LaMotte Test Methods for Soil

Titrimetric

Titrimetric tests can be used to determine the concentration of a substance in a sample solution. After the sample is treated with an indicator, a standard titrant is added until a color change indicates a completed reaction. LaMotte offers four separate types of titration methods, allowing a choice of precision and convenience.

Automatic Buret

The self-zeroing automatic buret is calibrated from 0 to 10 mL in 0.1 mL increments. It is available with a squeeze valve (pinchcock), glass stopcock, or Teflon® stopcock.

Direct Reading Titrators

The Direct Reading Titrator is a 1.0 mL microburet calibrated to allow direct reading of the test result. Each Titrator has a specific range, but may be refilled to test higher concentrations.

Dropper Pipet

The drop count test uses a pipet to provide fast, precise measurements in the field. The number of drops used before the color change is multiplied by a given number to produce the test result.

Dropper Bottle

The dropper bottle test uses bottle tips which deliver a consistent standard drop size to add titrant to the sample. As with the drop count test, the number of drops used to complete the reaction is multiplied by a given equivalence factor to determine the concentration.

Colorimetric:

There are two basic types of colorimetric tests:

1. Tests which determine the concentration of a substance are based on Beer's Law. Simply stated, this says that the higher the concentration of a substance, the darker the color developed in the test, so more light is absorbed by the sample.
   
2. pH tests use an indicator which changes color with changes in the concentration of hydrogen ions, or the acidity of the solution.

Visual Methods

LaMotte offers several visual colorimetric comparators:

The Octet Comparator and Octa-Slide contain eight color standards with built-in filters which eliminate optical distortion. The color standards in the Octet Comparator are arranged so that the sample can be compared to four standards at once. In the Octa-Slide, the standards are placed in a bar so that they can be compared to the sample individually.

There are two accessories which may be used with an Octet Comparator. The Bi-Color Reader (Code 2150) neutralizes sample color and/or turbidity to give more accurate readings. The Axial Reader (Code 2070) uses a mirror to extend the viewpath and intensify faint colors of low concentrations for easy distinction. Both accessories attach directly to the comparator and come with complete instructions for use.

LaMotte also offers an economical alternative to the Octet Comparator and Octa-Slide. The ColoRuler consists of plastic color standards mounted on a white plastic panel. The reacted sample is held against the panel and compared to the standards.

Electronic Methods

Electronic colorimeters measure the amount of light which travels through the reacted sample, and convert the measurement to an analog or digital reading as ppm or %T. In addition to colorimeters, LaMotte manufactures instruments to test pH, TDS/conductivity, dissolved oxygen, and turbidity.

Colorimeters - How Do They Work?

Many of the substances in water which must be measured are colorless and so they are invisible. So, how do you determine how much of that substance is there if you can't see it? A common method for testing how much of a substance is in the water is to run a colorimetric test. A colorimetric test is a test which forms a color. The amount of the color is then measured. In most tests the more color formed, the more of the test substance there is in the water.

But how do you measure the color which has been formed? You could look at the color with your eyes and compare the color formed with some standards. This is a visual measurement of the color. Visual methods can give very good results when applied in an appropriate manner. However, visual measurements of color are subjective. Everyone's eyes are different. Some people are color blind. Colors might look different when there is different lighting.

When your testing protocol requires the utmost accuracy and precision these problems must be solved. How do you do that? You use a colorimeter.

LaMotte colorimeters were developed to make measuring color differences in water easy. The detectors in these colorimeters are very sensitive and can detect very small differences in color. These measurements do not depend on the eyes of the person making the measurement. The intensity and color of the light used to make measurements are carefully controlled with electronic circuitry and light filters. This eliminates problems caused by different lighting situations.

To use a colorimeter for colorimetric testing, the color being measured must be proportional to the concentration of the substance being tested. The greater the intensity of the color observed, the greater the concentration of the substance. Beer's Law is used to make this relationship quantitative.

Beer's Law says that the absorbance measured with the colorimeter depends on three things: 1) the substance being measured; 2) the concentration of the substance; 3) the pathlength or distance the light travels through the substance.

Absorbance is the measure of how much light is absorbed by a substance compared with a blank (usually clear, colorless water).

The substance determines the color of light which is absorbed and consequently, the color which must be measured. When white light passes through a colored substance, the substance absorbs one color and transmits the rest. Your eyes see the transmitted color. You must measure the absorbed color since that is the color of the light which interacts with the substance being tested. For instance, a blue substance absorbs yellow light and transmits blue. You see the blue color but yellow light is used to measure the absorbance of the blue substance. LaMotte colorimeters use special light filters to select the color of light used to measure the absorbance.

As mentioned above, the concentration of the substance determines how much colored light is absorbed. The more substance present the more light is absorbed. LaMotte colorimeters are calibrated to determine concentrations directly.

The longer the distance (the pathlength) that the light travels through the substance the more light is absorbed. This means that a calibration for a colorimetric test must always be performed using the same pathlength. All LaMotte colorimeters have a fixed pathlength for each test.

LaMotte colorimeters apply the principles of Beer's Law to give you the result you want. The concentration, the color of light, and pathlength are set for the colorimetric test being performed. You measure the absorbance. The concentration is given from a calibrated look-up table, a calibrated direct-reading scale or a calibrated direct-reading display.

LaMotte's SMART Colorimeter uses innovative microprocessor technology to give the analyst 42 colorimetric tests at the press of a button. Plus, the analyst can enter up to 25 of his own colorimetric tests. The meter's memory will store 500 test results.

Maintenance Tip

Before inserting the colorimeter tube into the colorimeter, wipe it with a lint-free tissue to remove water drops, fingerprints, dust, and other particles. This simple step will reduce errors that fingerprints and dust cause, and help assure accurate results. Additionally, removing water drops will reduce the possibility of water damaging the meter components.

Testing Hint

Before starting a test or series of tests, run a reagent blank. To run a reagent blank simply substitute distilled or deionized water for the sample, and add the reagents as specified in the instructions. Insert the tube into the colorimeter and follow the instructions for performing a reagent blank in your meter manual. Remove the tube, dispose of the reagent blank, and use the same tube to run the test. Running a reagent blank eliminates errors that may result from small differences in reagent color. Using the same tube eliminates glassware errors.

Direct Reading Titrator

Instructions for use:

1. Fill the titration tube to the line with the water sample.
2. Add the reagents as specified in the instructions for the individual test method. Cap the tube with the special titration tube cap. Mix by swirling gently.
3. Depress the plunger of the Titrator to expel air.
4. Insert the Titrator into the plastic fitting of the titrating solution bottle.
5. To fill the Titrator invert the bottle and slowly withdraw the plunger until the bottom of the plunger is opposite the zero mark on the scale.
   NOTE: When filling the titrator from a container not fitted with a special plug, submerge the tip of the titrator below the surface of the solution and withdraw the plunger.
   NOTE: A small air bubble may appear in the Titrator barrel. Expel the bubble by partially filling the barrel and pumping the titration solution back into the inverted reagent container. Repeat this pumping action until the bubble disappears.
6. Turn the bottle right-side-up and remove the Titrator.
7. Insert the tip of the Titrator into the opening of the titrator tube cap. Slowly depress the plunger to dispense the titrating solution. Gently swirl tube to mix. A slight rotating or twisting motion may permit the plunger to move more smoothly.
8. Continue adding the titrating solution until the specified color change occurs. If no color change occurs by the time the plunger tip reaches the bottom of the scale, refill the Titrator to the zero mark. Continue the titration. Include both titration amounts in the final test result.
9. Read the test result directly from the scale opposite the bottom of the plunger tip.
10. If no additional tests are to be made, discard the titrating solution in the Titrator. Thoroughly rinse the Titrator and the titration tube.
    NOTE: The plunger tip should periodically be coated with silicon grease.
    NOTE: Store Direct Reading Titrators with the plunger slightly withdrawn.

Soil Testing

All of the LaMotte soil test kits measure the portion of the soil nutrient that would be available for the plant to use. Since extraction is not complete, the amount that is measured is relative, dependent on the extraction procedure. Pounds per acre represent the number of pounds of soil in an acre to the plow depth of 6-7 inches, or 2,000,000 lbs.

A number of variables must be considered when interpreting soil test results in addition to the values obtained. These variables include the composition of the soil, drainage, climate, previous fertilizer programs, and the type of plant to be grown. Samples must also be truly representative of the area being studied and must be carefully selected.

Fertilizer Recommedations

Most LaMotte soil test kits provide just basic information as to pH and nutrient requirements for a limited selection of plants. We strongly recommend consulting your local garden center or Cooperative Extension Service for specific recommendations that take into account the local soil type and specie of plant. Soils vary from one geographical area to another. Consult your local experts!

Mode EL Color Chart Values

The following information applies to values assigned to the color charts as supplied in the Model EL Garden Guide Test Kit:

Pounds per acre (lbs./A) represent the number of pounds of soil in an acre to the plough depth of 6-7 inches, or 2,000,000 lbs.

To convert phosphorus to the oxide, multiply the P value by 2.4

To convert potassium to the oxide, multiply the K value by 1.2

To convert lbs./A to ppm (parts per million) multiply lbs./A value by 0.5