Water that has high TDS values will taste bad. 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 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 filter out toxic metals and other toxic substances.
Reverse osmosis systems are used in the home, water plants, and at places that dispense drinking water at 25 cents a gallon.
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, potassum 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).
How is TDS measured?
Gravimetric method: To measure TDS using this method, the water sample is first passed through a filter that blocks anything bigger than 2 microns ( 2 micrometers or 2 millionths of a meter). This ensures the test measures dissolved solids not solids suspended in the water. Such things as sediment or specks of plant material are filtered out and therefore not counted in the "total dissolved solids".
Gravimetric method continued: A certain amount of the filtered water is then weighed out and the water is boiled away leaving the dissolved solids behind as a solid residue. This residue is weighed. This is called the gravimetric method because a balance is used. Balances need gravity to find the mass. So that's why it's called a gravimetric method.
In this lab you will be doing something similar, but instead of a Bunsen burner you will use an alcohol burner. Instead of a glass beaker, you will use an metal can.
Electrical Conductance Method: The other way TDS is tested is by measuring how well the water sample conducts electricity. If the dissolved solids are salts and minerals, they would have dissolved into plus and minus ions. This allows electricity to pass through the water. For example, sodium chloride (NaCl) becomes Na+ and Cl- when dissolved in water. The negative chloride ions (Cl-) will be attracted to the + side of the battery terminal to give up their electrons, and the positive sodium ions (Na+) will be attracted to the negative terminal to pick up electrons. In this manner, the ions will allow electrical current to pass through the water. The more electrical current flows through the solution if there are more ions in the solution. More ions mean there are more dissolved solids, so TDS goes up.
Electrical conductance is the measurement of how much a material (liquid, solid, or gas) conducts electricity.
Electrical Conductance Method: Measuring electrical conductance with a meter is a quick way to get a measurement of the total dissolved solids. There are two meters of this type. There is an electrical conductance meter (EC meter) and a TDS meter.
The electrical conductance methad does miss chemicals like sugar, alcohol, glycerin, and antifreeze. These chemicals do not form ions and therefore do not help conduct electricity, so they aren't measured using one of either of these meters; however, chemicals like these can be measured using the gravimetric method by weighing the water sample before, during, and after the water is boiled away.
TDS is sometimes measured in parts per million (ppm). In this situation, parts of dissolved solids would mean grams of dissolved solids found in a million grams of the water. If the "parts per million" is divided by 1000, grams of dissolved solids become milligrams, and a million grams becomes 1000 grams. So TDS is also expressed as milligrams of dissolved solids for every 1000 grams of the water sample. Since 1000 grams of water is the mass of 1 liter of water, TDS is often expressed as milligrams (mg) of dissolved solids per liter of water. Notice that the screen on this meter says "mg/L". Also, the buttons have mg/L.
Here are the concentrations written as fractions. The fraction bar is the "per".
parts = grams ÷ 1000 = milligrams = milligrams
million 1,000,000 grams ÷ 1000 1,000 grams Liter
The math shows us going from the generic"parts per million" to a more specific "grams per million grams." This is reduced by dividing numerator and denominator by 1000 to get milligrams per 1,000 grams. Knowing that a liter of water weighs 1000 grams, "Liter" replaces the 1,000 grams. So if you are given the concentration of total dissolved solids as 435 ppm, you can change that to 435 milligrams per Liter.
http://www.china-total.com/Product/meter/PH-meter/TDS-meter.htm
Using a multimeter on the ohm setting is a substitute for a dedicated TDS meter. This requires more math. Remember, an oh meter measures ohms, which is a measure of electrical resistance. TDS is related to electrical conductance. Conductance is the reciprocal of electrical resistance. For example, if something measures out as 100 ohms, the electrical conductance is 1/100 or 0.01. Sometimes electrical conductance is measured with a unit called a mho (ohm spelled backwards). However, the more official unit for electrical conductance is called "siemens" after German inventor Ernst Siemens. Therefore, you would take the ohm reading on the meter and divide that into 1 to get siemens (electrical conductance). In math, that is 1/ohm = siemens.
Conductivity measurements are temperature dependent. The degree to which temperature affects conductivity varies from solution to solution and can be calculated using the following formula:
Gt = Gtcal {1 + a(T-Tcal)}
where: Gt = conductivity at any temperature T in °C, Gtcal = conductivity at calibration temperature Tcal in °C, a = temperature coefficient of solution at Tcal in °C.
| Substance at 25°C | Concentration | Alpha (a) |
| HCl | 10 wt% | 1.56 |
| KCl | 10 wt% | 1.88 |
| H2SO4 | 50 wt% | 1.93 |
| NaCl | 10 wt% | 2.14 |
| HF | 1.5 wt% | 7.20 |
| HNO3 | 31 wt% | 31.0 |
One website said their TDS meter had manual temperature (beta) of 2%/ deg C. I've seen others that allow you to adjust that compensation from 0% to 6%/degC. So it seems to depend on the type of solutions you have and how you calibrate them.
The EC meter (electrical conductivity meter) measures in siemens (named after German inventor Ernst Siemens). On this meter the readout is 1220 and notice the unit is µS, which means microsiemens (one millionth of a siemens).
Siemens is directly related to the total dissolved solids. Using a calibration solution, these meters can be adjusted to read the proper electrical conductance (in siemens) or the proper TDS (total dissolved solids measured in parts per million or milligrams per liter).
A TDS meter is actually an electrical conductivity meter (EC meter) that calculates TDS from the siemens value and reports the TDS concentration in mg/L. That's why you see meters that are dual TDS and EC meters.
1. Once a solid is dissolved in water, is it still solid?
2a. If sugar is dissolved in water, does it get measured using a TDS meter?
2b. If sugar is dissolved in water, does it get measured using the gravimetric method for finding total dissolved solids?
3. What is the TDS level in snow?
4. If you take tap water and make ice cubes with it, does the TDS level change?
5. Would a bottling company like Coca-cola do TDS testing? (Why or why not?)
6. If a full glass of water sat outside in the sun for few days and half of the water evaporated, does the water in the glass now have higher, lower, or the same TDS level as the water when the glass was full?
7. Sodium nitrate is a common fertilizer. In water, this solid disassociates into Na+ and NO3-. Will these ions reduce or raise electrical conductance in water?
8. Which would have a higher TDS reading, water from a freshwater lake or water from the ocean?
(answers are at the very end of this page).
The basic approach here is simple. You will need to weigh out a known amount of water. Then boil the water leaving a solid residue. The water actually doesn't have to boil, it just needs to evaporate. In Phoenix, that's not hard to do either. The boiling is just to speed up the evaporation of the water so you can see and weigh the solid residue sooner.
Locate the metal can in your kit and weigh it to the nearest one hundredth of a gram. Record that mass.
Locate the bottle labeled "Salt solution". Use this solution to fill the metal can to about 1/4 full. Weigh the can again with the salt solution in it. Record that mass of the can plus the salt solution. Of course, you can subtract the can's mass from this mass to find the mass of just the salt solution.
Earlier I said that the water to be tested for TDS using gravimetric methods needs to be filtered through a 2 micron (2 micrometers or 2 millionths of a meter) filter. That has already been done.
Place the cap back on by turning it a quarter turn clockwise. Set the container on the piece of tile from the kit. The container does get hot so the tile will protect the table the alcohol burner is sitting on. Place the metal wind shield on the top of the container. Remove the rubber stopper that blocks the holes in the copper coil.
Set the metal stand over the alcohol burner and place the metal can with the salt solution on the metal stand.
Using matches or the gas barbecue lighter, light the alcohol burner. If it doesn't light right away, swirl the alcohol burner gently to get some liquid and vapors to go into the copper coil, then try lighting it again.
It will take about 20 minutes for the water to boil away and leave the salt behind. Like any fire, be sure to keep an eye on it. After the water has boiled away, blow out the flame of the alcohol burner. Allow about 3 minutes for the metal can to cool down before attempting to weigh it.
Turn on the digital balance and make sure it reads 0.00. If not, press the TARE button. Place the metal can with the salt residue onto the digital balance. Record the mass of the can plus salt residue.
You now have enough information to find the TDS value of this salt solution.
(Metal can + water) mass - (metal can) mass = water's mass
(Metal can + residue) mass - (metal can) mass = Residue mass.
Total dissolved solids is a concentration measurement. All concentrations are given as the amount of the solute (dissolved material) per set amount of solution. The "per" is indicated with a fraction bar "/". So solute/solvent (solute divided by solvent) is a measure of concentration. For example, if the residue (which is our total dissolved solids in this sample) was 1.25 grams and the mass of the salt solution was 45.78 grams, our concentration would be: 1.25grams solute/45.78 grams solvent. In other words, the concentration is 0.85 grams total dissolved solids for every (per) 45.78 grams of salt solution. This concentration is accurate but not standardized. For TDS, they want the total dissolved solids dissolved in 1000 grams of salt solution. 1000 grams of the solution normally assumed to be one liter (which is pretty close).
Example:
Solute = 0.25 g solute = 0.00584 g solute x 1000 = 5.84g = 5840mg
Solution 45.78 g solution 1 g solution 1000 1000 g 1 Liter
The total dissolved solids in this example is 5840mg/L. That's very high. Anything over 5,000 mg/L is considered saline water (salt water); however, saltwater aquariums are usually around 35,000mg/L in salt concentration. That could also be written as 35 grams/Liter or 35grams/1000grams or 35 ppt (parts per thousand).
TDS meters are popular because they are easy to use. There's just a few things to keep in mind..
| Taste/Health | High TDS results in undesirable taste which could be salty, bitter, or metallic. It could also indicate the presence of toxic minerals. The EPA's recommended maximum of TDS in water is 500mg/L (500ppm). |
| Filter performance | Test your water to make sure the filter system has a high rejection rate and know when to change your filter (or membrane) cartridges. |
| Hardness | High TDS indicates Hard water, which causes scale buildup in pipes and valves, inhibiting performance. |
| Aquaculture | A constant level of minerals is necessary for aquatic life. The water in an aquarium should have the same levels of TDS and pH as the fish and reef's original habitat. |
| Hydroponics | TDS is the best measurement of the nutrient concentration in a hydroponics' solution. |
| Pools and Spas | TDS levels must be monitored to prevent maintenance problems. |
| Commercial/Industrial | High TDS levels could impede the functions of certain applications. |
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.
The common laboratory conductivity meters employ a potentiometric method and four electrodes. Often, the electrodes are cylindrical and arranged concentrically. The electrodes are usually made of platinum metal. An alternating current is applied to the outer pair of the electrodes. The potential between the inner pair is measured. Conductivity could in principle be determined using the distance between the electrodes and their surface area using the Ohm's law but generally, for accuracy, a calibration is employed using electrolytes of well-known conductivity.
Industrial conductivity probes often employ an inductive method, which has the advantage that the fluid does not wet the electrical parts of the sensor. Here, two inductively-coupled coils are used. One is the driving coil producing a magnetic field and it is supplied with accurately-known voltage. The other forms a secondary coil of a transformer. The liquid passing through a channel in the sensor forms one turn in the secondary winding of the transformer. The induced current is the output of the sensor.
The conductivity of a solution is highly temperature dependent, therefore it is important to either use a temperature compensated instrument, or calibrate the instrument at the same temperature as the solution being measured. Unlike metals, the conductivity of common electrolytes typically increases with increasing temperature.
Over a limited temperature range, the way temperature affect conductivity of a solution can be modeled linearly using the following formula: (see wikipedia http://en.wikipedia.org/wiki/Electrical_conductivity_meter
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1. Some dissolved solids come from organic sources such as leaves, silt, plankton, and industrial waste and sewage. Other sources come from runoff from urban areas, road salts used on street during the winter, and fertilizers and pesticides used on lawns and farms.
2. Dissolved solids also come from inorganic materials such as rocks and air that may contain calcium bicarbonate, nitrogen, iron phosphorous, sulfur, and other minerals. Many of these materials form salts, which are compounds that contain both a metal and a nonmetal. Salts usually dissolve in water forming ions. Ions are particles that have a positive or negative charge.
3. Water may also pick up metals such as lead or copper as they travel through pipes used to distribute water to consumers.
Analytical Balances and other digital balances
What Are Total Dissolved Solids - TDS?
1. "Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in water other than the pure water (H20) molecule and suspended solids. (Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as wood pulp.)
2. In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water.
3. Parts per Million (ppm) is the weight-to-weight ratio of any ion to water.
4. Conductivity is usually about 100 times the total cations or anions expressed as equivalents. Total dissolved solids (TDS) in ppm usually ranges from 0.5 to 1.0 times the electrical conductivity.
Water can be classified by the amount of TDS per litre:[8]
- fresh water < 1500 mg/L TDS
- brackish water 1500 to 5000 mg/L TDS
- saline water > 5000 mg/L TDS
E l e c t r i c a l c o n d u c t i v i t y i s m e a s u r e d i n
millisiemens per centimeter (mS/cm) or millimhos
per centimeter (mmhos/cm). Some EC meters or
combination pH/EC meters express salt concentration in total dissolved solids (TDS). This is expressed as
parts per million (ppm), which is mS/cm multiplied
by 640 or 700. This and other conversion values are
listed in Table 3.
pH and EC values are only as good as the last
calibration and proper storage of the standards is
important. Most pH meters require two pH calibration
solutions of 7.0 and 4.0 (the range of most soilless
substrates), and EC meters generally
require one standard. If possible, use a
calibration solution reported in mS/cm
as most meters and analytical labs
report values in mS/cm. However,
converting values from ppm or µS/cm
(Table 3) is relatively easy. Always
store the meters and the solutions at
room temperature. Avoid fluctuating
temperatures and high humidity (do
not store in the greenhouse). Throw
away calibration solutions after use.
The solutions should never be recycled
back into the fresh solution container
EC is measured by an EC meter, which is also known as a solubridge. These meters normally look like pH meters, but instead of the glass bulb electrodes, EC meters have metal prongs or bridges to measure the electrical current that passes between them.
Before placing an object onto the weighing pan, "tare" the balance by pressing the TARE button. A different model Mettler balance uses the symbol "O/T" (meaning zero or tare).
The word "tare" is from Arabic "tarah" meaning to throw away. In essence by pressing the tare button, we are telling the balance to throw away any weight on the balance so we get a zero starting point.
Roll cursor over image on the right to see the other Mettler balance.