An Inexpensive Device for Collection of Samples of Water for Dissolved Oxygen Determination without Air Contact Lello Favaretto FFCL Rlbeirao Prelo-USP
Figure 5. Reaction vessel tor me calalmeter. A = glass rod; B = buret. C = nylon lid: D = 250 cm' Dewar flask:F = insulating material: T = tnerm stor (NTC resistor 10 K at 25 'C: Yellow Springs instrument Ca).
14.049-Ribeirao Preto, SP. Brasil This communication describes a simple and inexpensive device that permits the collection of water samples and fixation of dissolved oxygen by the Winkler method, while preventing additional contact of the water sample with atmospheric 0 2 . Samples are taken with a 50-mL glass syringe provided with a three-way disk valve (see figure). The outlet is fitted with plastic tubing of appropriate diameter and length. While maintaining the free end of the ~ l a s t i ctubing immersed in water, turn the key valve to position 1,and p ; ~an amount of water (about 20 mL) into the syringe. Turn the key to position 2, and press the plunger to empty the syringe throuzh the lateral nozzle of the valve. Thus, this first water sample that has had contact with air is discharged. This step should he repeated two or three times, to ensure "no contaminated" water within the system. Turn the key back to position 1,and pull the plunger until thevolume of water reaches the 50-mL mark. Close the valve by turning the key to position 3. The sample is now ready for stock or subsequent determination of dissolved oxveen .- bv- the Winkler method. The addition of the appropriate reagents (MnSOa, KI NaOH, H3POa) t o the sample is achieved by injecting them, one a t a time, into the glass syringe, through the lateral nozzle of the valve with the aid of three smaller disposable plastic syringes (3 mL). Pull the desiredvolume of the chemical solution into one small syringe, and fit tightly to the lateral nozzle of the valve. Turn the key to position 2, and press the plunger. This will allow the chemical to pass into the glass syringe. I t is desirable, during this step, to pull and press the plunger of the small syringe two or three times to ensure rapid mixing of the chemical with the sample. Press the plunger a last time, turn the key to position 3, and uncouple the plastic syringe. Repeat this procedure for the other chemicals. The sample is now ready for the iodimetric titration. With the outlet of the valve directed to the inside of a 100mL Erlenmeyer flask, turn the key to position 1,and release the sample. T o make certain that all the iodine is recovered, rinse the system with a small amount of distilled water, and add this to the sample already in the Erlenmeyer flask. ~~~
Figure 6. Schematicdiagram of the calorimeter module. R1 = 620 Ck R 2 = 3 Kn;R3=4.7KR;R4=R6=10KO;R5=6.81KR;R7=R8=100KCkDZ= Zener 9 V (temperature compensated);C1 = 1 pF116 V (elecboiytlc):PI =
ing essentially of a 250-mL Dewar flask in which to carry out a chemical reaction (neutralization or precipitation) while monitoring the temperature change with a thermistor (NTC: Resistor with Negative Temperature Coefficient).The high sensitivityof this last element enahles one to work with dilute solutions. The 250-mL Dewar flask D is placed in a protective sheath F as shown in Figure 5; a lid C, made out of a nylon (or Styrofoam)block, supports asmall motor M (Crouzet 82.334) that rotates the stirrer A (60 rpm) as well as the thermistor T (10 KO) stuck to the tip of a glass tube. A small hole allows a buret tip to he introduced for the titration. The thermistor is one element of a Wheaatone hridge (Fig. 6 ) , whichis set at -1.0 Vwith apotentiometer P1 fixed on the calorimeter PCB. The equilibrium of the bridge is tuned with the potentiometer P2 (10-turnpotentiometer fixed on the front panel of the basic module) while the nonequilihrium potential of the bridge is amplified by the operational amplifier (IC2). The variation in the thermistor resistance decreases exponentially with an increase in temperature, while the nonequilibrium potential of the hridge is a homographic function of the resistance variation. It turns out that the nonequilihrium potential of the hridge varies almost linearly with the temperature in a range of 3 to 4 T near the initial temperature. The unbalanced potential of the hridge read on the DPM voltmeter (range f 1.999 V) is therefore directly proportional to the temperature of the solution in the Dewar flask. Taking into account the characteristicsof the components given in Figure 6, the sensitivityis about 200 mV oer demee. Mure detailed information including complete schematics, drawingsufprinted circuiroand a moredemiled theory ofoperationsmay he obmined by contacting one of the authors (MRP).
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Syringe with threeway valve used to collen water samples Volume 67
Number 6
June 1990
