28 Inorganic Qualitative Analysis (QUAL) Heating You should not use a burner flame to directly heat liquid in a small test-tube. Such direct heating is too intense and too localized. It will usually cause the liquid to blow out of the test-tube. This practice can cause injury and, in any case, ruins the experiment. Heating is to be accomplished by resting the test-tubes in beakers of heated water. This heating is more even and keeps the temperature from exceeding 100 °C. If a test-tube heating platform (also called an aluminum holder) is available it should be used in the beaker. Otherwise use a beaker large enough to keep the test- tubes from protruding too far over the beaker lip. Adjust the water level in the beaker so that only the lower 1/2 to 3/4 of the test-tube is submerged. Move the burner flame in and out as needed to keep the water hot but to prevent violent boiling (see Figure D). pH Measurement and Control It is important in many reactions that the solution’s pH becarefullycontrolled.Thecontrolisachievedbyadding anacidorbasesolutiondrop- wise until the required pH is obtained. The pH of the solutionshouldbemeasured after  each  dropwise  addi- tion by using litmus or pH paper. Do not dip the pH paperintothesolution.This practice    can    lead    to chemicals  from  the  paper getting  into  the  solution. The proper procedure is to mix  the  solution  with  a stirring  rod  and  then  to touch the wet end of the rod to the test paper. The pH of a gas bubbling out of a solution can be tested by wetting the test paper with water and then holding it over the mouth of the test-tube. Contamination Even the smallest contamination of your ion or reagent solutions can change the test results. Contamination is a particular problem with these experiments because you reuse equipment so often. Clean and distilled water rinse each piece of equipment after each use. Thus you should clean a dropper before using it in a different solution, clean a stirring rod before using it to mix a second solution, etc. Often cleaning requires only that the test-tube, rod, etc. be rinsed several times with tap water then 2 to 3 times with distilled H₂O. The equipment does not have to be dry. Use a test-tube brush to clean out stubborn ppts. A dropper can be cleaned by removing the bulb and running water down the barrel. Wire gauze Hot water Figure D. Heating a sample Solution drop Figure E. Testing pH.