Soil Organic Content Using Uv-Vis Method Txdot Designation: Tex-148-E

SOIL ORGANIC CONTENT USING UV-VIS METHOD TXDOT DESIGNATION: TEX-148-E

Test Procedure for

SOIL ORGANIC CONTENT USING UV-VIS METHOD TxDOT Designation: Tex-148-E Effective Date: March 2016 – September 2020

1. SCOPE

1.1 This method determines the soil organic content based on the amount of humic acid present in the soil sample by using ultraviolet-visible (UV-Vis) spectroscopy.

1.2 The values given in parentheses (if provided) are not standard and may not be exact mathematical conversions. Use each system of units separately. Combining values from the two systems may result in nonconformance with the standard.

2. DEFINITIONS

2.1 Ultraviolet-visible spectroscopy (UV-Vis)—The process of measuring the soil organic content by the use of UV-Vis absorption properties from soil extracts.

2.2 Cuvette—A small, transparent tube of circular or square cross section, sealed at one end, made of glass or fused quartz, and designed to hold sample solution for spectroscopic analysis.

2.3 Filtrate solution—Soil and reagents that has passed through a filter.

2.4 Blank—Solution that consists of reagents without soil sample.

3. APPARATUS

3.1 UV-Vis spectrometer, capable of measuring at a wavelength of 300 nm.

3.2 Balance, Class G1 in accordance with Tex-901-K, minimum capacity of 300 g.

3.3 Sieves, U.S. Standard No. 4 (4.75 mm) and No. 40 (425 μm).

3.4 Mortar and pestle.

3.5 Crusher.

3.6 Cuvettes (glass or quartz), 1 cm UV-Vis.

3.7 Polypropylene centrifuge tubes, 50 mL.

EFFECTIVE DATE: MARCH 2016 – JULY CONSTRUCTION DIVISION 1 – 6 2020

SOIL ORGANIC CONTENT USING UV-VIS METHOD TXDOT DESIGNATION: TEX-148-E

3.8 Luer-lok syringes, 10 mL.

3.9 Syringe filter, 25-mm filter with 0.45-μm polypropylene membrane.

3.10 Graduated cylinder (glass or plastic), 10 mL.

3.11 Lint-free wipes.

3.12 Wash bottles, for distilled or deionized water, hydrochloric acid, and sodium pyrophosphate solutions (minimum of 3).

3.13 Glass or polyethylene bottles, minimum capacity, 1 L, for storage of hydrochloric acid and sodium pyrophosphate solutions (2 required).

3.14 Volumetric flasks, 500 mL and 1000 mL.

3.15 Funnel.

3.16 Latex gloves.

3.17 Sample splitter.

4. MATERIALS

4.1 Hydrochloric acid—Solution, 37% reagent grade.

4.2 Sodium pyrophosphate decahydrate—crystal, reagent grade.

4.3 Sodium hydroxide—concentrated, reagent grade.

4.4 Standard soil samples with a soil organic content of 0.5, 1.2, and 1.5%—Contact the Geotechnical, Soils, and Aggregates Branch of the Construction Division’s Materials and Pavements Section to obtain standard soil samples.

5. PROCEDURES

5.1 Preparing Test Sample.

5.1.1 Obtain a 300-g representative soil sample in accordance with Tex-100-E.

5.1.2 Air-dry the sample to constant weight. Constant weight is achieved when the weight loss is less than 0.1% of the sample weight in 4 hours of drying. Note 1—Do not oven-dry samples, as this may alter test results.

5.1.3 Break down the air-dried sample to pass the No. 40 (425-µm) sieve using a mortar and pestle or a suitable mechanical pulverizer with an opening set from 0.025 to 0.035 in. (635 to 889 µm).

EFFECTIVE DATE: MARCH 2016 – JULY CONSTRUCTION DIVISION 2 – 6 2020

SOIL ORGANIC CONTENT USING UV-VIS METHOD TXDOT DESIGNATION: TEX-148-E

Note 2—When the sample is larger than a No. 10 (2.00-mm) sieve, a crusher or other suitable means may be used to reduce the particle size to allow it to be placed in a pulverizer. Note 3—If using a pulverizer, break down any material still aggregated in lumps larger than No. 40 (425 µm) with a mortar and pestle.

5.1.4 Use a sample splitter to split the sample obtained in Section 5.1.3 to obtain a 10-g representative sample and weigh to the nearest 0.1 g.

5.1.5 Use a balance to weigh a 0.10 ± 0.01-g sample and place it in a 50-mL polypropylene centrifuge tube.

5.1.6 Repeat Section 5.1.5 to produce 2 additional test samples.

5.2 Preparing Blank and Standard Soil Samples.

5.2.1 Obtain four 50-mL polypropylene centrifuge tubes and label them appropriately.

5.2.2 Prepare one sample from each known standard soil sample listed in Section 4.4.

5.2.3 Use a balance to weigh a 0.10 ± 0.01-g sample and place it in one of the 50-mL polypropylene centrifuge tubes labeled in Section 5.2.1.

5.2.4 Repeat Section 5.2.3 to produce one sample of each known standard.

5.3 Preparing Reagents. CAUTION: Use latex gloves when handling solutions and prepared reagents.

5.3.1 Prepare 1N hydrochloric acid (HCl) solution using a 500-mL volumetric flask.

5.3.1.1 Add 250 mL of distilled or deionized water from a wash bottle to a 500-mL volumetric flask.

5.3.1.2 Add 41.43 mL of 37% reagent grade HCl to the 250 mL of distilled or deionized water in the 500-mL volumetric flask.

5.3.1.3 Add additional distilled or deionized water from a wash bottle to fill the flask to the 500-mL mark.

5.3.1.4 Pour the solution into a glass or polyethylene bottle.

5.3.1.5 Add the solution to a clean, empty wash bottle using a funnel, and label it appropriately.

5.3.2 Prepare sodium pyrophosphate decahydrate solution (Na2P2O7 × 10H2O) using a 1000-mL volumetric flask.

5.3.2.1 Add 500 mL of distilled or deionized water from a wash bottle to a 1000-mL volumetric flask.

EFFECTIVE DATE: MARCH 2016 – JULY CONSTRUCTION DIVISION 3 – 6 2020

SOIL ORGANIC CONTENT USING UV-VIS METHOD TXDOT DESIGNATION: TEX-148-E

5.3.2.2 hydroxideAdd 44.6 g ( NaOH)of dry reagent to the 500 Na 2mLP2O of7 × distilled 10H2O orand deionized 10 g of reagent water in grade the 1000 sodium-mL volumetric flask.

5.3.2.3 Add additional distilled or deionized water from a wash bottle to fill the flask to the 1000-mL mark.

5.3.2.4 Pour the solution into a glass or polyethylene bottle.

5.3.2.5 Add the solution to a clean, empty wash bottle using a funnel, and label it appropriately.

5.4 Extracting Soil Organic Content. CAUTION: Use latex gloves when handling solutions and prepared reagents.

5.4.1 Measure 5 mL of 1N HCl solution using a graduated cylinder and add to each polypropylene centrifuge tube for the blank, the 3 standard soil samples, and the 3 test samples.

5.4.2 Vigorously shake the centrifuge tubes of soil and HCl solution by hand for 10 seconds at 1-minute intervals, 5 times.

5.4.3 Place each centrifuge tube in a centrifuge tube holder after shaking.

5.4.4 Measure 20 mL of Na2P2O7 × 10H2O solution using a graduated cylinder and add to each polypropylene centrifuge tube for the blank, the 3 standard soil samples, and the 3 test samples.

5.4.5 Vigorously shake the centrifuge tubes by hand for 10 seconds at 1-minute intervals, 5 times.

5.4.6 Place the centrifuge tubes in a centrifuge tube holder for 10 minutes to allow the soil samples to settle and the air bubbles to dissipate.

5.4.7 Add approximately 10 mL of the solution from each centrifuge tube to clean 10-mL luer-lok syringes and attach a 0.45-µm syringe filter to each luer-lok syringe. Note 4—Ensure all equipment is clean and free of any residue from previous testing. Use a new syringe filter for each sample including the blank.

5.4.8 Fill each cuvette using the filtrate solution from Section 5.4.7.

5.5 Measuring Soil Organic Content.

5.5.1 Prepare the UV-Vis spectrometer for testing according to the manufacturer’s guidelines and recommendations.

5.5.2 Wipe the sides of each cuvette using a lint-free wipe to remove dirt, fingerprints, or any other residue that will obstruct the light beam from passing through the cuvette.

CAUTION: Use latex gloves when wiping the sides of the cuvettes. Note 5—Ensure the soil-solutions in the cuvettes are free from bubbles and particulates.

EFFECTIVE DATE: MARCH 2016 – JULY CONSTRUCTION DIVISION 4 – 6 2020

SOIL ORGANIC CONTENT USING UV-VIS METHOD TXDOT DESIGNATION: TEX-148-E

5.5.3 Place a cuvette into the UV-Vis spectrometer. Note 6—Test the cuvettes in the following order: blank; known standard soil samples 0.5%, 1.2%, and 1.5%; and test samples.

5.5.4 Test the sample at a wavelength of 300 nm.

5.5.5 Record the absorbance value from the test.

5.5.6 Repeat Sections 5.5.2 to 5.5.5 for each cuvette.

5.5.7 Plot the soil organic content versus the measured absorbance from the blank and the 3 standard soil samples as shown in Figure 1.

2.00 Absorbance vs SOC%

1.50

e y = mx + b 1.00 R2 = 0.9816 sorbanc

Ab 0.50

0.00 0.0 0.5 1.0 1.5 Soil Organic Content (SOC), %

Known Standards & Blank Best-fit Line

Figure 1—Example of Soil Organic Content (SOC) vs. Absorbance

5.5.8 Determine the best-fit line equation and the R-square value for the results plotted in Section 5.5.7. Note 7—The minimum R-square value for the best-fit line must be greater than or equal to 0.9500.

5.5.9 When the R-square value is less than 0.9500, prepare and test additional blank and standard soil samples to improve the best-fit line to achieve the minimum R-square value. Note 8—Refer to the manufacturer’s guidelines and recommendations for the UV-Vis spectrometer when an R-square value of 0.9500 or greater cannot be achieved.

5.5.10 Refer to Section 6 and calculate the soil organic content of each test sample using the equation of the best-fit line determined in Section 5.5.8 or 5.5.9.

EFFECTIVE DATE: MARCH 2016 – JULY CONSTRUCTION DIVISION 5 – 6 2020

SOIL ORGANIC CONTENT USING UV-VIS METHOD TXDOT DESIGNATION: TEX-148-E

6. CALCULATIONS

6.1 Calculate the soil organic content of each test sample:

푦 − 푏 푆푂퐶 = Where: 푚 SOC = soil organic content, % y = absorbance m = slope of best fit line b = y intercept

6.2 Record the soil organic content to the nearest tenth, 0.1.

7. TEST REPORT

7.1 Report the average soil organic content of the unknown soil samples as a percent to the nearest tenth, 0.1.

EFFECTIVE DATE: MARCH 2016 – JULY CONSTRUCTION DIVISION 6 – 6 2020