MIL-STD-1330D(SH)
20 September 1996
(a) Select a pH probe with a low sodium ion error and Teflon junction that is rated for a pH range of 0 to 14. The Teflon junction will not clog as quickly as the commonly used ceramic junction. Use of thick glass pH probes rated at 0 to 12 will result in pH errors of up to 0.5 units. Alternatively, an ion selective field effect transistor (ISFET) probe can be used. The ISFET probe eliminates the pH sensitive glass tip easing clean up and extending life.
(b) Check the calibration of the pH meter with pH 10 and pH 12 buffer solutions. The common practice of checking pH meter calibration with pH 7 and pH 10 buffer solutions will not yield accurate results.
(c) After measuring the pH of NOC, clean the probe by soaking in a saturated solution of calcium hydroxide for about 1 hour. A saturated solution of calcium hydroxide can be prepared by mixing
0.2 gm calcium hydroxide with 100 mL of water. The presence of some precipitate indicates the solution is saturated.
5.13.5 NOC alkalinity. When specified, the alkalinity (hydroxide concentration) in NOC shall be evaluated as follows:
5.13.5.1 Equipment.
(a) A balance accurate to 0.01 grams with a zero drift less than 0.01 grams, and a capacity sufficient to prepare reagent solutions as specified in 5.13.5.3. Suitable balances are available through laboratory supply houses such as Thomas Scientific.
(b) Beakers or erlenmeyer flasks of 100 to 250 mL capacity, plastic or glass; or equal.
(c) Graduated cylinder of 50 to 100 mL capacity, plastic or glass; or equal.
(d) Eye dropper or pipette, plastic or glass; or equal.
(e) Syringe or buret, 35 to 50 mL capacity, with 1 mL graduation
intervals, preferably plastic; or equal.
5.13.5.2 Reagents.
(a) Potassium acid phthalate or potassium biphthalate (KHP), CAS No.
877-24-7, primary standard or acidimetric standard, crystals, ACS
reagent grade or better, available from chemical supply houses
such as Sigma Chemical Co. (800)325-3010. Do not use powdered
KHP; powdered KHP will not completely dissolve.
(b) Thymolphthalein, pH indicator, CAS No. 125-20-2, powder, available
from chemical supply houses such as Sigma Chemical Co. (800)325-
3010.
(c) Grade B water.
(d) Isopropyl alcohol as specified in 4.4.2.
5.13.5.3 Reagent preparation.
(a) Prepare a 4 percent wt/vol KHP solution by adding 100.0 ± 1.0 mL of grade B water to each 4.0 ± 0.1 grams of KHP (see 5.13.5.2(a)).
The KHP may not dissolve in the water immediately. After adding the water to the flask gently swirl or stir the contents for about
1 minute being careful not to deposit solid KHP on the sides of the container. Let the container stand for 10 minutes and swirl or stir the flask for an additional minute. The KHP solution has a 3-month shelf life when stored in a tightly sealed glass container.
(b) Prepare a 0.2 percent wt/vol thymolphthalein pH indicator solution by adding 50.0 ± 1.0 mL of isopropyl alcohol (see 5.13.5.2(d)) to each 0.1 ± 0.01 grams of thymolphthalein (see 5.13.5.2(b)). The solution may have to sit for up to 4 hours to allow the thymolphthalein to completely dissolve. The pH indicator should
be clear, water white with no precipitate. The pH indicator has a
1-month shelf life when stored in a tightly sealed brown glass or
plastic container.
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