Sample Flow Velocity and Low Level Sodium Ion Measurement with the Glass Electrode Edgar L. Eckfeldt’ and William E. Proctor, Jr.2 Corporate Research Department, Leeds & Northrup Company, North Wales, Pa. 19454
Low level sodium ion measurements are useful in power generation and the electronics industry for continuously monitoring high purity process water to guard against contamination (1-5). Preferred conditions relating to base reagent addition for measurements down to 0.1 ppb have been described (6, 7). The desirability of having flowing samples, a t least at moderate flow rate, is generally recognized. The purpose of the present note is to elucidate the importance of maintaining fast flow conditions for low level measurements. By using a flow channel of special design, which greatly enhances velocity of solution flow past the sensor bulb of the glass electrode, conditions are established a t the electrode interface whereby the electrode is able to respond to fresh sample solution rather than to sample solution that has become altered by the presence of the electrode. As a result, measurements are more reliable than heretofore, and the necessary volumetric rate of sample flow can be cut to a fraction of that previously required. This effect may also be useful in other ion-selective electrode measurements as a way to improve measurement sensitivity and accuracy.
BACKGROUND
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The effect of sample flow rate on the unmodified L&N 7971-1 sodium ion analyzer is illustrated in Figure 1. During the first half of the indicated time period, sample flow rate was 10 ml/min. At point A, the flow rate was increased to and then held at 100 ml/min. As will be observed, the increase in flow rate caused the indicated sodium ion concentration to decrease significantly, although solution composition remained constant. The lower value at the higher flow rate is believed to be a truer representation of actual sodium ion concentration than that registered a t low flow rate. For this reason, operating instructions currently specify that flow rate should be 100 to 125 ml/min. The cause of this flow effect is believed to be related to chemical reactivity of the sensor membrane of the glass electrode, which results in spontaneous release of alkali metal ions from the constitution of the glass at a slow rate into the surrounding solution. The basic condition of the sample solution probably accelerates the loss. The reactivity of pH-responsive glasses with water has been well established and the possibility of error from leaching of alkali from the sensor bulb in measuring pH of unbuffered solutions is generally recognized (8). Perley (9) has recommended strong agitation and preferably a continuous flow-type system for making such measurements, Glasses sensitive to sodium ion are generally similar to sensor glasses used for pH measurement, and one might therefore expect the possibility of a corresponding interference arising from release of alkali metal ions from sodium ion sensor glasses. Low level sodium ion measurement is probably more susceptible in general to this kind of interference .than pH measurement, because low level sodium ion sample solutions are unbuffered with respect to the ion being measured, while a great many low level hydrogen ion (high Present address, 6 Lindenwold Terrace, Ambler, Pa. 19002. Present address, 1702 Sheffield Drive, Norristown, Pa. 19403.
pH) measurements are made on pH buffered solutions where the effect is not noticeable. The sensor bulb of the sodium ion electrode used (L&N 117201) is made of a lithium aluminosilicate glass. Lithium glasses are generally more stable than sodium glasses (10) and should therefore show less interference from release of alkali than sodium glasses (9). A lithium glass should therefore be a good choice for low level measurements. Nevertheless, lithium ions from the sensor bulb used will constitute an interference threat. The selectivity of the sensor for lithium ions is only slightly less than that for sodium ions. Evidence such as that of Figure 1 is consistent with the assumption that interference is coming from the electrode bulb. Increasing sample flow should diminish such interference and cause the instrument reading to decrease, as observed. In liquids, transport of ions away from an electrode by diffusion alone is a slow process. Hence, in a quiescent solution, a significant concentration of the monovalent cations may accumulate in the solution layer directly in contact with the glass membrane and may result in a false response of the electrode. Convection induced by solution stirring, ultrasonic waves, or sample flow velocity will accelerate transport of interfering ions away from the sensor membrane. The important factor is believed to be not the volumetric flow rate but rather the flow velocity in the region close to the electrode surface. A high flow velocity can be maintained even a t low volumetric flow rate by reducing the cross-sectional area of the annular stream of solution flowing past the electrode. This may be accomplished by miniaturizing the electrode and channel or by shaping the channel approximately to conform with the electrode bulb. The latter technique was tested experimentally.
EXPERIMENTAL Pertinent experimental details are illustrated in Figure 2 which shows the standard polyethylene flow block of the L&N 7971-1 sodium ion analyzer, along with a removable insert placed in the glass electrode chamber. The insert was constructed of polyethylene in accordance with Figure 3. The outside diameter was machined to provide a friction fit in the flow block. The taper of the cavity was made to match the conical contour of the sensor bulb of the electrode. The insert was designed to allow the tip of the electrode bulb to rest on the small flat area a t the bottom of the cavity. The four Kel-F pins were intended to center the electrode stem and hold the bulb centrally in the cavity, to provide uniform clearance between bulb and cavity. This clearance approximated 0.4 mm. In operation, sample solution entered the bottom of the electrode chamber, passed upward through several holes in the bottom of the insert, flowed into the narrow interspace surrounding the electrode bulb, and then emerged a t the top of the insert into the normal passageway of the flow block. The arrangement established a much higher velocity of solution flow past the electrode bulb, for a given volumetric flow rate, than would be present without the insert. All tests used the identical sample water of low sodium ion content. The water was prepared by flowing distilled water in series through Illco-way Universal ion exchange columns. Diisopropylamine (50 m1/5 gal. water) was added for hydrogen ion suppres-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
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Figure 2. Modified flow block of the L&N 7971-1 sodium ion analyzer
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