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7.1.2 Each trument/electrode system must be calibrated at a <br /> minimum of two points that bracket the expected pH of the samples and are <br /> approximately three pH units or more apart. Repeat adjustments on <br /> successive portions of the two buffersolutions until readings are within <br /> 0.05 pH units of the buffer solution value. <br /> 7.2 Sam le Rreparation and pH measurement of noncalcareous soils: <br /> 7.2.1 To 20 g of soil in a 50-mL beaker, add 20 mL of Type II water <br /> and stir the suspension several times during the next 30 min. <br /> 7.2.2 Let the soil suspension stand for about 1 hr to allow most of <br /> the suspended clay to settle out from the suspension. <br /> 7.2.3 Adjust the electrodes in the clamps of the electrode holder <br /> so that, upon lowering the electrodes into the beaker, the lass <br /> electrode will be immersed just deep enough into the clear supernatant <br /> solution to establish a good electrical contact through the ground-glass <br /> s <br /> joint or the fiber-capillary hole. Insert the electrodes into the <br /> solution in this manner. For combination electrodes, immerse Just below <br /> the suspension. <br /> 7.2.4 If the sample temperature differs by more than 2'C from the <br /> buffer solution, the measured pH values must be corrected. <br /> 7.2.5 Report the results as "soil pH measured in water." <br /> 7.3 Sam le reparation and H measurement of calcareous soils: <br /> 7.3.1 To 10' g of soil in a 50-mL beaker, add 20 mL of 0.01 M CaC12 <br /> (Step 5.5) solution and stir the suspension several times Burin <br /> 30 min. g the next <br /> 7.3.2 Let the soil suspension stand for about 30 min to allow mo <br /> of the suspended clay to settle out from the suspension. st <br /> 7.3.3 Adjust the electrodes in the clamps of the electrode holder <br /> so that, upon lowering the electrodes into the beaker, the las <br /> electrode will be immersed well into the partly settled suspensiong s <br /> the calomel electrode will be immersed o and <br /> supernatant solution to establish a Just deep enough into the clear. <br /> ground-glass joint or the fiber-capillary <br /> Insert <br /> through the <br /> into the sample solution in this manner. t the electrode <br /> 7.3.4 If the sample temperature differfrom , <br /> J buffer S61141VOn. «e seosUred pH values must be co-T-ected. -s by more than 2% l'` <br /> . 7.3.5 Report the results as "soil pH mea <br /> aured in 0.01 M Car.100 <br /> 9045 — 3 <br /> Revision p <br /> Date Septem er 1986 <br />